Nitric Oxide Scattering off Graphene Using Surface-Velocity Map Imaging

Abstract

We investigated the scattering of nitric oxide, NO, off graphene supported on gold. This is of fundamental importance to chemistry as collisions are the necessary first step to chemical reactions on graphene, and nitric oxide molecules are inherently radicals, with the potential to bond to graphene. We directed a molecular beam of NO in helium onto graphene and detected the directly scattered molecules using surface-velocity map imaging. In contrast to previous scattering studies off graphite, which detected only a modest reduction of the translational energy of the NO, we observe a loss of ∼80% of the molecules’ kinetic energy. Our classical molecular dynamics simulations still predict a loss of ∼60% of the translational energy in the scattering process. This energy appears to partly go into the NO rotations but mostly into collective motion of the carbon atoms in the graphene sheet. At 0° incidence angle, we also observe a very narrow angular scattering distribution. Both findings may be unique to pristine graphene on gold as (1) the two-dimensional (2D) honeycomb structure is perfectly flat and (2) the graphene is only loosely held to the gold at a distance of 3.4 Å, thus it can absorb much of the projectiles’ kinetic energy.

Molecular Dynamics Simulations of Nitric Oxide Scattering Off Graphene

“…We have recently investigated experimentally the scattering of nitric oxide, NO, off graphene [9] . This was in part motivated by the fact that NO is a diatomic radical, hence potentially reactive with graphene, but NO also allows rotational distributions to be observed, unlike monoatomic radicals.…”

“…We have recently investigated experimentally the scattering of nitric oxide, NO, off graphene. [9] This was in part motivated by the fact that NO is a diatomic radical, hence potentially reactive with graphene, but NO also allows rotational distributions to be observed, unlike monoatomic radicals. However, the by far largest contribution of scattered NO is due to direct inelastic scattering, and this is a process that can conveniently be modelled using MD simulations, allowing us to create snapshots of the actual scattering process.…”

Velocity-Selected Rotational State Distributions of Nitric Oxide Scattered off Graphene Revealed by Surface-Velocity Map Imaging

“…One can observe the signal from the molecular beam itself (within the blue rectangle in Figure ), the directly scattered component (red), and a slow (but crucially upward in the lab frame, pink) component. This slower component, which we assign to a trapping desorbing mechanism, is much weaker than the scattered component, and we in fact failed to observe it in our previous work . While it appears as if some components may overlap in the images, most notably the signal for the direct inelastic scattering and the slower trapping component, varying the delay time between the molecular beam and the REMPI laser allows us to separate the integrated signals.…”

“…Hase and co-workers found that only a small fraction of the available energy is channeled into rotations but none into vibrations. Our group has previously investigated the translational energy distribution of NO after scattering off graphene supported on gold and detected a significant loss of ∼80% of the molecules’ kinetic energy and a surprisingly narrow angular distribution . In addition to this, we can also learn from state-resolved scattering studies off graphite. , Walther and co-workers found that cold surfaces could lead to a cooling of the rotational temperature of the NO radicals in a rotationally hot molecular beam after collision with the graphite; however, a cold molecular beam of NO tends to result in a hotter rotational temperature for those NO radicals that are quasi-specularly scattered and an even hotter rotational distribution close to the surface temperature for the diffusely scattered NO molecules, i.e., the trapping–desorption component .…”

Near-Ambient Pressure Velocity Map Imaging

“…Due to the different application, it was much further from the extraction region. These strengths of imaging detection have recently been applied to molecular beam surface scattering experiments, where they can be used to probe the dynamics of the molecule-surface interactions 36,37 Using pulsed molecular beams and lasers even allows pumpprobe measurements of the kinetics of surface processes and reactions by directly probing the time-dependent flux of molecules leaving the surface. [38][39][40] Wodtke and co-workers are developing high repetition rate imaging experiments that can probe the time dependence within a single MB pulse, and will allow changes in the surface during reaction to be investigated.…”