Angular and velocity distributions of CO desorbed from adsorption layers on Ni(100) and Pt(111): examples of non-activated desorption

Angular and velocity distributions of CO thermally desorbing from Ni(100) and Pt(111) have been measured in a time-of-flight experiment. Coverages were averaged over ranges with approximately constant desorption parameters. In all cases, independent of coverage, an angular dependence ∼ cos(φ{symbol}) was found up to desorption angles of 85°. Desorption fluxes are excellently described by Maxwellian velocity distributions. The average kinetic energy of desorption, however, deviates significantly from 2kT_s (where Ts is the mean surface temperature). The values measured vary between 1.88kT_s and 1.91kT_s. Where checked (Pt) no dependence on coverage was found. This result of slight translational cooling has to be expected both from theory for this class of systems as well as from sticking measurements via detailed balance.