Coverage dependence in the electron-stimulated desorption of neutral NO from Pt(111)

We report a strong coverage dependence in the yield of a neutral molecule desorbed by low-energy excitations produced by electron impact. For NO/Pt(lll), there are two distinct desorption channels revealed by state-specific translational energy distributions, measured by time-of-flight laser resonance ionization. The dominant low-energy channel (Etrans — — 0.05 eV) has been previously assigned to a simple 50- excitation. The high-energy channel (Etnm— —0.35 eVj involves complex (multielectron) excitations. Here we present the coverage dependence of the specific yield for the two channels. Strikingly different behavior is seen in that a strong inverse dependence occurs for low-energy NO but relative independence of coverage is seen for high-energy NO. A model is developed which describes the coverage dependence in terms of varying localization (or self-trapping) probabilities as a function of nearest-neighbor site occupancy. Comparison to experiment using a random vacancy model is rather good, with deviations from randomness consistent with higher coordination than random at intermediate coverages.