Scattering Dynamics of Nitromethane and Methyl Formate on Highly Oriented Pyrolytic Graphite (HOPG)

Abstract

The gas–surface scattering dynamics of nitromethane (CH3NO2) and methyl formate (HCOOCH3) on a highly oriented pyrolytic graphite (HOPG) surface have been investigated as part of a broader effort to evaluate the efficacy of a funnel-like neutral gas concentrator that has been proposed as a mass spectrometer inlet for the characterization of tenuous planetary atmospheres or plumes. Molecular beams of CH3NO2 and HCOOCH3 with incidence energies, Ei, of 106.5 and 98.8 kJ mol–1, respectively, were directed at the surface with incidence angles, θi, of 70, 45, and 30°. A rotatable mass spectrometer, employing electron-impact ionization, was used to collect angle-resolved time-of-flight (TOF) distributions of the molecules that scattered inelastically from the surface, allowing angular distributions of the scattered product flux and translational energy distributions at a given final angle, θf, to be obtained. The TOF distributions of the scattered products detected the parent ion mass-to-charge ratios and their respective dominant ion fragments were identical, indicating that CH3NO2 and HCOOCH3 fragmented in the ionizer of the detector and not while colliding with the surface. The scattering dynamics suggested that the parallel momentum of the molecules was conserved during impact with the surface. The translational energy and angular distributions of CH3NO2 and HCOOCH3 were identical when θi = 70°. For θi = 45 and 30°, the HCOOCH3 angular distributions were shifted to a slightly larger θf than the CH3NO2 distributions. The molecules scattered from the surface through impulsive scattering (IS) and quasitrapping (QT) pathways. The IS molecules retained a large fraction of their incidence translational energy when colliding with the surface. The QT molecules transferred more energy, but they did not come completely into thermal equilibrium with the surface before scattering into the vacuum. The QT molecules had a lobular angular distribution with a maximum flux far from the surface normal, indicating that they retained some memory of their incident conditions despite losing a significant amount of energy at the surface. The results presented in this article demonstrate that for Ei near 100 kJ mol–1, these molecules would not dissociate upon impact with the surfaces of a gas concentrator constructed of HOPG. Although the observed scattering dynamics suggest that such a concentrator could perform well for a variety of molecular species, accurate concentration factors are ultimately molecule-specific and determined by the details of the molecule–surface interaction potential.