DYNAMICS OF LOW-ENERGY-ELECTRON STIMULATED DESORPTION OF METASTABLE PARTICLES FROM N-2 CONDENSED ON XE AND KR FILMS

Electron-impact desorption of metastable molecular nitrogen (N-2) fro m N-2 condensed on Xe or Kr multilayer films is investigated as a func tion of electron energy (0-70 eV), rare-gas film thickness, and N-2 co verage. The different behaviors are explained with a simple mathematic al expression which rakes into account the parameters influencing the magnitude of the desorption yields. Two basic mechanisms are identifie d to contribute to the observed N-2 yields: the direct excitation and electronic energy transfer. The former mechanism proceeds via electro nic excitation of adsorbed N-2 by the primary electron beam followed b y exciton motion to the surface and desorption. In the energy-transfer mechanism, primary electrons first create excitons in the rare-gas fi lms which then transfer their energy to the adsorbed N-2 molecules. N- 2 desorption at the N-2-film-vacuum interface can proceed via intramo lecular to molecule-surface bond vibrational energy exchange and via c avity expulsion. When energy transfer dominates desorption, the N-2 y ield functin clearly bears the characteristics of exciton creation in the rare-gas film. The relative contributions of these two mechanisms depend on the impact electron energy, and layer thickness of N-2 and r are-gas film. The different energy-transfer efficiencies between excit ed rare-gas atoms and N-2 molecules is found to be the major cause for the observed difference in N-2 yield between the Xe and Kr film subs trates.