Channels of energy redistribution in short-pulse laser interactions with metal targets

L. V. Zhigilei and D. S. Ivanov

Appl. Surf. Sci., in press, 2005.

Abstract

The kinetics and channels of laser energy redistribution in a target irradiated by a short, 1 ps, laser pulse is investigated in computer simulations performed with a model that combines molecular dynamics (MD) simulations with a continuum description of the laser excitation and relaxation of the conduction band electrons, based on the two-temperature model (TTM). The energy transferred from the excited electrons to the lattice splits into several parts, namely the energy of the thermal motion of the atoms, the energy of collective atomic motions associated with the relaxation of laser-induced stresses, the energy carried away from the surface region of the target by a stress wave, the energy of quasi-static anisotropic stresses, and, at laser fluences above the melting threshold, the energy transferred to the latent heat of melting and then released upon recrystallization. The presence of the non-thermal channels of energy redistribution (stress wave and quasi-static stresses), not accounted for in the conventional TTM model, can have important implications for interpretation of experimental results on the kinetics of thermal and mechanical relaxation of a target irradiated by a short laser pulse as well as on the characteristics of laser-induced phase transformations. The fraction of the non-thermal energy in the total laser energy partitioning increases with increasing laser fluence.

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Mesoscopic model for dynamic simulations of carbon nanotubes

L. V. Zhigilei, C. Wei, and D. Srivastava

Phys. Rev. B 71, 165417, 2005.

Abstract

A mesoscopic model is developed for static and dynamic simulations of nanomechanics of carbon nanotubes (CNTs). The model is based on a coarse-grained representation of CNTs as "breathing flexible cylinders" consisting of a variable number of segments. Internal interactions within a CNT are described by a mesoscopic force field (MFF) designed and parameterized based on the results of atomic-level molecular dynamics simulations. The radial size of the CNTs and external interactions among multiple CNTs and molecular matrix are introduced through a computationally efficient "virtual surface" method that does not require explicit representation of the CNT's surfaces. The mesoscopic model is shown to reproduce well the dynamic behavior of individual CNTs predicted in atomistic simulations at a minor fraction of the computational cost, opening the way for investigation of mesoscopic structure and collective dynamics of nanotubes in real devices and in nanocomposites.

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Molecular dynamics simulation of sputtering from a cylindrical track: EAM vs. pair potentials

Orenthal J. Tucker, Dmitriy S. Ivanov, Leonid V. Zhigilei, Robert E. Johnson, and Eduardo M. Bringa

Nucl. Instr. Meth. B 228, 163-169, 2005.

Abstract

Molecular dynamics simulations implementing the thermal spike model for sputtering by energetic particle bombardment are performed for a gold target represented with a many-body Embedded Atom Method (EAM) potential. A linear dependence of sputtering yield on the effective energy deposition is observed in a broad range of sufficiently high excitation energies, suggesting that the conclusions of earlier simulations performed with pair potentials have a general character and are not sensitive to the choice of interatomic potential. At the same time, significant differences in cluster ejection are observed between the simulations performed with EAM and pair potentials. Clusters constitute a much larger fraction of the total yield in the EAM simulations, which is related to the environmental dependence of the interatomic interaction in metals that is correctly reproduced by EAM potential. An apparent disagreement between the analytical thermal spike model and its implementation in MD simulations cannot be attributed to the choice of interatomic potential but reflects a difference in the ejection mechanisms. A thermally-activated evaporation from the surface is assumed in the analytical thermal spike model, whereas a prompt ejection from a relatively deep part of the excited region and fast non-diffusive cooling of the spike region takes place in MD simulations.

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Molecular-dynamics study of thermal boundary resistance: evidence of strong inelastic scattering transport channels

Robert J. Stevens, Pamela M. Norris, and Leonid V. Zhigilei

Proceedings of the 2004 ASME International Mechanical Engineering Congress (IMECE'04), paper IMECE2004-60334, 2004.

Abstract

With the ever-decreasing size of microelectronics, growing applications of superlattices, and development of nanotechnology, thermal resistances of interfaces are becoming increasingly central to thermal management. Although there has been much success in understanding thermal boundary resistance (TBR) at low temperature, the current models for room temperature TBR are not adequate. This work examines TBR using molecular dynamics (MD) simulations of a simple interface between two FCC solids. The simulations reveal a temperature dependence of TBR, which is an indication of inelastic scattering in the classical limit. Introduction of point defects and lattice-mismatch-induced disorder in the interface region is found to assist the energy transport across the interface. This is believed to be due to the added sites for inelastic scattering and optical phonon excitation. A simple MD experiment was conducted by directing a phonon wave packet towards the interface. Inelastic scattering, which increases transport across the interface, was directly observed. Another mechanism of energy transport through the interface involving localization of optical phonon modes at the interface was also revealed in the simulations.

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Computer modeling of laser melting and spallation of metal targets

Leonid V. Zhigilei, Dmitriy S. Ivanov, Elodie Leveugle, Babak Sadigh, and Eduardo M. Bringa

High-Power Laser Ablation V, Proc. SPIE 5448, 505-519, 2004.

Abstract

The mechanisms of melting and photomechanical damage/spallation occurring under extreme superheating/deformation rate conditions realized in short pulse laser processing are investigated in a computational study performed with a hybrid atomistic-continuum model. The model combines classical molecular dynamics method for simulation of non-equilibrium processes of lattice superheating and fast phase transformations with a continuum description of the laser excitation and subsequent relaxation of the conduction band electrons. The kinetics and microscopic mechanisms of melting are investigated in simulations of laser interaction with free-standing Ni films and bulk targets. A significant reduction of the overheating required for the initiation of homogeneous melting is observed and attributed to the relaxation of laser-induced stresses, which leads to the uniaxial expansion and associated anisotropic lattice distortions. The evolution of photomechanical damage is investigated in a large-scale simulation of laser spallation of a 100 nm Ni film. The evolution of photomechanical damage is observed to take place in two stages, the initial stage of void nucleation and growth, when both the number of voids and the range of void sizes are increasing, followed by the void coarsening, coalescence and percolation, when large voids grow at the expense of the decreasing population of small voids. In both regimes the size distributions of voids are found to be well described by the power law with an exponent gradually increasing with time. A good agreement of the results obtained for the evolution of photomechanical damage in a metal film with earlier results reported for laser spallation of molecular systems and shock-induced back spallation in metals suggests that the observed processes of void nucleation, growth and coalescence may reflect general characteristics of the dynamic fracture at high deformation rates.

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Computational investigation of the effect of cluster impact energy on the microstructure of films grown by cluster deposition

Avinash M. Dongare, Derek D. Hass, and Leonid V. Zhigilei

Proceedings of the International Symposium on Clusters and Nano-Assemblies (ISCANA'03), in press.

Abstract

The microstructure of thin film growth during low-energy cluster beam deposition is studied in a series of molecular dynamics simulations. The films are grown by depositing Ni clusters on a Ni (111) substrate at room temperature. The deposition of a single Ni cluster is first studied, followed by a detailed analysis of the effect of the impact velocity of the deposited clusters on the microstructure of the growing film. The observed differences in the microstructure are related to the differences in the impact-induced processes. In the case of the lower incident energy only a partial transient melting of a small contact region between the incoming cluster and the film takes place. Epitaxial growth is seen to occur for the first few layers of the clusters in contact with the substrate, above which the clusters largely retain their crystal structure and orientation. The films grown by deposition of low-energy clusters have a low density (~50% of the density of a perfect crystal) and a porous "foamy" structure with a large number of interconnected voids. The higher-energy impacts lead to the complete melting and recrystallization of the whole cluster and a large region of the film, leading to the epitaxial growth, smaller number of localized voids, and a higher overall density of the growing film.

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Combined atomistic - continuum model for simulation of laser interaction with metals: application to calculation of melting thresholds in Ni targets of varying thickness

Dmitriy S. Ivanov and Leonid V. Zhigilei

Appl. Phys. A 79, 977-981, 2004.

