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 Xe^54+ ions with Kr and Xe atoms by measuring the x-ray intensity ratios I (1snp1s^2) /I (np1s) (n=2, 3, and 4) of the resulting Xe^52+* and Xe^53+* 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 (1s2p1s^2): I (1s3p1s^2): I (1s4p1s^2) of Xe^52+* ions were found to be nearly identical to the ratios I (2p1s): I (3p1s): I (4p1s) of Xe^53+* 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 Xe^54++Xe collisions is significantly larger than that for Xe^54++Kr collisions. We speculate that the resonant electron transfer plays an important role in the symmetric Xe^54++Xe collisions and the double-electron capture therein cannot be regarded as two independent one-electron capture events.