Barbara J. Garrison

Microscopic mechanisms of laser ablation of organic solids in the thermal and stress confinement irradiation regimes

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 ...

Laser ablation of organic polymers: Microscopic models for photochemical and thermal processes

Irradiation of organic polymers by short pulses of far‐UV (e.g., 193 nm) laser light causes ablative photodecomposition (APD) of the material. This etching process occurs cleanly leaving behind a well‐defined pit. Longer wavelength (e.g., 532 nm) laser light also ablates material from a polymeric solid. However, this process is distinct from APD in that the sample near the pit is distorted and melted. Microscopic models are presented here for both the photochemical and thermal processes. The photochemical model predicts that well‐defined pits will be formed, that narrow angular distributions of the ablated material should be observed, and that the average perpendicular ejection velocity will be 1000–2000 m/s. The thermal model predicts melting or distortion of the solid and a broad angular distribution of the ejected material.

Low energy ion impact phenomena on single crystal surfaces

The dynamics of a solid bombarded by a 600 eV Ar+ ion have been studied classically by computer simulation. The model uses a crystallite of about 250 atoms described by pair potentials derived from elastic constants and which reproduce the surface binding energy of the solid. The relative calculated yield of secondary atom emission from the three low index faces of Cu follow the previously determined experimental order (111) > (100) > (110). We find major differences in the sputtering mechanisms for these faces. On (110), the impacted atom is ejected most frequently, while on (111) and (100) it almost never leaves the solid. We report the energy distribution of the sputtered particles for each face. The simulation successfully predicts the shape of the curve including the low energy maximum which is observed experimentally near 2 eV. In addition our model shows that many low energy atoms attempt to leave the crystal but are subsequently trapped to the solid at large distances from their original sites. This mechanism of radiation enhanced diffusion inevitably occurs in conjunction with sputtering or any other heavy secondary particle emission or scattering process.

Molecular Dynamics Simulations of Dimer Opening on a Diamond {001}(2x1) Surface

Computer simulations of hydrocarbon and related molecules using empirical force fields have become important tools for studying a number of biological and related processes at the atomic scale. Traditional force fields, however, cannot be used to simulate dynamic chemical reactivity that involves changes in atomic hybridization. Application of a many-body potential function allows such reactivity to occur in a computer simulation. Simulations of the reaction of small hydrocarbon molecules adsorbed on a reconstructed diamond {001}(2x1) surface suggest that these hydrocarbons are highly reactive species and that initial stages of diamond growth proceed through a dimer-opening mechanism. Rates estimated from transition state theory of two interconversions between states where the dimer is open and closed are given.

Computational view of surface based organic mass spectrometry

Surface based mass spectrometric approaches fill an important niche in the mass analysis portfolio of tools. The particular niche depends on both the underlying physics and chemistry of molecule ejection as well as experimental characteristics. In this article, we use molecular dynamics computer simulations to elucidate the fundamental processes giving rise to ejection of organic molecules in atomic and cluster secondary ion mass spectrometry (SIMS), massive cluster impact (MCI) mass spectrometry, and matrix-assisted laser desorption ionization (MALDI) mass spectrometry. This review is aimed at graduate students and experimental researchers.

Formation of small metal clusters by ion bombardment of single crystal surfaces

The mechanism for the formation of small metal clusters ejected from an ion bombarded metal surface is examined in detail. The analysis is performed by classical trajectory methods which determine the positions and momenta of all particles in a model microcrystallite as a function of time. The calculation utilizes pair potentials for Cu derived from elastic constants of the solid and is performed for 600 eV Ar+ ion at normal incidence to the crystal. The results show that cluster species do not leave the surface as intact parts of the solid but form in a region above the surface. A trajectory for Cu5 formation is traced in detail showing a typical mechanism which is valid for Cun formation where n?7.

Microscopic model for the ablative photodecomposition of polymers by far‐ultraviolet radiation (193 nm)

Short pulses of far‐ultraviolet (193 nm) laser radiation are capable of etching organic polymer films without melting the remaining sample. The mechanism proposed for this ablative photodecomposition attributes ablation to the increase in volume that accompanies the photolysis of the polymer. A model of the microscopic process is presented here. The predictions of the model include ablation without melting, a mean perpendicular ejection velocity of 1300 m/s, and an angular distribution of the ablated material which has a narrow peak normal to the surface.

Structure sensitive factors in molecular cluster formation by ion bombardment of single crystal surfaces

Abstract The dynamics of molecular cluster formation from a solid bombarded by a 600 eV Ar + ion have been studied classically by computer simulation. The dimers and trimers are found to establish their identity as clusters within interaction range of the solid, but not by a direct ejection of a bound molecule. The Cu 2 /Cu and Cu 3 /Cu ratios are found to be strongly dependent on crystal orientation. The (111) face is 2–3 times more likely to produce multimers than the (100) face. We find 9 trimers from (111) but none from (110). The relationship between cluster composition and the original arrangement of those atoms on the surface is presented in detail. We find that each multimer forms from atoms that originate within a roughly circular region of area ∼70 A 2 or less. This region is not necessarily centered on the ion impact point. A consequence of this observation is that dimers can consist of atoms that were several Angstroms apart on the surface but that most trimers contain at least one nearest neighbor pair of atoms. The calculated energy distribution for the dimers matches well with similar experimental studies.

Chemical sputtering of Si related to roughness formation of a Cl‐passivated Si surface

Chemical sputtering of Si in a chlorine environment has been examined with molecular dynamics simulations. It is found that chemical sputtering correlates with the roughness formation of Cl‐passivated Si surfaces during low‐energy ion bombardment. The chlorine passivation of the Si surface prevents the flattening of the surface due to the high activation barrier for surface diffusion. The rough surface contains reactive intermediates that can be desorbed into the gas phase when, after an ion impact, the region has a large energy content. The observed products and the increase of the sputtering yield are in agreement with experimental observations.

Generalized Langevin theory for gas/solid processes: Dynamical solid models

A new class of smooth and structured solid models is developed from the generalized Langevin theory of gas/solid processes [S. A. Adelman and J. D. Doll, J. Chem. Phys. 64, 2375 (1976)], and numerical results for scattering off the simplest of these model solids are presented. The models, which may be refined to arbitrary precision, allow one to treat the many‐body or lattice effect in gas/solid dynamics in a qualitatively correct but computationally simple manner. Scattering calculations based on the models may be carried out using standard classical trajectory methodology; the many‐body dynamics modifies the usual classical equations of motion through noise terms and auxiliary variables. Collisional studies based on the simplest of the new models reveal the importance of many‐body dynamics on energy transfer and trapping thresholds. The percentage of energy transfer due to many‐body effects is found to be a rapidly increasing function of solid Debye temperature ΘD; at ΘD≳225°K the many‐body contribution...