A high-resolution photoionization study of ⁵⁶Fe using a vacuum ultraviolet laser

The photoionization efficiency (PIE) spectrum for ⁵⁶Fe formed by laser ablation has been measured using tunable vacuum ultraviolet (VUV) laser radiation in the energy range of 63000-74700 cm^-1, covering the threshold regions for the photoionization transitions of Fe⁺(3d ⁶4s ⁶ D) ← Fe(3d ⁶4s ^{2 5} D) and Fe⁺(3d ⁶4s ⁴ D) ← Fe(3d ⁶4s ^{2 5} D). The fact that no step-like structures in the PIE spectrum are observed at these ionization thresholds indicates that direct photoionization plays a very minor role in the photoionization of Fe in this VUV energy range. Comparing the VUV-PIE and the VUV-absorption spectra of Fe shows that all prominent absorption bands of Fe correspond to strong autoionizing structures in the PIE spectrum of Fe. Due to the significantly narrower VUV laser optical bandwidth of 0.12 cm^-1 used in the present study, complex autoionizing resonances are resolved in the PIE spectrum. Two autoionizing Rydberg series (3d ^{7 4} F_7/2)np (n = 9-27) and (3d ^{7 4} F_9/2)np (n = 10-32) formed by two-electron excitations from the (3d ⁶4s ^{2 5} D₄) ground state are identified to converge to the respective Fe⁺(3d ^{7 4} F _7/2) and Fe⁺(3d ^{7 4} F_9/2) ion levels. The photoionization cross sections and well resolved autoionizing Rydberg resonances observed in this study are relevant to astrophysics for understanding the Fe contribution to the VUV opacity in the solar atmosphere, and for benchmarking theoretical calculations under the Opacity Project and the IRON Project.