Abstract

The threshold laser fluence for the onset of surface melting is calculated for Ni films of different thicknesses and for a bulk Ni target using a combined atomistic-continuum computational model. The model combines classical molecular dynamics (MD) method for simulation of non-equilibrium processes of lattice superheating and fast phase transformations with a continuum description of the laser excitation and subsequent relaxation of the conduction band electrons based on the two-temperature model (TTM). In the hybrid TTM-MD method, MD substitutes the TTM equation for the lattice temperature and the diffusion equation for the electron temperature is solved simultaneously with MD integration of the equations of motion of atoms. The dependence of the threshold fluence on the film thickness predicted in TTM-MD simulations qualitatively agrees with TTM calculations, while the values of the thresholds for thick films and bulk targets are ~10% higher in TTM-MD. The quantitative differences between the predictions of TTM and TTM-MD demonstrate that the kinetics of laser melting as well as the energy partitioning between the thermal energy of atomic vibrations and energy of the collective atomic motion driven by the relaxation of the laser-induced pressure should be taken into account in interpretation of experimental results on surface melting.

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Microscopic mechanisms of short pulse laser spallation of molecular solids

Elodie Leveugle and Leonid V. Zhigilei

Appl. Phys. A 79, 753-756, 2004.

Abstract

The mechanisms of photomechanical spallation are investigated in a large-scale MD simulation of laser interaction with a molecular target performed in the irradiation regime of the inertial stress confinement. The relaxation of laser-induced thermoelastic stresses is found to be responsible for the nucleation, growth and coalescence of voids in a broad sub-surface region of the irradiated target. The depth of the region subjected to the void evolution is defined by the competition between the evolving tensile stresses and thermal softening of the material due to the laser heating. The initial void volume distribution obtained in the simulation of laser spallation can be well described by a power law. A similar volume distribution is obtained in a series of simulations of uniaxial expansion of the same molecular system performed at a strain rate and temperature realized in the irradiated target. Spatial and time evolution of the laser-induced pressure predicted in the MD simulation is related to the results of integration of a thermoelastic wave equation and the scope of applicability of the continuum calculations is discussed.

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Molecular dynamics simulations of shocks including electronic heat conduction and electron-phonon coupling

Dmitriy S. Ivanov, Leonid V. Zhigilei, Eduardo M. Bringa, Maurice De Koning, Bruce A. Remington, Maria Jose Caturla, and Stephen M. Pollaine

Shock Compression of Condensed Matter - 2003, AIP Conference Proceedings 706, 225-228, 2004.

Abstract

Shocks are often simulated using the classical molecular dynamics (MD) method in which the electrons are not included explicitly and the interatomic interaction is described by an effective potential. As a result, the fast electronic heat conduction in metals and the coupling between the lattice vibrations and the electronic degrees of freedom can not be represented. Under conditions of steep temperature gradients that can form near the shock front, however, the electronic heat conduction can play an important part in redistribution of the thermal energy in the shocked target. We present the first atomistic simulation of a shock propagation including the electronic heat conduction and electron-phonon coupling. The computational model is based on the two-temperature model (TTM) that describes the time evolution of the lattice and electron temperatures by two coupled non-linear differential equations. In the combined TTM-MD method, MD substitutes the TTM equation for the lattice temperature. Simulations are performed with both MD and TTM-MD models for an EAM Al target shocked at 300 kbar. The target includes a tilt grain boundary, which provides a region where shock heating is more pronounced and, therefore, the effect of the electronic heat conduction is expected to be more important. We find that the differences between the predictions of the MD and TTM-MD simulations are significantly smaller as compared to the hydrodynamics calculations performed at similar conditions with and without electronic heat conduction.

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Photomechanical spallation of molecular and metal targets: molecular dynamics study

Elodie Leveugle, Dmitriy S. Ivanov, and Leonid V. Zhigilei

Appl. Phys. A 79, 1643-1655, 2004.

Abstract

Microscopic mechanisms of photomechanical spallation are investigated in a series of large-scale molecular dynamics simulations performed for molecular and metal targets. A mesoscopic breathing sphere model is used in simulations of laser interaction with molecular targets. A coupled atomistic-continuum model that combines molecular dynamics method with a continuum description of the laser excitation and subsequent relaxation of the conduction band electrons is used for metal targets. Similar mechanisms of the laser-induced photomechanical spallation are observed for molecular and metal targets. For both target materials, the relaxation of compressive stresses generated under conditions of stress confinement is found to be the main driving force for the nucleation, growth and coalescence of voids in a sub-surface region of an irradiated target at laser fluences close to the threshold for fragmentation. The mechanical stability of the region subjected to the void nucleation is strongly affected by the laser heating and the depth of the spallation region in bulk targets is much closer to the surface as compared to the depth where the maximum tensile stresses are generated. Two stages can be identified in the evolution of voids in laser spallation, the initial void nucleation and growth, with the number of voids of all sizes increasing, followed by void coarsening and coalescence, when the number of large voids increases at the expense of quickly decreasing population of small voids. The void volume distributions are found to be relatively well described by power law N(V) ~ V-t, with exponent gradually increasing with time. Comparison of the simulation results obtained for Ni films of two different thicknesses and bulk Ni targets suggests that the size/shape of the target plays an important role in laser spallation. The reflection of the laser-induced pressure wave from the back surface of a film results in higher maximum tensile stresses and lower threshold fluence for spallation. As the size of the film increases, the location of the spallation region and the region of the maximum tensile stresses are splitting apart and the threshold fluence for spallation increases.

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Materials science under extreme conditions of pressure and strain rate

B. A. Remington, G. Bazan, J. Belak, E. Bringa, M. Caturla, J. D. Colvin, M. J. Edwards, S. G. Glendinning, D. S. Ivanov, B. Kad, D. H. Kalantar, M. Kumar, B. F. Lasinski, K. T. Lorenz, J. M. McNaney, D. D. Meyerhofer, M. A. Meyers, S. M. Pollaine, D. Rowley, M. Schneider, J. S. Stölken, J. S. Wark, S. V. Weber, W. G. Wolfer, B. Yaakobi, and L. V. Zhigilei

Metall. Mater. Trans. A 35, 2587-2607, 2004.

Abstract

Solid-state dynamics experiments at very high pressures and strain rates are becoming possible with high-power laser facilities, albeit over brief intervals of time and spatially small scales. To achieve extreme pressures in the solid state requires that the sample be kept cool, with Tsample < Tmelt. To this end, a shockless, plasma-piston "drive" has been developed on the Omega laser, and a staged shock drive was demonstrated on the Nova laser. To characterize the drive, velocity interferometer measurements allow the high pressures of 10 to 200 GPa (0.1 to 2 Mbar) and strain rates of 106 to 108 s1 to be determined. Solid-state strength in the sample is inferred at these high pressures using the Rayleigh-Taylor (RT) instability as a "diagnostic." Lattice response and phase can be inferred for single-crystal samples from time-resolved X-ray diffraction. Temperature and compression in polycrystalline samples can be deduced from extended X-ray absorption fine-structure (EXAFS) measurements. Deformation mechanisms and residual melt depth can be identified by examining recovered samples. We will briefly review this new area of laser-based materials-dynamics research, then present a path forward for carrying these solid-state experiments to much higher pressures, P > 103 GPa (10 Mbar), on the National Ignition Facility (NIF)laser at Lawrence Livermore National Laboratory.

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The limit of overheating and the threshold behavior in laser ablation

Barbara J. Garrison,Tatiana E. Itina, and Leonid V. Zhigilei

Phys. Rev. E, 68, 041501, 2003.

Abstract

Constant temperature and pressure molecular-dynamics simulations in conjunction with constant pressure and enthalpy simulations, designed to examine the threshold behavior in laser ablation, demonstrate that the rate of homogeneous nucleation (explosive boiling) increases sharply in a very narrow temperature range at approximately 90% of the critical temperature. Moreover, the homogeneous nucleation is sufficiently rapid to prevent the superheated liquid from entering the spinodal region at densities greater than the critical density.

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The effect of pressure relaxation on the mechanisms of short pulse laser melting

Dmitriy S. Ivanov and Leonid V. Zhigilei

Phys. Rev. Lett. 91, 105701, 2003.

Abstract

The kinetics and microscopic mechanisms of laser melting of a thin metal film are investigated in a computational study that combines molecular dynamics simulations with a continuum description of the laser excitation and subsequent relaxation of the conduction band electrons. Two competing melting mechanisms, homogeneous nucleation of liquid regions inside the crystalline material and propagation of melting fronts from external surfaces, are found to be strongly affected by the dynamics of the relaxation of the laser-induced pressure.

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Combined atomistic-continuum modeling of short pulse laser melting and disintegration of metal films

Dmitriy S. Ivanov and Leonid V. Zhigilei

Phys. Rev. B 68, 064114, 2003.

