Water window soft X-ray spectra of Ce- to Gd-like Bi ions in an electron beam ion trap

Abstract

The water window emission lines (20–40 Å) of highly charged bismuth (Bi) ions in laser produced plasmas are useful as light sources for biological microscopy of living cells. However, broad distributions of charge states and overlapping of transition arrays in the laser produced high density plasmas make precise line identification difficult. In this paper, we identified strong emission lines in the water window range from Bi ions decomposing to each charge state with a compact electron beam ion trap (CoBIT). To this end, we constructed a collisional radiative model accounting for detail atomic processes of the Bi ions interacting with mono-energetic electron beams. Each transition array observed in the experimental spectra was carefully identified with the present calculations. Distributions of the observed peaks show distinct features depending on charge state abundance in the CoBIT. We found that line emissions mediated via meta-stable excited states play an important role.