Multiple ion scattering

The energy spectra and the angular distributions of noble gas ions, reflected from a metal surface, yield information about a number of important properties of this surface. A large number of investigations have been carried out in the past decades, not only to get insight into the interaction mechanism but also to develop methods for applying of the knowledge gained. To obtain information about the outermost surface layer, it appears necessary to use noble gas ions as primary particles, and to detect the scattered particles in the charged state only; the fraction of noble gas ions reaching the detector, after reflection from target atoms in the second layer, is very small because of the high probability of neutralization. However, this is only valid if the initial energy of the incoming ions is relatively low, namely ? 10 KeV. Under certain experimental circumstances it appears justifiable, down to a lower limit of about 20 eV, to conceive the interaction of these ions with the target atoms as single collisions. The relation between the initial energy and the post-collision energy is then very simple if the collision is an elastic one; it depends upon the scattering angle and upon the ratio of the two given masses only and not upon the interaction potential. The shift of the peaks in an energy spectrum is caused by inelastic collisions and is relatively small in the considered energy region. The causes of this shift will be discussed. As opposed to single collisions, the post-collision energies after a multiple collision depend largely upon the interaction potential. Attention will be paid to the search for these potentials. Utilization of the multiple collision phenomenon in the study of surface geometry is hampered by the vibrational motion of the surface atoms. As a result, the energy spectra are blurred and a shift of the so-called quasi-single and quasi-double peaks can occur. Under certain conditions a third peak emerges which can give additional information about the surface vibrations. The intensity of the scattered ions depends upon the cross section for scattering. Recently it has been shown that the relation between this cross section and the initial energy has an oscillating character for certain combinations of incident ions and target atoms. This phenomenon has very important consequences, e.g. in the use of single ion scattering as an analytical tool. To investigate surface structures it appears that single and multiple ion scattering, combined with LEED and AES, can provide valuable information.