Abstract

The kinetics and microscopic mechanisms of laser melting and disintegration of thin Ni and Au films irradiated by a short, from 200 fs to 150 ps, laser pulse are investigated in a coupled atomistic-continuum computational model. The model provides a detailed atomic-level description of fast non-equilibrium processes of laser melting and film disintegration and, at the same time, ensures an adequate description of the laser light absorption by the conduction band electrons, the energy transfer to the lattice due to the electron-phonon coupling, and the fast electron heat conduction in metals. The interplay of two competing processes, the propagation of the liquid-crystal interfaces (melting fronts) from the external surfaces of the film and homogeneous nucleation and growth of liquid regions inside the crystal, is found to be responsible for melting of metal films irradiated by laser pulses at fluences close to the melting threshold. The relative contributions of the homogeneous and heterogeneous melting mechanisms are defined by the laser fluence, pulse duration, and the strength of the electron-phonon coupling. At high laser fluences, significantly exceeding the threshold for the melting onset, a collapse of the crystal structure overheated above the limit of crystal stability takes place simultaneously in the whole overheated region within ~2 ps, skipping the intermediate liquid-crystal coexistence stage. Under conditions of the inertial stress confinement, realized in the case of short τ ≤ 10 ps laser pulses and strong electron-phonon coupling (Ni films), the dynamics of the relaxation of the laser-induced pressure has a profound effect on the temperature distribution in the irradiated films as well as on both homogeneous and heterogeneous melting processes. Anisotropic lattice distortions and stress gradients associated with the relaxation of the laser-induced pressure destabilize the crystal lattice, reduce the overheating required for the initiation of homogeneous melting down to T ≈ 1.05 Tm, and expand the range of pulse durations for which homogeneous melting is observed in 50 nm Ni films up to ~150 ps. High tensile stresses generated in the middle of an irradiated film can also lead to the mechanical disintegration of the film.

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Evolution of a plume in laser ablation: dependence on the spot size

Michael I. Zeifman, Barbara J. Garrison, and Leonid V. Zhigilei

Proceedings of the 36th AIAA Thermophysics Conference, AIAA Paper 2003-3493, 2003.

Abstract

The interaction of a powerful laser beam with a target material may result in the formation of a plume, which consists of gas molecules and molecular clusters directly ejected from the surface. A hybrid Molecular Dynamics - direct simulation Monte Carlo scheme capable to model the formation and evolution of the multi-component plume has been proposed recently. In the present study, the first objective is precise characterization of various types of collisions and other reactions among the molecules and clusters with the aid of molecular dynamics simulations. The updated scheme is then applied to study the effect of laser spot size on the evolution of the plume. The increased number of reactions per particle for larger spot size smoothed out a typical two-fold monomer density profile observed for smaller spot sizes.

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Atomistic simulation study of misfit strain relaxation mechanisms in heteroepitaxial islands

Avinash M. Dongare and Leonid V. Zhigilei

Mat. Res. Soc. Symp. Proc. 749, W10.12.1-W10.12.6, 2003.

Abstract

The mechanisms of the misfit strain relaxation in heteroepitaxial islands are investigated in two-dimensional molecular dynamics simulations. Stress distributions are analyzed for coherent and dislocated islands. Thermally-activated nucleation of misfit dislocations upon annealing at an elevated temperature and their motion from the edges of the islands towards the positions corresponding to the maximum strain relief is observed and related to the corresponding decrease of the total strain energy of the system. Differences between the predictions of the energy balance and force balance criteria for the appearance of misfit dislocations is discussed. Simulations of an island located at different distances form the edge of a mesa indicate that the energy of the system decreases sharply as the island position shifts toward the edge. These results suggest that there may be a region near the edge of a mesa where nucleation and growth of ordered arrays of islands is favored.

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Computer simulations of laser ablation of molecular substrates

Leonid V. Zhigilei, Elodie Leveugle, Barbara J. Garrison, Yaroslava G. Yingling, and Michael I. Zeifman

Chem. Rev. 103, 321-348, 2003.

Abstract

Computer modeling is playing an increasingly important role in the development of a better understanding of complex processes involved in laser ablation of molecular systems. In order to provide an adequate description of the diverse processes induced by pulsed laser irradiation, a range of computational models has been adapted to study various aspects of laser ablation with appropriate spatial and temporal resolution. The methods include the breathing sphere model for molecular-level simulation of the initial stage of laser ablation, the direct simulation Monte Carlo method for simulation of the multi-component ablation plume expansion, and the atomic-level molecular dynamics technique for investigation of the redistribution of the deposited laser energy among the translational and internal degrees of freedom of molecules. The results obtained to date include prediction of a fluence threshold for ablation, identification of the processes responsible for material ejection in the regimes of thermal and stress confinement, a detailed microscopic picture of the dynamics of the early stages of the ablation plume formation and subsequent long-term plume evolution, and a description of the abundance of clusters and their distribution in the ejected plume. Velocity distributions of the ejected clusters and individual molecules of different masses and the shapes and amplitudes of the acoustic waves propagating from the absorption region have been also studied and related to the ablation mechanisms and experimental data. In this paper we review the computational methods and the basic physical picture that emerges from the simulations, present new results, and discuss future prospects for computational investigations of laser ablation at different time and length scales.

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Dynamics of the plume formation and parameters of the ejected clusters in short-pulse laser ablation

Leonid V. Zhigilei

Appl. Phys. A 76, 339-350, 2003.

Abstract

The dynamics of the early stages of the ablation plume formation and the mechanisms of cluster ejection are investigated in large-scale MD simulations. The cluster composition of the ablation plume has a strong dependence on the irradiation conditions and is defined by the interplay of a number of processes during the ablation plume evolution. At sufficiently high laser fluences, the phase explosion of the overheated material leads to the formation of a foamy transient structure of interconnected liquid regions that subsequently decomposes into a mixture of liquid droplets, gas-phase molecules, and small clusters. The ejection of the largest droplets is attributed to the hydrodynamic motion in the vicinity of the melted surface, especially active in the regime of stress confinement. Spatially-resolved analysis of the dynamics of the plume formation reveals the effect of segregation of the clusters of different sizes in the expanding plume. A relatively low density of small/medium clusters is observed in the region adjacent to the surface, where large clusters are being formed. Medium-size clusters dominate in the middle of the plume and only small clusters and monomers are observed near the front of the expanding plume. Despite being ejected from deeper under the surface, the larger clusters in the plume have substantially higher internal temperatures as compared to the smaller clusters. The cluster size distributions can be relatively well described by a power law Y(N) ~ N-t with exponents different for small, up to ~15 molecules, and large clusters. The decay is much slower in the high mass region of the distribution.

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Substrate-assisted laser-initiated ejection of proteins embedded in water films

Yusheng Dou, Nicholas Winograd, Barbara J. Garrison, and Leonid V. Zhigilei

J. Phys. Chem. B 107, 2362-2365, 2003.

Abstract

Molecular dynamics simulations have been employed to investigate laser initiated liftoff of the protein enkephalin embedded in a H2O film adsorbed onto a gold substrate. The laser energy is deposited solely into the gold substrate at different heating rates. For fast heating rates on the picosecond timescale, the results show that large clusters of the H2O molecules are ejected from the film entraining the enkephalin molecule away from the metal surface. For heating on the nanosecond timescale, the H2O overlayer evaporates in the form of individual molecules and small clusters that are unable to entrain the enkephalin molecule.

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A hybrid MD-DSMC model of picosecond laser ablation and desorption

Michael I. Zeifman, Barbara J. Garrison, and Leonid V. Zhigilei

Proceedings of the 23rd International Symposium on Rarefied Gas Dynamics, AIP Conference Proceedings 663, 939-946, 2003.

Abstract

A two-stage computational model of the evolution of a plume generated by laser ablation of an organic solid is presented and discussed. The first stage of the laser ablation involves laser coupling to the target and ejection of the plume and is described by the molecular dynamics (MD) simulation. The following stage of a long-term expansion of the ejected plume is modeled by the direct simulation Monte Carlo (DSMC) method. The results of the MD simulations demonstrate that the physical mechanism of material ejection at sufficiently high laser fluences is a phase explosion of the overheated material followed by a homogeneous decomposition of the expanding plume into a mixture of liquid droplets (molecular clusters) and gas phase molecules. The extremely low proportion of large-size clusters hinders both statistical description of their parameters from the results of MD simulations and the following representation of each cluster size as a separate species, as required in the conventional DSMC. Therefore, a new computational scheme, which treats the size of large clusters as a random variable, is developed. The results of the hybrid model demonstrate that even for low laser fluences and short pulse duration, the evolution of the plume differs considerably from that predicted by pure thermal desorption models. For high fluences, the phase explosion of the target material and intensive processes of particle interactions within the plume are responsible for dramatic changes in the plume evolution as compared to that at low fluences.

