Theoretical interpretation of equilibrium charge-state distributions of ~4He ions backscattered from metallic foils using extended Bohr theory

摘要

Equilibrium charge-state distributions (ECSD's) for He~0, He ~+, and He~(++) backscattered by the foils made of silver have been measured using a post-acceleration system, and simulated by the extended Bohr theory. The kinematics behavior of the backscattered ions is understood from Rutherford backscattering spectrometry (RBS). The results from these measurements have been integrated with the data available in the literature to cover a wide energy range of the incident singly charged helium ion He ~+. The detected charge states increase from 0 to +Z with the incident ion energy. A formula for calculating He~0 distribution, for the studied energy range, has been proposed. The ratio of singly to doubly ionized helium ions backscattered from thin Ag-foils, F(He~+/F(He ~(++)), has been measured as a function of incident ion velocity. The use of a semi-classical approach, based on the theory of Bohr, allows a satisfactory theoretical description of F(He~+/F(He~(++)) ratios. It has been found that the measured and simulated ratios are strongly decreased with the incident velocity. The present study has demonstrated that the satisfactory fits of ECSD's and the ratio of the fractions are mainly accounted to the contribution of both free valance electrons of the target and screened target nucleus in the calculation of total electron loss cross section.