Influences of the incident He* velocity on metastable de-excitation processes at a Cs-covered Si(1 0 0) surface

A Si(1 0 0) surface covered with cesium at saturation coverage were exposed to helium metastable-atom beams. The spectra of the electrons induced by metastable-atom de-excitation were detected as a function of the velocity of the incident He* in the velocity range 1-5 km/s. We discuss the influence of the incident He* velocity on the metastable de-excitation processes that take place at the Cs/Si(1 0 0) surface of very low work function. With increasing He* velocity, the relative weight of the peak from the decay of He*-(1s2s2) in the spectrum was decreased, which means the auto-detachment process becomes more important than the Auger de-excitation process. The low-energy threshold of the spectrum shifted to higher energies with higher He* velocities. This implies that the surface electronic states are distorted by incident atoms, and, consequently, the surface potential barrier is increased