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Multiscale modeling of laser ablation: applications to nanotechnology

Leonid V. Zhigilei and Avinash M. Dongare

CMES: Computer Modeling in Engineering & Sciences 3, 539-555, 2002.

Abstract

Computational modeling has a potential of making an important contribution to the advancement of laser-driven methods in nanotechnology. In this paper we discuss two computational schemes developed for simulation of laser coupling to organic materials and metals and present a multiscale model for laser ablation and cluster deposition of nanostructured materials. In the multiscale model the initial stage of laser ablation is reproduced by the classical molecular dynamics (MD) method. For organic materials, the breathing sphere model is used to simulate the primary laser excitations and the vibrational relaxation of excited molecules. For metals, the two temperature model coupled to the atomistic MD model provides an adequate description of the laser energy absorption into the electronic system and fast electron heat conduction. A combined MD - finite element method and the dynamic boundary condition are used to avoid reflection of the laser-induced pressure wave from the boundary of the MD computational cell. The direct simulation Monte Carlo method is used for simulation of the long term ablation plume expansion, and the MD method is used to simulate film growth by cluster deposition from the ablation plume. The proposed multiscale approach is applied to investigate the mechanisms of cluster formation in laser ablation and to analyze the distributions of clusters of different sizes in the ejected plume. MD simulations of cluster deposition are performed for different impact velocities and a strong dependence of the structure of the growing films on the parameters of the deposited clusters is revealed. A new technique for controlled implantation of functional organic molecules into sub-micron regions of a polymer substrate is investigated in molecular-level simulations and different regimes of molecular transfer are discussed.

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Metal ablation by picosecond laser pulses: A hybrid simulation

Carsten Schäfer, Herbert M. Urbassek, and Leonid V. Zhigilei

Phys. Rev. B 66, 115404, 2002.

Abstract

We investigate picosecond laser ablation of metals using a hybrid simulation scheme. Laser energy input into the electron system and heat conduction within it are modeled using a finite-difference scheme for solving the heat conduction equation. Atomic motion in the near surface part (72 nm) of the sample is modeled using molecular dynamics. Energy transfer between the electronic and atomic sub-systems due to electron-phonon coupling is taken into account. For the special case of 0.5 ps UV laser irradiation of copper, we investigate the fluence dependence of the ablation yield, the temperature and pressure evolution in the target, and the ablation mechanism.

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Combined molecular dynamics - direct simulation Monte Carlo computational study of laser ablation plume evolution

Michael I. Zeifman, Barbara J. Garrison, and Leonid V. Zhigilei

J. Appl. Phys. 92, 2181-2193, 2002.

Abstract

A two-stage computational model of evolution of a plume generated by laser ablation of an organic solid is proposed and developed. The first stage of the laser ablation, that involves laser coupling to the target and ejection of molecules and clusters, is described by the molecular dynamics (MD) method. The second stage of a long-term expansion of the ejected plume is modeled by the direct simulation Monte Carlo (DSMC) method. The presence of clusters, which comprise a major part of the overall plume at laser fluences above the ablation threshold, presents the main computational challenge in the development of the combined model. An extremely low proportion of large-size clusters hinders both the statistical estimation of their characteristics from the results of the MD model and the following representation of each cluster size as a separate species, as required in the conventional DSMC. A number of analytical models are proposed and verified for the statistical distributions of translational and internal energies of monomers and clusters as well as for the distribution of the cluster sizes, required for the information transfer from the MD to the DSMC parts of the model. The developed model is applied to simulate the expansion of the ablation plume ejected in the stress-confinement irradiation regime. The presence of the directly ejected clusters changes drastically the evolution of the plume as compared to the desorption regime. A one-dimensional self-similar flow in the direction normal to the ablated surface is developed within the entire plume at the MD stage. A self-similar two-dimensional flow of monomers forms in the major part of the plume by the time of about 40 ns, while its counterpart for large clusters forms much later, leading to the plume sharpening effect. The expansion of the entire plume becomes self-similar by the time of about 500 ns, when inter-particle interactions vanish. The velocity distribution of particles cannot be characterized by a single translational temperature; rather, it is characterized by a spatially- and direction-dependent statistical scatter about the flow velocity. The cluster size dependence of the internal temperature is mainly defined by the size dependence of the unimolecular dissociation energy of a cluster.

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Direct simulation Monte Carlo calculation: strategies for using complex initial conditions

Michael I. Zeifman, Barbara J. Garrison, and Leonid V. Zhigilei

Mat. Res. Soc. Symp. Proc. 731, W3.8.1-W3.8.6, 2002.

Abstract

Modeling of phenomena is increasingly being used to obtain an understanding of important physical events as well as to predict properties that can be directly tied to experimental data. For systems with relatively low densities of particles, the Direct Simulation Monte Carlo (DSMC) method is well suited for modeling gases with non-equilibrium distributions, coupled gas-dynamic and reaction effects, emission and absorption of radiation. On the other hand, if the density of particles is large such as in dense gases or condensed matter, the DSMC method is not appropriate and techniques such as molecular dynamics (MD) simulations are employed. There are phenomena such as laser ablation, however, in which the system evolves from a condensed state appropriate to be studied with MD to an expanding rarified gas appropriate to be studied with DSMC. The work presented here discusses the means of transferring information from a MD simulation of laser ablation to a DSMC simulation of the plume expansion. The presence of clusters in the MD output poses the main computational challenge. When the laser fluence is above the ablation threshold, the cluster size distribution is very broad (up to 10,'000's of particles per cluster) but there are relatively few of each cluster size. We have developed a method for statistical processing of the MD results and have represented the cluster size as a random variable. Various aspects of the coupling between the MD and DSMC models are discussed and several examples are presented.

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Multiscale simulation of laser ablation of organic solids: evolution of the plume

Michael I. Zeifman, Barbara J. Garrison, and Leonid V. Zhigilei

Appl. Surf. Sci. 197-198, 27-34, 2002.

Abstract

A computational approach that combines the molecular dynamics (MD) breathing sphere model for simulation of the initial stage of laser ablation and the direct simulation Monte Carlo (DSMC) method for simulation of the multi-component ablation plume development on the time- and length-scales of real experimental configurations is presented. The combined multiscale model addresses different processes involved in the laser ablation phenomenon with appropriate resolutions and, at the same time, accounts for the interrelations among the processes. Preliminary results demonstrate the capabilities of the model and provide new insights into complex processes occurring during the ablation plume expansion. The spatial distribution of monomers in the plume is found to be strongly affected by the presence of large clusters. Interaction between the clusters and monomers can result in splitting of the monomer distribution into faster and slower components. The overall spatial mass distribution is found to have little relation with the monomer distribution.

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Pressure-transmitting boundary conditions for molecular dynamics simulations

C. Schäfer, H. M. Urbassek, L. V. Zhigilei, and B. J. Garrison

Comp. Mater. Sci. 24, 421-429, 2002.

Abstract

A scheme for establishing boundary conditions in molecular-dynamics simulations that prevent pressure wave reflection out of the simulation volume is formulated. The algorithm is easily implemented for a one-dimensional geometry. Its efficiency is tested for compressive waves in Cu.

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Big Molecule Ejection - SIMS vs. MALDI

B. J. Garrison, A. Delcorte, L. V. Zhigilei, T. E. Itina, K. D. Krantzman, Y. G. Yingling, C. M. McQuaw, E. J. Smiley, and N. Winograd

Appl. Surf. Sci. 203-204, 69-71, 2003.

Abstract

Using the results of molecular dynamics simulations, we discuss the question of whether the observed difference in mass limits in SIMS (Secondary Ion Mass Spectrometry) and MALDI (Matrix Assisted Laser Desorption Ionization) are inherently related to the underlying physics of ejection or rather insufficient experimentation. The simulations show clearly that the physics of large molecule emission in SIMS and MALDI is very different. Consequently, we conclude that larger molecules can be ejected in MALDI than in SIMS.

