Strong double K–K transfer channel in near symmetric collision of Si + Ar at intermediate velocity range

We present a combined study of single and double K–K electron transfer cross sections along with the single and double K-shell ionization of Ar induced by Si projectiles in the energy range 0.9–4.0 MeV u−1. The charge-state dependence of the normal and hypersatellite x-rays was used to derive the cross sections for the oneand two-electron processes, respectively. The enhancement in the fluorescence yields due to multiple vacancies was measured from the energy shifts and intensity ratios of the characteristic x-ray lines to derive K-shell vacancy production cross sections from x-ray production cross sections. The ratio of double to single K–K transfer cross sections is found to be quite large for this nearly symmetric collision system, whereas the ratio of double to single ionization cross sections is quite small. The measured single K–K transfer cross sections are reproduced very well by the two-centre close-coupling calculations whereas the double K–K transfer data are underestimated by the theory based on the independent-electron approximation (IEA). The K-shell ionization cross sections are found to deviate strongly from the calculations based on the continuum distorted wave eikonal initial state (CDW-EIS) and ECPSSR models. The CDW-EIS calculations along with the IEA model grossly underestimate the double ionization cross sections. It is stressed that in the case of two-electron processes the independentelectron model breaks down and the possible role of correlations between K-electrons is discussed.