Molecular Dynamics Study of Vibrational Excitation Dynamics and Desorption in Solid O₂

Abstract

Molecular dynamics calculations were performed to describe vibrational to translational energy transfer processes leading to desorption in a low cohesive energy solid excited by a laser pulse. In this study solid oxygen crystals were vibrationally excited and the redistribution of energy in the solid was followed for several nanoseconds. In a closed system, representing bulk processes, the energy transfer to the lattice occurred slowly at first while the vibrational modes equilibrated rapidly. This was followed by a short period of very rapid energy transfer to the lattice and full equilibration. The anharmonic nature of the O 2 potential, the density of excitation, and the lattice structure were identified as the factors determining the rate of internal vibration to lattice energy transfer. In addition, melting, leading to molecular diffusion, was shown to lead to very rapid (catastrophic) equilibration in the closed system. The addition of an interface with the vacuum allowed desorption/ablation. Such a sample could expand, dramatically slowing the equilibration process. The vibrational energy excitation rate, which changes with laser pulse length, was shown to affect both the conversion from the internal vibrational to lattice heating and the desorption efficiency. The competition between energy transfer to the lattice and material expansion was such that desorption is a thermal process at low excitation densities corresponding to low energy transfer rates. Ablation occurs at high excitation densities for which the vibrational to translational energy transfer rates produce a pressure pulse causing rapid expansion. The ejections in the two regimes were shown to exhibit distinctly different properties. At high excitation density the ejection in the early stage is dominated by forward directed, "hot" clusters that evaporate, so the ablation process may be thought of as forming a nanospray.

Computer Simulations of Laser Ablation of Molecular Substrates

“…59 The anharmonic nature of the interatomic potential, the energy density created by the excitation, as well as the lattice structure and melting transition have been identified as the main factors determining the rate of the internal to translational energy transfer. When the energy deposition into an O 2 crystal was simulated in the presence of a free surface, molecular ejection was observed.…”

“…61 In effect, one can control the rate of the conversion of internal energy of the molecules excited by the laser to the translational and internal motion of the other molecules. The rate of the vibrational relaxation of excited molecules is an input parameter in the model and can be either estimated from experimental data [79][80][81] or modeled in atomistic 59,[88][89][90] or ab initio 105 molecular dynamics simulations.…”

Computational model for multiscale simulation of laser ablation

“…Part A represents atomic-level simulations that can be used to study the channels and rates of the vibrational relaxation of molecules excited by photon absorption [14,16,28]. The information on the rates of the conversion of the internal energy of the excited molecules to the translational and internal motion of the other molecules can be verified in pump-probe experiments and can be used for parameterization of the coarser-grained breathing sphere model designed for large-scale MD simulation of laser ablation, Part B of Figure 1.…”

“…During the last several years extensive experimental [1][2][3][4][5][6], computational [7][8][9][10][11][12][13][14][15][16], and theoretical [2,17] efforts have resulted in considerable progress in understanding of many aspects of laser ablation of organic materials. In a big part this progress is due to the development of advanced computational methods and their application to various processes induced by pulsed laser irradiation.…”

“…In particular, a molecular-level breathing sphere model has yielded a wealth of information on the microscopic mechanisms of laser ablation [7,9,13], parameters of the ejected plume (velocity distributions of matrix and analyte molecules in MALDI [8,10], cluster ejection [11,12,13]) and their dependence on the irradiation conditions (laser fluence [7,9,11], pulse duration [13], initial temperature of the sample [12]). At smaller time-and length-scales, conventional atomic-level MD simulations have demonstrated the ability of this technique to provide detailed information on the dynamics of intermolecular redistribution of the deposited laser energy [14,16] and conformational changes in the molecules undergoing laser desorption [15]. Atomic-level MD simulation technique has been also applied to laser ablation of inorganic materials and first interesting results have been reported [18,19,20].…”