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Molecular Dynamics Simulations of Matrix Assisted Laser Desorption - Connections to Experiment

Leonid V. Zhigilei, Yaroslava G. Yingling, Tatiana E. Itina, Tracy A. Schoolcraft, and Barbara J. Garrison

Int. J. Mass Spectrom. 226, 85-106, 2003.

Abstract

Molecular dynamics simulation technique has been applied to study various aspects of matrix assisted laser desorption. In this paper we focus on direct comparisons of the results from the simulations with experimental data and on establishing links between the measured or calculated parameters and the basic mechanisms of molecular ejection. The results on the fluence dependence of the ablation/desorption yields and composition of the ejected plume are compared with mass spectrometry and trapping plate experiments, implications of the prediction of a fluence threshold for ablation are discussed. The strongly forward-peaked velocity and angle distributions of matrix and analyte molecules, predicted in the simulations, are related to the experimental distributions. The shapes and amplitudes of the acoustic waves transmitted from the absorption region through the irradiated sample are compared to recent photoacoustic measurements and related to the ejection mechanisms. The conformational changes and the ejection velocities of analyte molecules are studied and the directions for future investigations are discussed. Finally, we demonstrate that the molecular dynamics simulation technique can be used to model a diverse range of systems and processes relevant to mass spectrometry applications, such as laser disintegration of aerosol particles, explosive boiling of a water film adjacent to a hot metal surface, and laser ablation in the presence of photochemical reactions.

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Microscopic Mechanisms of Matrix Assisted Laser Desorption of Analyte Molecules: Insights from Molecular Dynamics Simulation

Tatiana E. Itina, Leonid V. Zhigilei, and Barbara J. Garrison

J. Phys. Chem. B, 106, 303-310, 2002.

Abstract

A hybrid model, which combines a bead-and-spring approach with the breathing sphere model, is developed for a molecular dynamics study of matrix-assisted laser desorption of analyte molecules. The combined model is used to investigate the initial stage of analyte molecular ejection at different laser fluences. Analyte molecules embedded near the irradiated surface are lifted off at laser fluences corresponding to the ablation threshold. Higher fluences are required to eject analyte molecules embedded deeper below the surface. At all considered laser fluences, analyte molecules are ejected within matrix clusters, thus solvated. The degree of solvation decreases with increasing laser fluence and during the ablation plume expansion. Possible mechanisms of analyte desolvation in the ejected plume are discussed. Analyte fragmentation is found to be negligible under all explored conditions.

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The Role of Inertial and Spatial Confinement in Laser Interaction with Organic Materials

Leonid V. Zhigilei, Barbara J. Garrison, Masahiro Goto, Jonathan Hobley, Maki Kishimoto, Hiroshi Fukumura, and Alexei G. Zhidkov

Proceedings of the 5th World Multi-Conference on Systemics, Cybernetics and Informatics (SCI 2001 / ISAS 2001), (Institute of Informatics and Systemics: Orlando, Florida), 215-220, 2001.

Abstract

Short-pulse laser irradiation of organic material performed under conditions of inertial or spatial confinement can result in laser damage or material ejection (ablation) at relatively low laser fluences. A computational investigation of the mechanisms of the efficient transformation of the deposited laser energy into the energy of material ejection is used in this work to discuss a number of potential applications of the regime of confined laser ablation. In particular, we show that a direct irradiation of a bulk organic sample by a laser pulse performed in the regime of stress confinement can lead to the ejection of a layer of a relatively intact material with thickness determined by the laser penetration depth and fluence. Further irradiation of the spalled layer by an intense femtosecond laser pulse leads to efficient emission of picosecond bunches of energetic ions directed toward the target. In another example, a controlled deposition of functional organic molecules into a designated region of a polymer substrate is achieved by laser irradiation of a microscopic amount of molecular substance spatially confined in the tip of a micropipette.

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Computational Model for Multiscale Simulation of Laser Ablation

Leonid V. Zhigilei

-in: Advances in Materials Theory and Modeling-Bridging Over Multiple-Length and Time Scales, edited by V. V. Bulatov, F. Cleri, L. Colombo, L. J. Lewis, N. Mousseau, (Mat. Res. Soc. Symp. Proc. 677), invited paper, AA2.1.1-AA2.1.11, 2001.

Abstract

Multiscale computational approach that combines different methods to study laser ablation phenomenon is presented. The methods include the molecular dynamics (MD) breathing sphere model for simulation of the initial stage of laser ablation, a combined MD - finite element method (FEM) approach for simulation of propagation of the laser-induced pressure waves out from the MD computational cell, and the direct simulation Monte Carlo (DSMC) method for simulation of the ablation plume expansion. The multiscale approach addresses different processes involved in laser ablation with appropriate resolutions and, at the same time, accounts for the interrelations between the processes. A description of the ablation plume appropriate for making a connection between the MD simulation of laser ablation and the DSMC simulation of the ablation plume expansion is discussed.

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Laser Expulsion of an Organic Molecular Nano-jet from a Confined Domain onto a Polymer Surface

Masahiro Goto, Leonid Zhigilei, Jonathan Hobley, Maki Kishimoto, Barbara Garrison, and Hiroshi Fukumura

J. Appl. Phys. 90, 4755-4760, 2001.

Abstract

Functional organic molecules have been manipulated into fluorescent features as small as 470 nm on a polymer film using a method derived from laser ablation and laser implantation. The technique utilises a piezo driver to position a pipette, having a 100nm aperture and doped at the tip with organic molecules, tens of nano-meters above a polymer film. The pipette is subsequently irradiated using 3-nanosecond (FWHM) laser pulses guided down to the tip by a fibre optic. This method of ablation confinement gives fine spatial control for placing functional organic molecules in a designated region and will have application in opto-electronics. It could also be applied to drug delivery or biotechnology, because in principle, different molecules of diverse function can be manipulated in the same way for various purposes.

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The Role of Photochemical Fragmentation in Laser Ablation: A Molecular Dynamics Study

Yaroslava G. Yingling, Leonid V. Zhigilei, Barbara J. Garrison

J. Photochem. Photobiol. A 145, 173-181, 2001.

Abstract

Despite numerous studies, the mechanistic understanding of the role of the photochemical processes and their coupling with the thermal processes in UV laser ablation is still far from being complete. In this work the effects of the photochemical reactions on the laser ablation mechanism are delineated based on the results of molecular dynamics simulations of 248-nm laser irradiation of solid chlorobenzene. Photochemical reactions are found to release additional energy into the irradiated sample and decrease the average cohesive energy, therefore decreasing the value of the ablation threshold. The yield of emitted fragments becomes significant only above the ablation threshold. Below the ablation threshold only the most volatile photoproduct, HCl, is ejected in very small amounts, whereas the remainder of photoproducts are trapped inside the sample. Results of the simulations are in a good qualitative agreement with experimental data on the ejection of photoproducts in the laser ablation of chlorobenzene.

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Laser Ablation of Bi-component Systems: A Probe of Molecular Ejection Mechanisms

Yaroslava G. Yingling, Leonid V. Zhigilei, Barbara J. Garrison, Antonis Koubenakis, John Labrakis, and Savas Georgiou

Appl. Phys. Lett. 78, 1631-1633, 2001.

Abstract

A combined experimental and molecular dynamics simulation study of laser ablation of a model bi-component system with solutes of different volatility provides a consistent picture of the mechanisms of material ejection. The comparison of the ejection yields clearly shows that there are two distinct regimes of molecular ejection, desorption at low laser fluences and a collective ejection of a volume of material or ablation at higher fluences. Ejection of volatile solutes dominates in the desorption regime, whereas non-volatile solutes are ejected only in the ablation regime. The simulations indicate that the volatile molecules are mainly ejected as monomers, and the non-volatile solutes tend to eject as part of molecular clusters.

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Photochemical Fragmentation Processes in Laser Ablation of Organic Solids

Yaroslava G. Yingling, Leonid V. Zhigilei, Barbara J. Garrison

Nucl. Instr. Meth. B 180, 171-175, 2001.

