Differential scattering cross-sections, inelastic energy losses and ion fractions in backscattering of keV He + ions from monolayer metal adsorbates on solid surfaces measured by means of CAICISS

Abstract

Energy spectra of He atoms and He + ions backscattered at an angle of ∼180° by monolayer metal adsorbates (Ag, Sn, Sb, Pb and Bi) on the Si(1 1 1)- 3 × 3 surfaces and monolayer Si atoms on the graphite surface have been measured by means of the coaxial impact collision ion scattering spectroscopy technique combined with low energy electron diffraction, Auger electron spectroscopy and Rutherford backscattering spectrometry techniques in the energy range from 0.5 to 3.0 keV. It is found from their data analysis that the ratios of the experimental scattering cross-section to the magic formula of the Thomas–Fermi cross-section for different adsorbates deviate from unity: for instance 1.2 for Si, 0.8 for Ag, 1.5 for Sn, 1.2 for Sb, 1.2 for Pb and 1.6 for Bi. It is also found that the average inelastic energy loss for Si increases monotonically with increasing the incident energy, while those for the other adsorbates increase stepwisely at around 1.0 keV and thereafter gradually. Moreover, it is found that the stopping cross-sections at 2 keV and at zero impact parameter obtained in the present study are by an order of magnitude smaller than, but, in their functional dependence on Z 2, very similar to the empirical data of electronic stopping cross-sections by Ziegler. Finally, it is found that the ion fractions for Sn, Sb, Pb and Bi show oscillatory variations with the inverse velocity of primary ions which are explained in terms of the quasi-resonant charge exchange between the He 1s state and the 4d states or 5d states of the adsorbate atoms. On the other hand, it is also found that the ion fraction for Ag decreases very gradually from 1/3 and those for Si are less than 0.05 in the energy range below 3 keV. The latter two results on the inelastic energy losses and the ion fractions are discussed in terms of promotion of the quasi-molecular orbitals and charge exchange between the quasi-molecular orbitals formed during the close collisions of He and adsorbates.