Interpretation of the relative line intensities in the autoionizing spectrum of Al III emitted from a Penning discharge

We attempt to interpret the relative line intensities in the soft X-ray spectrum of Al III ions emitted from a Penning discharge, on the assumption that the 2p⁵3s² and 2p⁵(3s3p ³P) autoionizing levels are excited predominantly by inner-shell ionization from Al II ground and metastable levels, respectively. The plasma has been spectroscopically diagnosed and the Al III soft X-ray spectrum predicted using a collisional radiative model incorporating the experimental plasma parameters and theoretical predictions for the properties of the autoionizing levels. The model includes collisional inner-shell ionization from Al II levels by fast electrons, inelastic collisions with cold Maxwellian electrons and radiative and autoionization decays. The comparison between the experimental and the predicted relative line intensities seems to confirm that most of the 2p⁵(3s3p ³P) Al III levels have large radiative branching ratios. This conclusion is supported also by a comparison between Al III and Mg II spectra obtained in similar excitation conditions. Several 2p⁶3p ²P-2p⁵(3s3p ³P)²P, ²D Al III transitions having radiative yields close to one and large gain cross sections, may be of interest for soft X-ray lasing.