Abstract

Studies on ultraviolet (UV) laser ablation of molecular solids have received considerable attention due to its proven and potential applications. Despite its active practical use the mechanisms of laser ablation are still being studied and debated. One crucial mechanistic discussion is on the relative importance of direct photodissociation of chemical bonds and of thermal ejection following rapid conversion of light energy into heat in the ablation process. It is generally believed that those two processes are coupled in UV ablation resulting in difficulty in analyzing the mechanisms. In this simulation the breathing sphere model is enhanced allowing the photon absorption event to break a bond in the molecule and then have subsequent abstraction and recombination reactions. The initial system to model is chlorobenzene. Chlorobenzene is chosen because of simplicity of its fragmentation, entailing exclusively scission of the carbon-chlorine bond to yield phenyl and chlorine radicals. The model presented here allows us to study the photochemical processes themselves and their coupling with the thermal processes.

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Matrix Assisted Pulsed Laser Evaporation of Polymeric Materials: Molecular Dynamic Simulation

T. E. Itina, L. V. Zhigilei, B. J. Garrison

Nucl. Instr. Meth. B 180, 238-244, 2001.

Abstract

A new matrix assisted pulsed laser evaporation (MAPLE) technique has been recently developed for deposition of high quality thin films for a wide range of polymeric materials. To analyze the evaporation of polymer molecules in MAPLE, we present a molecular dynamics (MD) simulation of laser ablation where the target material is modeled as a solution of polymer molecules in a molecular matrix. The breathing sphere model is used for MD simulation of laser ablation of the molecular matrix. Polymer molecules are described using a bead-spring model, where each bead represents one or several polymer groups. The initial stage of polymer ejection is investigated at different laser fluences and pulse durations. The influence of polymer molecules on the stability of clusters formed in the plume and the processes that can lead to polymer decomposition are discussed.

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Thickness effects of water overlayer on its explosive evaporation at heated metal surfaces

Yusheng Dou, Leonid V. Zhigilei, Zbigniew Postawa, Nicholas Winograd, and Barbara J. Garrison

Nucl. Instr. Meth. B 180, 105-111, 2001.

Abstract

Molecular dynamics simulations have been employed to investigate the thickness effects of water overlayers on its explosive boiling at a heated Au surface. The simulations are performed for five systems differing in the thickness of water overlayer adsorbed on metal substrate heated to 1000 K. For each system, an explosive evaporation occurs in the water film near the metal surface and the upper part of film is pushed off by the force generated. The highest temperature of water film decreases as the film thickness increases. In contrast, the lowest temperature achieved by the fast cooling because of explosive evaporation shows an inverse trend. The significance of the simulation results to matrix-assisted laser desorption and ionization mass spectrometry is discussed.

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Explosive Boiling of Water Films Adjacent to Heated Surfaces: A Microscopic Description

Yusheng Dou, Leonid V. Zhigilei, Nicholas Winograd, and Barbara J. Garrison

J. Phys. Chem. A 105, 2748-2755, 2001.

Abstract

Molecular dynamics simulations are employed to investigate the response of water films adjacent to a hot surface. The simulations clearly show that the water layers nearest the surface overheat and undergo explosive boiling. For thick water films, the expansion of the vaporized molecules forces the outer water layers away from the surface. These results have potential applicability to mass spectrometry of biological molecules, steam cleaning of surfaces, and medical procedures.

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Laser Ablation in a Model Two-phase System

Gareth J. Williams, Leonid V. Zhigilei, and Barbara J. Garrison

Nucl. Instr. Meth. B 180, 209-215, 2001.

Abstract

Short pulse laser ablation of a model two-component system has been simulated using a coarse-grained molecular dynamics model. A system with two disparate components (hard and soft) has been considered, based broadly on the properties of hard tissue. The existence of distinct regimes of material ejection has been identified and the processes leading to the ablation have been investigated. In the lower fluence regime gaseous particles of the soft component stream from the hard matrix but there is no ejection of the hard component. In the higher fluence regime chunks of the hard component are ejected simultaneously with the soft component.

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Short-laser-pulse Driven Emission of Energetic Ions into a Solid Target from a Surface Layer Spalled by a Laser Prepulse

A. G. Zhidkov, L. V. Zhigilei, A. Sasaki, and T. Tajima

Appl. Phys. A, submitted, 2001.

Abstract

An efficient emission of energetic protons and carbon ions from a thin layer spalled from a solid by a laser prepulse is demonstrated numerically. We combine the molecular dynamics technique and multi-component collisional particle-in-cell method with plasma ionization to simulate the laser spallation and ejection of a thin (~20-30 nm) solid layer from an organic target and its further interaction with an intense femtosecond laser pulse. In spite of small thickness, a layer produced by laser spallation efficiently absorbs ultra-short laser pulses with the generation of hot electrons that convert their energy to ion energy. The efficiency of the conversion of the laser energy to ions can be as high as 20%, and 10% to MeV ions. A transient electrostatic field created between the layer and surface of the target is up to 10 GV/cm.

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Molecular Dynamics Simulations of Laser Disintegration of Amorphous Aerosol Particles with Spatially Nonuniform Absorption

Tracy A. Schoolcraft, Gregory S. Constable, Bryan Jackson, Leonid V. Zhigilei, and Barbara J. Garrison

Nucl. Instr. Meth. B 180, 245-250, 2001.

Abstract

A series of molecular dynamics simulations are preformed in order to provide qualitative information on the mechanisms of disintegration of aerosol particles as used in aerosol mass spectrometry. Three generic types of aerosol particles are considered, strongly absorbing particles with homogeneous composition, transparent particles with absorbing inclusion, and absorbing particles with transparent inclusion. To study the effect of the mechanical properties of the aerosol material on the disintegration process, the results for crystalline (brittle) and amorphous (ductile) particles are compared. For large laser fluences, nearly complete dissociation of the absorbing material is observed, whereas the nonabsorbing portions remain fairly intact. Because large fluences can cause photofragmentation of constituent molecules, multiple pulses at low laser fluence and/or lasers with different wavelengths are recommended for the best representative sampling of multicomponent aerosol particles in laser desorption / ionization mass spectrometry.

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Molecular Dynamics Simulation of the Laser Disintegration of Aerosols Particles

Tracy A. Schoolcraft, Gregory S. Constable, Leonid V. Zhigilei, and Barbara J. Garrison

Anal. Chem., Accelerated Article, 72, 5143-5150, 2000.

Abstract

The mechanisms of disintegration of submicron particles irradiated by short laser pulses are studied by molecular dynamics simulation technique. Simulations at different laser fluences are performed for particles with homogeneous composition and particles with transparent inclusions. Spatially nonuniform deposition of laser energy is found to play a major role in defining the character and the extent of disintegration. The processes that contribute to the disintegration include overheating and explosive decomposition of the illuminated side of the particle, spallation of the backside of large particles, and disruption of the transparent inclusion caused by the relaxation of the laser-induced pressure. The observed mechanisms are related to the nature of the disintegration products and implications of the simulation results for aerosol time-of-flight mass spectrometry are discussed. Application of multiple laser pulses is predicted to be advantageous for efficient mass spectrometry sampling of aerosols wi th a large size to the laser penetration depth ratio.

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Microscopic Mechanisms of Laser Ablation of Organic Solids in the Thermal and Stress Confinement Irradiation Regimes

Leonid V. Zhigilei and Barbara J. Garrison

J. Appl. Phys. 88, 1281-1298, 2000.

Abstract

The results of large-scale molecular dynamics simulations demonstrate that the mechanisms responsible for material ejection as well as most of the parameters of the ejection process have a strong dependence on the rate of the laser energy deposition. For longer laser pulses, in the regime of thermal confinement, a phase explosion of the overheated material is responsible for the collective material ejection at laser fluences above the ablation threshold. This phase explosion leads to a homogeneous decomposition of the expanding plume into a mixture of liquid droplets and gas phase molecules. The decomposition proceeds through the formation of a transient structure of interconnected liquid clusters and individual molecules and leads to the fast cooling of the ejected plume. For shorter laser pulses, in the regime of stress confinement, a lower threshold fluence for the onset of ablation is observed and attributed to photomechanical effects driven by the relaxation of the laser-induced pressure. Larger and more numerous clusters with higher ejection velocities are produced in the regime of stress confinement as compared to the regime of thermal confinement. For monomer molecules, the ejection in the stress confinement regime results in broader velocity distributions in the direction normal to the irradiated surface, higher maximum velocities, and stronger forward peaking of the angular distributions. The acoustic waves propagating from the absorption region are much stronger in the regime of stress confinement and the wave profiles can be related to the ejection mechanisms.

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Mechanisms of Laser Ablation from Molecular Dynamics Simulations: Dependence on the Initial Temperature and Pulse Duration

Leonid V. Zhigilei and Barbara J. Garrison

Appl. Phys. A 69, S75-S80, 1999.

