Anomalous behavior of the intensity ratio of resonance and intercombination lines from He-like ions in laser-produced plasmas

Space-resolved measurements of x-ray line emission from He-like ions of magnesium plasma, produced by a 20-J, 5-ns Nd:glass laser, expanding in a buffer gas are presented. In the expansion region, the intensity ratio α of the resonance line ($1^1$${\mathrm{S}}_0$ –$2^1$${\mathrm{P}}_1$) to the intercombination line ($1^1$$S_0$–$2^3$$P_1$) is observed to be smaller than (1/3, which is the lower limiting value predicted by existing models. This anomalously small value of α is explained in terms of a preferential population of higher levels (n≥3) through three-body recombination and charge-exchange processes. Small values of the ratio observed here could be used to study the roles of different atomic processes in achieving population inversion in excited states of highly charged ions.