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Scattering Dynamics of Glycine, H₂O, and CO₂ on Highly Oriented Pyrolytic Graphite

Murray, Vanessa J., Zhou, Linsen, Xu, Chenbiao, Wang, Yingqi, Guo, Hua, Minton, Timothy K.
Journal of physical chemistry 2019 v.123 no.6 pp. 3605-3621
carbon dioxide, energy transfer, graphene, ionization, molecular dynamics, spectrometers, temperature
The dynamics of H₂O, CO₂, and glycine (GLY) colliding with highly oriented pyrolytic graphite (HOPG) have been explored with beam-surface scattering techniques and molecular dynamics (MD) simulations that were carried out using a reactive force field. A supersonic, continuous molecular beam containing H₂O, CO₂, and GLY with incidence translational energies of 38.9, 87.5, and 149.5 kJ mol–¹, respectively, was directed at an HOPG surface held at a temperature of 677 K. Angular and translational energy distributions of the inelastically scattered molecules were derived from time-of-flight distributions collected with a rotatable mass spectrometer employing electron bombardment ionization. The experimental results indicated that H₂O and CO₂ retained their incident parallel energy during the gas–surface interaction. The scattering dynamics of GLY were more complicated, as a substantial fraction of the molecules exchanged a significant amount of energy during the gas–surface interaction but did not come into thermal equilibrium with the surface. The MD simulations revealed that each of the three molecules scattered from the surface via three mechanisms, which could be distinguished by the number of inner turning points (ITPs) within the trajectory: impulsive scattering (one ITP), extended impulsive scattering (two or more ITPs), and trapping (molecule remained on the surface when the simulation was terminated). The results show that the scattering dynamics are heavily dependent on the strength of molecule–surface interaction. Molecules with a stronger attraction tend to have longer residence times on the surface and consequently experience more translational energy transfer and vibrational excitation. This study is part of a broader effort focused on evaluating the efficacy of a funnel-like neutral-gas concentrator designed for increasing the flux of gas into a mass spectrometer intended for the characterization of tenuous planetary atmospheres. Complex scattering dynamics, such as those observed in this study, must be considered carefully when designing a neutral-gas concentrator that can collect a variety of molecules.