Resonances in electron stimulated desorption yield of cesium atoms from germanium monolayer-covered tungsten

The electron-stimulated desorption ESD yields and energy distributions for Cs atoms from a cesium layer adsorbed on germanium monolayer-covered tungsten surfaces have been measured as a function of electron energy, cesium coverage , and substrate temperature. Cs is adsorbed at low temperature, 160 K. The measurements have been carried out using a time-of-flight method and surface ionization detector. The Cs atom appearance threshold is about 24 eV at 0.1 ML which correlates well with the Cs 5s level excitation energy. The cesium atom appearance threshold shifts to about 30 eV at 0.15 ML which can be associated with the Ge 3d level excitation energy; a resonantlike peak with a maximum at Ee about 38 eV that can be associated with the W 5p3/2 level excitation energy also appears. This peak is seen only at 0.3 ML and at a substrate temperature T 300 K. A second resonantlike peak appears at 0.3 and at Ee about 50 eV. A third resonantlike peak is observed at 0.5 ML, at Ee about 80 eV. The positions of these two peaks can be associated with W 5p1/2 and W 5s level excitation energies, respectively. The intensities of all resonant features pass through maxima at =1 as the cesium coverage increases and are almost independent of the substrate temperature. The energy distribution for Cs atoms consists of a bell-shaped peak with a maximum at a kinetic energy of 0.55 eV at 0.15 ML. It is transformed into a distribution containing two peaks at 0.15: a wide one with a maximum at a kinetic energy of 0.5 eV and a narrow one with a maximum at a kinetic energy of 0.35 eV. The results are interpreted to arise from three ESD channels for Cs atoms associated with excitations of Cs, Ge, and W core levels, respectively. © 2007 American Vacuum Society. DOI: 10.1116/1.2746043