Formation of lithium-like boron and nitrogen ions in the 4 P 5/2 state in gaseous media

Abstract

We experimentally determined the fraction of αv of lithium-like boron B2+ and nitrogen N4+ ions in the 4P5/2 state having a velocity of 3.6 au that are formed upon capture of two (α2) electrons by hydrogen-like B4+ and N6+ ions and upon capture of one (α1) electron by helium-like (1s2s)1,3S metastable B3+ and N5+ ions in gaseous media (H2, He, N2, Ar), as well as upon passage through a celluloid film. In light-element media (H2, He), α2 increases proportional to the target thickness Tg and reaches a maximum at Tg ≈ 1016 atom/cm2 (for B ions, α2 ≈ 0.2 in H2 and α2 ≈ 0.4 in He). For boron and nitrogen ions passing through thin layers of heavier gases (N2, Ne), α2 depends considerably more weakly on Tg, and, in Ar, becomes practically constant. It is assumed that, since hydrogen and helium do not contain electrons with parallel spins, autoionizing lithium-like ions are formed as a result of successive (one by one) capture of electrons, whereas, in the heavier gases, simultaneous capture of two electrons predominates. At Tg ∼ 1015 atom/cm2, the fraction α1 of boron ions is the highest in He, ∼0.15, and the lowest in Ar, ∼0.07, being in qualitative agreement with calculations.