Electron-stimulated desorption of samarium from oxidized tungsten

The electron-stimulated desorption (ESD) yields for neutral samarium (Sm) atoms from layers adsorbed on oxidized tungsten surfaces have been measured as a function of electron energy, rare-earth metal coverage, degree of oxidation of tungsten and substrate temperature. The results are compared to previous ESD studies of europium (Eu). The measurements have been carried out using a time-of-flight method and surface ionization detector in the temperature range 160-600 K. The fare-earth metal atom ESD yield has a resonant character as a function of electron energy. Overlapping resonant-like Sm atom yield peaks are seen at electron energies of 34 and 46 eV that may possibly be connected with Sm 5p and 5s level excitations. Similar resonant-like Eu atom peaks are observed at electron energies of 36 and 41 eV that might be associated with Eu 5p and 5s level excitations. The rare-earth metal atom yield is a complicated function of rare-earth metal coverage and substrate temperature. The temperature dependent shape of the Sm atom yield depends on the Sm coverage. At low Sm coverage (below 0.07) the Sm atom yield decreases slightly with temperature up to 270 K and then falls to zero at a temperature above 360 K. At higher Sm coverages the Sm atom yield passes through a maximum with increasing substrate temperature. There are qualitative similarities but quantitative differences seen in ESD of Eu. At higher coverages of both Eu and Sm, additional features (resonant-like peaks) are observed at electron energies of 42, 54 and 84 eV that can be associated with tungsten 5p and 5s level excitations. It is possible that the ESD yield connected with tungsten core level excitations includes desorbed SmO molecules;. these yields reach maxima at rare-earth metal coverages of about 1 ML. (C) 2002 Elsevier Science B.V. All rights reserved.