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 / u Xe54+ 54 + ions with Kr and Xe atoms by measuring the x-ray intensity ratios I (1 s 'n p-* 1s2)/I('np s 2 ) / I ( 'n p-* 1s) s ) ('n 'n = 2, 3, and 4) of the resulting Xe52+& lowast; 52 +& lowast; and Xe53+& lowast; 53 +& lowast; 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 / u the intensity ratios I (1 s 2 p-* 1s2) s 2 ) : I (1 s 3 p-* 1s2) s 2 ) : I (1 s 4 p-* 1s2) s 2 ) of Xe52+& lowast; 52 +& lowast; ions were found to be nearly identical to the ratios I (2 p-* 1s) s ) : I (3 p-* 1s) s ) : I (4 p-* 1s) s ) of Xe53+& lowast; 53 +& lowast; 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 / u the relative double-electron capture intensity for Xe54+ 54 + + Xe collisions is significantly larger than that for Xe54+ 54 + + Kr collisions. We speculate that the resonant electron transfer plays an important role in the symmetric Xe54+ 54 + + Xe collisions and the double-electron capture therein cannot be regarded as two independent one- electron capture events.