ELECTRON-STIMULATED DESORPTION OF POTASSIUM AND CESIUM ATOMS FROM OXIDIZED MOLYBDENUM SURFACES

The electron-stimulated desorption (ESD) yields and energy distributio ns for potassium (K) and cesium (Cs) atoms have been measured from K a nd Cs layers adsorbed at 300 K on oxidized molybdenum surfaces with va rious degrees of oxidation. The measurements were carried out using a time-of-flight method and surface ionization detector. The ESD appeara nce threshold for K and Cs atoms is independent of the molybdenum oxid ation state and is close to the oxygen 2s level ionization energy of 2 5 eV. Additional thresholds for both K and Cs atoms are observed at ab out 40 and 70 eV in ESD from layers adsorbed on an oxygen monolayer-co vered molybdenum surface; they are associated with resonance processes involving Mo 4p and 4s excitations. The ESD energy distributions for K and Ca atoms consist of single peaks. The most probable kinetic ener gy of atoms decreases in going from cesium to potassium and with incre asing adsorbed metal concentration; it lies in the energy range around 0.35 eV. The K and Cs atom ESD energy distributions from adlayers on an oxygen monolayer-covered molybdenum surface are extended toward ver y low kinetic energies. The data can be interpreted by means of the Au ger stimulated desorption model, in which neutralization of adsorbed a lkali-metal ions occurs after filling of holes created by incident ele ctrons in the O 2s, Mo 4s or Mo 4p levels. (C) 1997 Elsevier Science B .V.