Abstract

The effect of the initial sample temperature and laser pulse duration on the mechanisms of molecular ejection from an irradiated molecular solid is investigated by large-scale molecular dynamics simulations. The results of simulations performed for two initial temperatures are found to be consistent with the notion of two distinct regimes of molecular ejection separated by a threshold fluence. At low laser fluences, thermal desorption from the surface is observed and the desorption yield is described by an Arrhenius-type dependence on the laser fluence. At fluences above the threshold, a collective multilayer ejection or ablation occurs and the ablation depth follows a critical density of the deposited energy. The same activation energy for desorption and critical energy density for ablation provide a good description of the fluence dependence of the total yield in simulations with different initial temperatures. Comparison of the simulation results for two pulse durations is performed to elucidate the d ifferences in the ejection mechanism in the regimes of thermal and stress confinement. We find that in the regime of stress confinement, high thermoelastic pressure can cause mechanical fracture/cavitation leading to energetically efficient ablation and ejection of large relatively cold chunks of material.

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Mesoscopic Breathing Sphere Model for Computer Simulation of Laser Ablation and Damage

Leonid V. Zhigilei and Barbara J. Garrison

in Proceedings of the International Conference on Modeling and Simulation of Microsystems, Semiconductors, Sensors, and Actuators (MSM'99), (Computational Publications: Boston), 138-141, 1999.

Abstract

A breathing sphere model has been developed for molecular dynamics simulations of laser ablation of organic solids. The novel feature of this model is an approximate representation of the internal molecular motion, which permits a significant expansion of the time and length scales of the model, and yet still allows one to reproduce a realistic rate of the vibrational relaxation of excited molecules. A dynamic boundary condition that accounts for the laser induced pressure wave as well as the direct laser energy deposition in the boundary region allows one to focus the computational effort to the region where active processes of laser ablation and damage occur. The model has been applied to study the mechanisms of laser ablation of molecular solids, velocity distributions of ejected molecules, and laser damage in the case of spatially localized absorbers. The results for different laser fluences and pulse durations have been analyzed and related to available experimental data.

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Pressure waves in microscopic simulations of laser ablation

Leonid V. Zhigilei and Barbara J. Garrison

in Multiscale Modelling of Materials, edited by T. Diaz de la Rubia, T. Kaxiras, V. Bulatov, N.M.Ghoniem, and R. Phillips, (Mat. Res. Soc. Symp. Proc. 538), 491-496, 1999.

Abstract

Laser ablation of organic solids is a complex collective phenomenon that includes processes occurring at different length and time scales. A mesoscopic breathing sphere model developed recently for molecular dynamics simulation of laser ablation and damage of organic solids has significantly expanded the length-scale (up to hundreds of nanometers) and the time-scale (up to nanoseconds) of the simulations. The laser induced buildup of a high pressure within the absorbing volume and generation of the pressure waves propagating from the absorption region poses an additional challenge for molecular-level simulation. A new dynamic boundary condition is developed to minimize the effects of the reflection of the wave from the boundary of the computational cell. The boundary condition accounts for the laser induced pressure wave propagation as well as the direct laser energy deposition in the boundary region.

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Molecular dynamics simulation study of the fluence dependence of particle yield and plume composition in laser desorption and ablation of organic solids

Leonid V. Zhigilei and Barbara J. Garrison

Appl. Phys. Lett. 74, 1341-1343, 1999.

Abstract

Two distinct regimes of molecular ejection separated by a well-defined threshold fluence are observed in molecular dynamics simulation of pulsed laser irradiation of an organic solid. At fluences above the threshold a collective multilayer ejection or ablation occurs where large liquid droplets are ejected and the total yield of the ablated material follows a critical volume density of the deposited energy. Below threshold thermal desorption from the surface is observed and the desorption yield has an Arrhenius-type dependence on the laser fluence. The yield of monomers does not have a step increase at the threshold and thus deceptively does not give a straightforward interpretation of the ejection mechanisms.

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Phase Transition at Low Fluences in Laser Desorption of Organic Solids: A Molecular Dynamics Study

Prasad B. S. Kodali, Leonid V. Zhigilei, and Barbara J. Garrison

Nucl. Instrum. Methods Phys. Research B 153, 167-171, 1999.

Abstract

Molecular dynamics study of short pulse laser irradiation of a molecular crystal is performed to investigate the dependence of the mechanism of ejection on laser fluence. We find that at low laser fluences molecules are desorbed from a thin surface layer of solid sample. An increase in the laser fluence leads to the melting of the surface region and desorption occurs from an overheated melted state. An additional increase of the fluence leads to the ablation when the laser induced pressure and phase explosion of the overheated liquid drives a collective ejection of a significant volume of irradiated material.

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A combined molecular dynamics and finite element method technique applied to laser induced pressure wave propagation

Julia A. Smirnova, Leonid V. Zhigilei, and Barbara J. Garrison

Comput. Phys. Commun. 118, 11-16, 1999.

Abstract

Analysis of a variety of dynamic phenomena requires simultaneous resolution at both atomistic and continuum length scales. A combined molecular dynamics and finite element method approach, which we discuss in this paper, allows us to find the balance between the necessary level of detail and computational cost. The combined method is applied to the propagation of a laser-induced pressure wave in a solid. We find good agreement of the wave profile in the molecular dynamics and finite element regions. This computational approach can be useful in cases where a detailed atomic-level analysis is necessary in localized spatially separated regions whereas continuum mechanics and thermodynamics is sufficient in the remainder of the system.

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Velocity Distributions of Analyte Molecules in Matrix-assisted Laser Desorption from Computer Simulations

Leonid V. Zhigilei and Barbara J. Garrison

Rapid Commun. Mass Spectrom. 12, 1273-1277, 1998.

Abstract

The mass dependence of the velocity distributions of analyte molecules in matrix assisted laser desorption is analyzed based on the results of molecular dynamics simulations. The spread of the velocities along the direction of the flow is found to be independent on the mass of the analyte molecules and reflect the entrainment of the analyte molecules in the expanding matrix. The radial velocity distributions for both matrix molecules and analyte molecules of different masses, on the other hand, fit well to a Maxwell-Boltzmann distribution with the same temperature, suggesting the association of the spread in the radial velocities with the thermal motion in the plume. A consistent analytical description of the complete velocity distribution for matrix molecules and analyte molecules of different masses is proposed based on the approximation of a range of stream velocities and a single temperature in the ejected plume.

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Microscopic Simulation of Short Pulse Laser Damage of Melanin Particles

Leonid V. Zhigilei and Barbara J. Garrison

Laser-Tissue Interaction IX, S.L.Jacques, Editor, Proc. SPIE 3254, 135-143, 1998.

Abstract

Microscopic mechanisms of short pulse laser damage to melanin granules, the strongest absorbing chromophores of visible and near - IR light in the retina and skin, are studied using the molecular dynamics simulations. The pulse width dependence of the fracture/cavitation and vaporization processes within the small particles, their coupling to the surrounding medium and the resulting tissue injury are discussed based on the simulation results. The effect of laser irradiation on an isolated submicron particle at different laser fluences and pulse durations is first analyzed. The mechanical disruption of the particle due to the laser induced pressure is found to define the character of damage for short pulse widths (tens of picoseconds) at laser fluences that are significantly lower than those required for boiling. Thermal relaxation and explosive disintegration of the overheated particle at higher laser fluencies are the processes that dominate at longer laser pulses (hundreds of picoseconds). Damage of an absorbing particle embedded into a transparent medium with different mechanical characteristics is then simulated. Coupling of the acoustic and thermal pulses generated within absorbing particles to the surrounding medium is studied and the possible cumulative effects from an ensemble of absorbing particles are discussed. The simulation results provide the basis for future work in which the microscopic and continuum descriptions are combined for multiscale modeling of laser tissue interaction.

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A Microscopic View of Laser Ablation

Leonid V. Zhigilei, Prasad B. S. Kodali, and Barbara J. Garrison

J. Phys. Chem. B, Feature Article, 102, 2845-2853, 1998.

Abstract

Recent applications of the breathing sphere model for molecular dynamics simulations of laser ablation of organic solids have yielded detailed microscopic data of the processes involved. The results to date include a prediction of a fluence threshold for ablation, an explanation for the presence of clusters in the plume and a consistent analytical description of the velocity distribution for both matrix molecules and heavier analyte molecules in matrix assisted laser desorption. In this paper we review the approach and the basic physical picture which emerges from the simulations, present new results, and discuss future prospects for microscopic simulations of laser ablation.

