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

Abstract

The gas−surface scattering dynamics of nitromethane (CH 3 NO 2 ) and methyl formate (HCOOCH 3 ) 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 CH 3 NO 2 and HCOOCH 3 with incidence energies, E i , of 106.5 and 98.8 kJ mol −1 , respectively, were directed at the surface with incidence angles, θ i , 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, θ f , 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 CH 3 NO 2 and HCOOCH 3 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 CH 3 NO 2 and HCOOCH 3 were identical when θ i = 70°. For θ i = 45 and 30°, the HCOOCH 3 angular distributions were shifted to a slightly larger θ f than the CH 3 NO 2 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 E i 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.

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

“…Although a grazing θ i reduces the internal excitation of the molecule, it increases the probability for the molecule to undergo multiple collisions with the surface. − Each subsequent collision might be expected to increase the translational energy transfer and result in an increased trapping probability. However, the incident parallel momentum of N 2 and O 2 colliding with HOPG was conserved even though the molecules suffered multiple collisions before scattering. , Parallel momentum conservation after multiple atom-surface interactions was also observed for Ar scattering from a hot Pt(111) surface by Head-Gordon et al and more recently for methyl formate and nitromethane scattering from HOPG by some of the current authors . The results of the latter experiments suggest that a portion of these polyatomic molecules transfer a significant amount of energy during the molecule–surface interaction.…”

“…The experiments were conducted with the use of a crossed molecular beams machine reconfigured for beam-surface scattering. A description of this apparatus has been presented in detail in earlier publications, , and only the relevant information will be presented here. A continuous molecular beam of H 2 O, CO 2 , and GLY seeded in He was directed at a sample of ZYA quality HOPG (Momentive Inc., mosaic spread of 0.4 ± 0.1°).…”

“…A test particle model that utilized the experimental results of molecular beam-surface scattering experiments, referred to as the Statistical Program for Aerodynamics and Radiation Pressure Coefficient Simulation, predicted that a concentrator constructed of highly oriented pyrolytic graphite (HOPG) will perform well because O, O 2 , and Ar undergo nearly elastic collisions and scatter from the surface with well-defined angular distributions that are directed beyond the specular angle . Subsequent experimental work has shown that methyl formate and nitromethane survive collisions with HOPG without dissociating during the gas–surface interaction despite having a relatively high average incidence energy, ⟨ E i ⟩, near 100 kJ mol –1 . However, the details of the scattering dynamics of life-signature organic molecules, for example, the amino acid glycine (GLY), must be better understood before the concentrator can be considered as a viable mass spectrometer inlet for the analysis of tenuous atmospheres.…”

“…This scattering mechanism, referred to as quasi-trapping, produced lobular angular distributions directed beyond the specular angle despite a significant amount of translational energy transfer. There was some evidence that the molecules became vibrationally excited during this process, , which could lead to molecular dissociation or change the EI fragmentation pattern in the mass spectrum of a particular molecular species. − …”

Dynamics and Sorption Kinetics of Methanol Monomers and Clusters on Nopinone Surfaces

“… 20 Yet, interactions of organic molecules/clusters with organic surfaces have never been studied by molecular beam techniques, in spite of previous numerous studies on cluster–surface collisions 21 − 23 and current adaptations of molecular/ion beam techniques to environmental and organics-related topics. 24 − 26 …”