Nonradiative double-electron capture in fast collisions of bare Xe54+ ions with Kr and Xe gaseous targets

We studied the nonradiative double-electron capture in collisions of 95, 146, and 197 MeV/u Xe⁵⁴⁺ ions with Kr and Xe atoms by measuring the x-ray intensity ratios I(1snp→1s²)/I(np→1s) (n=2, 3, and 4) of the resulting Xe52+* and Xe53+* ions. It was found that the double-electron capture becomes more important with decreasing projectile energy, although the single-electron capture is always dominant. At the lower energies of 95 and 146 MeV/u the intensity ratios I(1s2p→1s²):I(1s3p→1s²):I(1s4p→1s²) of Xe52+* ions were found to be nearly identical to the ratios I(2p→1s):I(3p→1s):I(4p→1s) of Xe53+* ions for the Kr target, which implies that the double-electron capture can be regarded as two uncorrelated one-electron capture processes, whereas this feature disappears for the heavier Xe target. It was also found that at 95 and 146 MeV/u the relative double-electron capture intensity for Xe⁵⁴⁺+Xe collisions is significantly larger than that for Xe⁵⁴⁺+Kr collisions. We speculate that the resonant electron transfer plays an important role in the symmetric Xe⁵⁴⁺+Xe collisions and the double-electron capture therein cannot be regarded as two independent one-electron capture events.