X-ray signatures of charge exchange in L-shell iron and sulfur

The X-ray signatures of L-shell charge exchange in sulfur and iron were studied in the laboratory. Charge states from S11+ to S14+ and Fe18+ to Fe24+ were created in electron beam ion traps (EBITs) and were left to interact via charge exchange with neutral gases. The measurements were monitored with a high-resolution microcalorimeter spectrometer for sulfur and a moderate-resolution SiLi solid-state detector for iron. Comparison of the charge exchange (CX) spectra with those obtained under electron-impact excitation showed marked differences. In particular, we show that CX shifts the centroid energy of the dominant n = 3 → n = 2 emission. We explain this by an enhancement of the intensity of the 3s → 2p transitions and a shift towards lower energy of the 3d → 2p peak.