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Understanding Energy Transfer in Gas–Surface Collisions from Gas-Phase Models

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journal contribution
posted on 06.02.2014, 00:00 by Juan J. Nogueira, William L. Hase, Emilio Martínez-Núñez
Large-scale trajectory simulations of different projectiles colliding with an organic surface, as well as a gas–surface model for energy transfer, are employed to investigate the effects of the mass, size, shape, and vibrational frequency­(ies) of the projectile and of the projectile–surface interaction potential on the energy-transfer dynamics. The gas–surface model employed in this work relies on simple gas-phase scattering models. When energy transfer is analyzed in the limit of high incident energies, the following results are found in this study. The percent of energy transfer to vibration (and rotation) of light diatomic projectiles decreases as the projectile’s mass increases, while this transfer is almost independent of the mass for heavier projectiles. Transfer to final translation of diatomic projectiles is a U-shaped function of the projectile’s mass, as predicted by the hard cube model. For larger projectiles, the partitioning of the energy transferred to the internal degrees of freedom (dof) between vibration and rotation depends on the projectile’s size. In other words, transfer to rotation is more important for the smaller projectiles, while transfer to vibration dominates for the bigger ones, which have more vibrational dof. For small projectiles (less than 10 atoms), transfer to vibration increases as a function of the projectile’s size. However, for larger projectiles, the percent transfer to vibration is nearly constant, a result that can be attributed to a mass effect and also to the fact that only a reduced subset of “effective” vibrational dof is being activated in the collisions. For linear hydrocarbons colliding with the perfluorinated self-assembled monolayer (F-SAM), the number of “effective” modes was estimated to be around 18, which corresponds to a percent energy transfer to vibration of 20–22%. The percent transfer to vibration of the more compact cyclic molecules is a bit higher than that for their linear counterparts.