Trajectory and charge dependence of L x-ray fluorescence following Ar++Ar collisions

The coincidence technique has been employed to determine the yield of L x-rays emitted following Ar++Ar collisions of known incident energy T0, scattering angle theta = 140, and scattered ion charge m. The range of T0 studied, 50 to 575 keV, corresponds to a distance of closest internuclear separation r0 from 0.18 to 0.03 A. As in the case of our earlier Auger-electron coincidence experiments on Ar++Ar over this range, the present work uncovers evidence for two different L-vacancy-creating processes. The first process has a threshold at r0 = 0.25 A and is the result of the transfer of two electrons out of the 4fsigma molecular orbital (MO). The second process, which appears when r0 is made less than 0.13 A, is due to rotational transitions of up to six electrons from the 3dπ and 3dsigma MO's into the 3ddelta MO. We find that there is a large increase in x-ray fluorescence associated with the 3dπ, 3dsigma→3ddelta transition, an increase which is considerably in excess of that expected from available estimates. A qualitative explanation of this phenomenon is offered that takes into account combinations of long-lived autoionizing states involving outer-shell electrons, preferential pumping of L electrons up to the 3d radiative level, and finally, outer-shell-electron depopulation by prior Auger transitions. This interpretation is also consistent with the differing ion-charge x-ray-yield correlations that we find are characteristic of the 4fsigma and 3dπ,3dsigma processes