Electron-Stimulated Desorption and Surface Species Conversion: The Observation of a Desorption Resonance for Atomic Oxygen on the Pd(111) Surface

摘要

The influence of electron bombardment on an oxygen-covered Pd(111) surface at 87 and at 300 K has been studied by temperature programmed desorption. It is found that electron bombardment of an O2(a) adlayer at 87K leads to the depletion of O2(a) and the conversion to O(a) with about the same cross sections, approx. 1X10 to the -17th power/sq cm (Ve = 200eV). The O2(a)-depletion process is preferred at higher O2(a) coverage as the availability of open sites limits the 02(a) conversion. The new channel for conversion to O(a) due to electron bombardment results in a maximum increase of O(a) coverage by 88% and an additional desorption feature at 704 K compared to the usual temperature programmed desorption behavior due to O(a) recombination. The cross sections for desorption and conversion of O2(a) exhibit different electron energy dependence, suggesting that different primary electron excitations are involved in the two processes. The cross section for the electron-simulated desorption of atomic oxygen is observed to be strongly dependent on electron energy, with its maximum value being approx. 1.2X10 to the -17th power/sq.cm at an electron energy of 10 + or - 2 eV. The behavior of the cross section as a function of electron energy suggests the involvement of a surface resonance near 10 eV. A possible mechanism for resonance enhanced desorption is suggested, based on semiclassical wave packet propagation over potential energy curves which are consistent with the excitation spectrum of oxygen negative ions. Reprints. (AW)