Beating Tunnel Vision: Near-Surface Velocity-Map Imaging

We demonstrate a new methodology for applying velocity-map imaging (VMI) to the study of gas-surface dynamics that allows direct imaging of the scattering plane. By bringing the surface near to the ionization laser inside the VMI electrodes, we demonstrate a route to overcoming "tunnel vision", a term used to describe the limited angular acceptance of measurements taken at too great distance from where the gas-surface interaction takes place. Using a molecular beam of NO molecules colliding with a surface of highly oriented pyrolytic graphite as an exemplar system, we show the correlation between the laser-surface distance and the accessible scattering angular range. We demonstrate the capabilities of near-surface VMI (NS-VMI) through the analysis of speed and angular distributions derived from the images.