Electron-stimulated desorption of sodium atoms from an oxidized molybdenum surface

Electron-stimulated desorption (ESD) yields and energy distributions for 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 energy and sodium coverage using a time-of-flight method and a surface ionization detector. The ESD appearance threshold for Na atoms is about 25 eV, independent of the molybdenum oxidation state and sodium coverage. Additional features for Na atoms are observed at about 40 and 70 eV in ESD from layers with sodium coverages above 0.125 and 0.25, correspondingly, 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 coverage increases above 0.125. The low-energy "tail" increases with increasing sodium coverage. The most probable kinetic energy of Na atoms 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 ions are neutralized by Auger electrons produced after filling of vacancies created by incident electrons in the O 2s, Mo 4p, or Mo 4s levels. The appearance of additional features and energy distributions extended toward low energies for neutral atoms in ESD, after deposition of some critical adsorbate concentration, has been demonstrated.