Note: Velocity map imaging the scattering plane of gas surface collisions

Abstract

The ability of gas-surface dynamics studies to resolve the velocity distribution of the scattered species in the 2D scattering plane has been limited by technical capabilities and only a few different approaches have been explored in recent years. In comparison, gas-phase scattering studies have been transformed by the near ubiquitous use of velocity map imaging. We describe an innovative means of introducing a dielectric surface within the electric field of a typical velocity map imaging experiment. The retention of optimum velocity mapping conditions was validated by measurements of iodomethane-d photodissociation and SIMION calculations. To demonstrate the system's capabilities, the velocity distributions of ammonia molecules scattered from a polytetrafluoroethylene surface have been measured for multiple product rotational states.

Nitric Oxide Scattering off Graphene Using Surface-Velocity Map Imaging

“…The grounded surface is positioned ∼5 cm below the laser beam axis close to a grounded shield surrounding the ion optics, and as such, does not disturb the VMI field in the ionization region noticeably, see Figure b. Previous research on the scattering off liquid surfaces by Greaves and co-workers employing a similar geometry to image the scattering plane used an elegant arrangement involving charged razor blades to ensure that the field lines between the ion plates are not distorted . We note, however, that the distance between the ionization region and the surface in those experiments is much shorter (i.e., the surface is placed in-between the first two ion plates), but thanks to the ease with which NO can be laser-ionized here, we can afford to locate the surface much further away from the laser beam and in fact close to the ground shield surrounding the ion optics, such that the field lines in the central region of the VMI spectrometer are not distorted.…”

“…Previous research on the scattering off liquid surfaces by Greaves and co-workers employing a similar geometry to image the scattering plane used an elegant arrangement involving charged razor blades to ensure that the field lines between the ion plates are not distorted. 33 We note, however, that the distance between the ionization region and the surface in those experiments is much shorter (i.e., the surface is placed in-between the first two ion plates), but thanks to the ease with which NO can be laserionized here, we can afford to locate the surface much further away from the laser beam and in fact close to the ground shield surrounding the ion optics, such that the field lines in the central region of the VMI spectrometer are not distorted. Wodtke and co-workers use a shorter flight distance in a similar setup and crucially a fs laser to nonresonantly ionize desorbed molecules, and can conveniently switch between spatial and velocity map imaging, 34 while on the other hand, the ease of (1 + 1) REMPI detection in our setup negates the need for a focusing lens, and we do not need to scan the probe laser across the ionization region.…”

“…Velocity map imaging (VMI) is one of the most successful recent techniques to probe reaction dynamics; , VMI is typically applied to gas-phase dynamics, but more recently expanded to surface processes, − and we have ourselves applied it to the laser-desorption of NO from gold single crystals. , Surface-VMI not only delivers the velocity (i.e., speed and angular) distribution of scattering products after surface collision but due to the resonance-enhanced multiphoton ionization (REMPI) scheme frequently employed, can also establish the vibrational and rotational state distribution of the scattered products. We hence chose to perform surface-VMI collision experiments by scattering nitric oxide off graphene because (1) NO is intrinsically a radical and can hence potentially react with the graphene substrate, and (2) because NO as a diatomic molecule allows us to also investigate the internal energy distribution after scattering off graphene.…”

Translational and Rotational Energy Distributions of NO Photodesorbed from Au(100)

“…Recently, a number of groups began to exploit the advantages of the velocity map imaging (VMI) techniqueso far predominantly used for gas-phase studiesfor surface dynamics studies. − We previously reported on the use of a combination of time-of-flight (TOF) and VMI techniques for resolving 3-dimensional velocity distributions of NO photodesorbed from a gold single crystal using 355 nm photons . NO molecules were found to leave the surface preferentially along the surface normal with a very narrow angular and rather fast velocity distribution, indicating a nonthermal desorption process.…”

Ultrahigh-density spin-polarized hydrogen isotopes from the photodissociation of hydrogen halides

“…Ion Imaging [99], later developed into velocity mapped imaging (VMI) [100], and still actively being improved upon, is a standard method to measure the velocities of photodissociative or scattering products [101][102][103][104][105][106][107][108][109][110][111][112][113]. In this study, we have utilized this technique to investigate the alignment behavior of the molecular axis of H 2 in its (E,F) state in the presence of a laser-generated electric field.…”