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

We present a combined study of single and double K-K electron transfer cros s sections along with the single and double K-shell ionization of Ar induce d 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 c ross sections for the one- and two-electron processes, respectively The enh ancement in the fluorescence yields due to multiple vacancies was measured from the energy shifts and intensity ratios of the characteristic x-ray lin es to derive K-shell vacancy production cross sections from x-ray productio n 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, whe reas 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 transf er data are underestimated by the theory based on the independent-electron approximation (IEA). The K-shell ionization cross sections are found to dev iate strongly from the calculations based on the continuum distorted wave e ikonal 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 ind ependent-electron model breaks down and the possible role of correlations b etween K-electrons is discussed.