Spatial resolution of a velocity-selected ion imaging microscope for surface reaction kinetics mapping

Experimental validation of complex microkinetic models derived from quantum chemistry is crucial for the advancement of bottom-up approaches to heterogeneous catalysis. State-of-the-art velocity-resolved kinetics experiments have made tremendous progress in this arena but integrate reactivity over centimeter-scale single-crystal catalytic surfaces even when complex spatial phenomena may perturb the kinetic results. We report a new design, optimization, and analysis of an ion imaging microscope that can collect spatially resolved kinetic data from a catalytic surface. In its simplest configuration, gaseous reaction products are ionized by a laser line or sheet above a catalytic surface. The resulting ions are extracted and strongly lensed to an intermediate velocity-mapped plane where a pinhole of radius r only transmits ions produced from reaction products with desorption velocities within a narrow solid angle centered on the surface normal. Transmitted ions re-expand through an electrostatic zoom lens to form a spatial image of the initial reaction product distribution with reduced blur from desorption velocity components parallel to the surface. The ion hits that define the magnified and deblurred spatial image can be used to determine spatiotemporal flux and speed-distributions of gas leaving the catalyst surface. Electrostatic trajectory simulations are performed and verify that transmission is ∝r2/TSurface. However, calculated global point spread functions acting on the magnified image have a width that is ∝r and largely independent of TSurface. Thus, velocity-filtered ion imaging microscopy can deliver a consistent resolution as the TSurface is varied, which is a great advantage because many catalytic reactions require elevated temperatures.