Multichannel quantum-defect analysis of the bound even-parity spectrum of neutral barium

Abstract

Extensive new data on the bound even-parity spectrum of neutral barium have been provided recently by two-step optogalvanic spectroscopy. Highly excited members of the Rydberg series $6sns^{3}S_{1}$, $6snd^{3}D_{1}$, $^{3}D_{3}$, and $6sng$ as well as perturbing levels of these series belonging to $5dnd$ configurations have been observed. By means of multichannel quantumdefect theory (MQDT), we have performed the analysis of all available energy data relevant to the $J=1,3,4,5$ even-parity bound spectra of barium. The behavior of $6sns^{3}S_{1}$, $6snd^{3}D_{1}$, $^{3}D_{3}$ series perturbed by levels pertaining to the $5dns (n=7,8)$ and $5dnd (n=6,7)$ configurations is clearly interpreted. The MQDT analyses of $J=1 \mathrm{and} 3$ spectra complementary to that previously performed on the $J=2$ spectrum offer the possibility of studying the anomalous fine structures of highly excited $6snd$ configurations. The MQDT analyses of $J=3\ensuremath{-}5$ spectra explain the observed energies of $6sng^{3}G_{3,4,5}$ and $^{1}G_{4}$ Rydberg levels; the fine structures of the $6sng$ series are only partially resolved in the vicinity of $5d7d$ $J=3 \mathrm{to} 5$ perturbing levels. Since the $J=0$ and $J=2$ bound even-parity spectra of Ba have already been analyzed by MQDT, we have now a full MQDT description of the bound even-parity spectrum for $J$ values ranging from 0 to 5.