A comparison of the electron-stimulated desorption of positive ions from NaCl-type alkali chloride thin films

摘要

Electron-stimulated desorption of positive ions from thin layers of alkali chlorides deposited onto Si(111) are investigated by the time-of-flight (TOF) technique. All the experiments have been performed under the same experimental conditions, using very low electron current densities for which the obtained data are not distorted by the charging-up effect, which in consequence allows a direct comparison of the relative ion yields, the kinetic-energy distributions of emitted chlorine and alkali ions, and their isotope effects. The determined isotope effect of the chlorine (Cl-35(+)/Cl-37(+)) desorption yield appears to be independent of the alkali component mass and its values are 1.21 +/- 0.02, 1.297 +/- 0.005, 1.23 +/- 0.02, and 1.24 +/- 0.05 for LiCl, NaCl, KCl, and RbCl, respectively. On the other hand, the chlorine ion emission yields and average kinetic energies of the ions increase with increasing mass of the alkali component, which suggests that the lattice relaxation processes influence the desorption process. The most probable kinetic energies of Cl-35(+) are 0.7, 1.4, 1.7, and 2.3 eV for LiCl, NaCl, KCl, and RbCl, respectively. The observed isotope effect for alkali ions decreases somewhat with increasing ion mass. The respective values are 1.282 +/- 0.004, 1.04 +/- 0.02, and 1.07 +/- 0.02 for Li-6(+)/Li-7(+), K-39(+)/K-41(+), and Rb-85(+)/Rb-87(+). The alkali ion emission yield tends at first to increase with mass, reaching a maximum for potassium, but then for rubidium the yield drops to the value almost the same as for lithium. The desorption yields of alkali ions appear to be correlated with their most probable kinetic energies, which are 1.4, 1.1, 1.8, and 1.5 eV for Li-7(+), Na-23(+), K-39(+), and Rb-85(+), respectively. The obtained results and the role of the different parameters are discussed in view of possible mechanisms and processes involved in chlorine ion desorption. Copyright (C) 2006 John Wiley & Sons, Ltd.