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Computer Simulation Study of Damage and Ablation of Submicron Particles from Short Pulse Laser Irradiation

Leonid V. Zhigilei and Barbara J. Garrison

Applied Surface Science 127-129, 142-150, 1998.

Abstract

The dynamics of damage and ablation of individual particles of ~100 nm in size due to short pulse laser irradiation is studied using a breathing sphere model and molecular dynamics simulations. The fluence thresholds for damage and ablation of irradiated particles have a strong pulse duration dependence. For 15 ps laser pulses the laser induced pressure buildup and the focusing of the pressure wave in the center of irradiated particle leads to low thresholds for mechanical damage and ablation. The pressure driven particle disruption provides an effective mechanism for transfer of the laser energy into the energy of radial expansion of the ablation products. For 300 ps laser pulses the explosive thermal decomposition of the particle is due to overheating and occurs at significantly higher laser fluences. Implications of the results of the simulations for the mass spectrometric aerosol characterization experiments and ablation of tissue in the case of inhomogeneous absorption of laser energy are discussed.

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Velocity Distributions of Molecules Ejected in Laser Ablation

Leonid V. Zhigilei and Barbara J. Garrison

Appl. Phys. Lett. 71, 551-553, 1997.

Abstract

Based on the results of molecular dynamics simulations we propose an analytical expression for the velocity distributions of molecules ejected in laser ablation. The Maxwell - Boltzmann distribution on a stream velocity, commonly used to describe the measured velocity distributios, is modified to account for a range of stream velocities in the ejected plume. The proposed distribution function provides a consistent description of the axial and radial velocity distributions. The function has two parameters that are independent of the desorption angle and have clear physical meaning, namely, the temperature of the plume and the maximum stream velocity or velocity of the plume propagation.

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On the Threshold Behavior in the Laser Ablation of Organic Solids

Leonid V. Zhigilei, Prasad B. S. Kodali, and Barbara J. Garrison

Chem. Phys. Lett. 276, 269-273, 1997.

Abstract

The microscopic mechanisms of the fluence threshold behavior in laser ablation of organic solids have been delineated using a breathing sphere model and molecular dynamics simulations. Below threshold, evaporation is identified to occur and primarily single molecules are desorbed. Above threshold, collective ejection or ablation occurs in which large molecular clusters constitute a significant portion of the ejected plume. The laser induced pressure buildup and the phase explosion due to overheating of the irradiated material are identified as the key processes that determine the dynamics of laser ablation.

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Molecular Dynamics Model for Laser Ablation of Organic Solids

Leonid V. Zhigilei, Prasad B. S. Kodali, and Barbara J. Garrison

J. Phys. Chem. B 101, 2028-2037, 1997.

Abstract

A breathing sphere model is developed for molecular dynamics simulations of laser ablation and desorption of organic solids. An approximate representation of the internal molecular motion permits a significant expansion of the time and length scales of the model and still allows one to reproduce a realistic rate of the vibrational relaxation of excited molecules. We find that the model provides a plausible description of the ablation of molecular films and matrix-assisted laser desorption. A well-defined threshold fluence has been found to separate two mechanisms for the ejection of molecules: surface vaporization at low laser fluences and collective ejection or ablation at high fluences. Above threshold the laser-induced high pressure and the explosive homogeneous phase transition leads to the strongly forwarded emission of ablated material and high, from 500 up to 1500 m/s, maximum velocities of the ejected plume expansion. Large analyte molecules get axial acceleration from an expanding plume and move along with the matrix molecules at nearly the same velocities. Big molecular clusters are found to constitute a significant part of the ejected plume at fluences right above the ablation threshold. The processes in the plume are found to have a strong influence on the final velocities of ejected molecules and molecular clusters.

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Intermediate Metastable Structure of the C{111}/(1x1)H - C{111}/(2x1) Surface Phase Transition

Leonid V. Zhigilei, Deepak Srivastava, and Barbara J. Garrison

Phys. Rev. B 55, 1838-1843, 1997.

Abstract

The pathway of the {111} diamond surface transformation between the (2x1) pi-bonded chain and the (1x1) bulk terminated structures is investigated using the molecular dynamics technique. The metastable surface structure that mediates the H adsorption-induced phase transition from the (2x1) to the (1x1) surface reconstruction, and the crucial role played by hydrogen in the stabilization of this intermediate structure, are proposed. Atomic configurations formed by adjacent CH bonds on the mostly (2x1) structure are responsible for the energy barrier separating the metastable phase from the hydrogen-terminated (1x1) structure. Calculated vibrational spectra for the various surface reconstructions are correlated with experimental observations of Prof. Shen and co-workers, Phys. Rev. B 45, 1522 (1992). The occurrence of the additional higher-frequency metastable peak, its intensity variation during hydrogen absorption, and possible reasons for the irreversible character of the surface transition are discussed based on the results of the molecular dynamics simulation.

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Vibrational Dynamics of the CH Stretching Mode of H-Terminated Diamond Surfaces

Leonid V. Zhigilei, Deepak Srivastava, and Barbara J. Garrison

Surface Science 374, 333-344, 1997.

Abstract

The vibrational dynamics of hydrogen on low-index diamond surfaces is studied by the molecular dynamics method using an empirical Molecular Mechanics (MM3) potential. We find that the CH stretching peak positions in the vibrational spectra of hydrogen are sensitive to the surface structure and can be used for the experimental in situ analysis of the growing CVD diamond phase. The differences in the spectral features corresponding to the CH bending vibrations have been correlated with the results of the vibrational energy relaxation rate estimates. We find that the major contributions to the lifetimes of the excited CH stretching states come from the anharmonic coupling between the CH stretching and CCH bending vibration modes. The latter is highly coupled with the substrate phonons which leads to the formation of broad spectral regions associated with CCH bending vibrations. The dynamic analysis of the motion induced by the symmetric and antisymmetric stretching excitation on C{100}/(2x1)H surface have been performed and the reasons for the unstability of the pure antisymmetric vibrations are discussed.

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Molecular dynamics study of medium-range order in metallic glasses

Vladimir A. Likhachev, Andrei I. Mikhailin and Leonid V. Zhigilei

Philosophical Magazine A 69, 421-436, 1994.

Abstract

Quenching of nickel from the melt leading to its amorphization has been simulated using the Andersen-Nose molecular dynamics technique. The effects of quenching rate and annealing on the glass structure and properties have been studied. Analysis of the amorphous structures through tesselation into Delaunay simplices has shown that they contain extended tetrahedral clusters. These clusters are the most dense, rigid and energetically favourable regions of amorphous material. They are composed of bulky icosahedron-like elements and spiral tetrahedral chains which are identified with dispiration nuclei. The coiled tetrahedral chains are the most stable non-crystalline elements responsible for the resistance of the amorphous structure to crystallization. The tetrahedral clusters determine the scale of medium-range order in an amorphous structure.

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Statistics of the fluctuations of the kinetic energy of atoms in a solid (computer experiment)

Andrei I. Mikhailin, Leonid V. Zhigilei and Alexander I. Slutsker

Phys. Solid State 37, 972-976, 1995.

Abstract

Fluctuations of the kinetic energy (Ek) in an fcc crystal at constant temperature and pressure have been investigated by computer simulation (molecular dynamics method). The distribution of the instantaneous values of Ek and the magnitude distribution of the fluctuations of Ek are determined. It is found that these distributions correspond to the Maxwell-Boltzmann distribution. The average weighting time of the fluctuations of the kinetic energy of an atom was determined as a function of the magnitude of the fluctuations.

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Molecular Dynamics simulation of two-dimensional disclinations

Leonid V. Zhigilei, Andrei I. Mikhailin and Alexei E. Romanov

Phys. Met. Metall. 66, 56-63, 1988.

Abstract

An investigation is maid of the core structure of total positive and negative wedge disclinations of strength 60° in two-dimensional crystallites containing up to 476 atoms. The total energy is determined as a function of the size of the crystallite and the position of the disclination in it. A comparison is made between the computer results and predictions of the linear theory of elasticity. The change in structure of the disclination core when vacancies or interstices are introduced into it and when edge dislocations are nucleated in the crystallite is analysed.

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Thank you for your interest.  I can foward reprints if you email a request to   lz2n@virginia.edu

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