Energy dependence of the metastable fraction in ${\mathrm{B}}^{3+}{(1s}^2{}^{1}S,1s2s{}^{3}S)$ beams produced in collisions with thin-foil and gas targets

The fraction of metastable ions in ${\mathrm{B}}^{3+}{(1s}^2{}^{1}S,1s2s{}^{3}S)$ beams produced in collisions with thin-foil and gas targets has been measured as a function of the incident energy in the range of 0.85–9 MeV. This was done by comparing the electron yield of doubly excited states formed in the collision of ${\mathrm{B}}^{3+}$ with hydrogen and helium targets. Significant differences were observed in the energy dependence of the metastable fraction between production in foil and gas targets. It was shown that the production of $1s2s{}^{3}S$ metastable ions in a foil yields a constant fraction over the investigated energy range, unlike the fraction of metastable $1s2s{}^{3}S$ ions produced in collisions with gas targets, which strongly depends on the incident beam energy. A theoretical study of the processes contributing to the formation of the ${}^{3}S$ metastable ions in collisions with foil and gas has been made. K-vacancy production in the ion-beam stripping process has been identified as a dominant mechanism and used to explain the observed difference in the energy dependence of the metastable fraction between production in collisions with gas and foil targets. A model is proposed for the calculation of the metastable fraction for He-like beams.