Radical Redistribution of the 4d Oscillator Strength Observed in the Photoabsorption of the Ba, Ba+, and Ba++ Sequence

Abstract

The vacuum-ultraviolet absorption spectra of isolated Ba, ${\mathrm{Ba}}^{+}$, and ${\mathrm{Ba}}^{++}$ have been measured. In Ba and ${\mathrm{Ba}}^{+}$ most of the $4d$ absorption oscillator strength is in the continuum, but in ${\mathrm{Ba}}^{++}$ several very strong discrete transitions are observed. Such behavior has never before been observed in the $4d$ photoabsorption of vapors, solids, or compounds. An interpretation is given in terms of the partial collapse of the $\mathrm{nf}$ bound states in ${\mathrm{Ba}}^{++}$.