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

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 $J=1,3,4,5\mathrm{}$ 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 $5dns \mathrm{}(n=7,8\mathrm{})\mathrm{}$ and $5dnd \mathrm{}(n=6,7\mathrm{})\mathrm{}$ configurations is clearly interpreted. The MQDT analyses of $J=1\mathrm{} \mathrm{and} 3$ spectra complementary to that previously performed on the $J=2\mathrm{}$ spectrum offer the possibility of studying the anomalous fine structures of highly excited $6snd$ configurations. The MQDT analyses of $J=3-5\mathrm{}$ 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 $5d7d$ $J=3\mathrm{} \mathrm{to} 5$ perturbing levels. Since the $J=0\mathrm{}$ and $J=2\mathrm{}$ 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.