Abstract. Multiple electron capture and photon emission in slow (velocity v ~ 0.3 au) collisions by highly charged Ta^q+(q = 41–49) projectile ions and atomic targets of He and Xe were investigated. The absolute photon yield, or number of x-rays emitted from the projectile ion, showed considerably different behaviours in two distinctive regions of projectile charge state q: the closed M-shell region (q < 45) and the open M-shell region (q > 45). We find that for a closed Ta M shell, in the case of He but not Xe, the absolute photon yield is smaller when two electrons are captured than in the case of single electron capture. For an open Ta M shell it is the opposite for both atomic targets. The branching ratios between three processes, radiative stabilization, internal dielectronic excitation (IDE) and Auger transition, are understood to have a critical influence on these different behaviours. Furthermore, it is found that these branching ratios are strongly influenced by the states into which electrons are captured, thus by the target atom species, and by the projectile core configuration. From our measurement of absolute photon yields for different capture channels, we derived the Auger probabilities after capture from He and Xe to be 0.5(2) and 0.7(2), respectively. We investigated multiple capture in the case of Xe when more than six electrons are transferred in the collision. A simple relation for the increase of the photon yield with increasing number of captured electrons is given.

Print publication: Issue 16 (28 August 2004)
Received 21 January 2004, in final form 9 June 2004
Published 23 July 2004