ELECTRON-STIMULATED DESORPTION OF SODIUM ATOMS FROM AN OXIDIZED MOLYBDENUM SURFACE

Electron-stimulated desorption (ESD) yields and energy distributions f or sodium (Na) atoms have been measured from Na layers adsorbed at 300 K on oxidized molybdenum surfaces with various degrees of oxidation. ESD measurements have been made as a function of incident electron ene rgy and sodium coverage using a time-of-flight method and a surface io nization detector. The ESD appearance threshold for Na atoms is about 25 eV, independent of the molybdenum oxidation state and sodium covera ge. Additional features for Na atoms are observed at about 40 and 70 e V in ESD from layers with sodium coverages above 0.125 and 0.25, corre spondingly, adsorbed on an oxygen-monolayer-covered molybdenum surface . The ESD energy distributions for Na atoms consist of a single peak. However, they are extended toward low kinetic energies as the sodium c overage increases above 0.125. The low-energy ''tail'' increases with increasing sodium coverage. The most probable kinetic energy of Na ato ms decreases from 0.23 to 0.17 eV as the sodium coverage increases in the range from 0.125 to 0.75. The data can be interpreted by means of the Auger-stimulated desorption model in which the adsorbed sodium ion s are neutralized by Auger electrons produced after filling of vacanci es created by incident electrons in the O 2s, Mo 4p, or Mo 4s levels. The appearance of additional features and energy distributions extende d toward low energies for neutral atoms in ESD, after deposition of so me critical adsorbate concentration, has been demonstrated.