3P PHOTOABSORPTION OF FREE AND BOUND CR, CR+, MN, AND MN+

A dual-laser-plasma technique has been used to measure photoabsorption by atomic Cr and Mn and their ions at photon energies between 40 and 70 eV, where the dominant absorption mechanism is excitation of the 3p subshell. No dramatic differences are observed between the absorption spectra of Mn and Mn+, and these spectra are similar to those of Mn metal and MnCl2. The spectra of Cr and Cr+ are strikingly dissimilar, the mean excitation energy being about 5 eV greater in the ion. We attribute this to strong mixing of the localized 3d6 configuration with 3d5nd Rydberg configurations, an effect that is also responsible for the anomalous appearance of the Cr spectrum with respect to those of the other iron-period elements. The absorption spectra of Cr metal and CrCl2 take forms intermediate between those of Cr and Cr+. We give spectroscopic assignments to most of the sharp absorption features of Cr+ and determine the 3p ionization thresholds from quantum-defect analysis.