Multichannel Quantum Defect Analysis of the Autoionized Even-parity Spectrum of Neutral Barium below the 5d²D3/2Limit

Extensive new data on the even-parity autoionized spectrum of Ba has been provided by two-step optogalvanic spectroscopy. The spectral range below the 5 d 3/2 ionization limit presents a complicated scheme of interacting levels, which is analyzed using the multichannel quantum defect theory (MQDT). The analysis concerns the 5 d 3/2 nd and 5 d 3/2 ns series converging on the 5 d 3/2 limit and their perturbers belonging to 5 d 5/2 nd and 5 d 5/2 ns series converging on the higher 5 d 5/2 limit. This is a favourable case for MQDT since the number of perturbing levels is very large: for each 5 d 5/2 nl perturbing series, nine levels are located below the 5 d 3/2 limit. A J by J treatment of the energies of the spectra with J value ranging from 0 to 5 was carried out taking into account the 5 dnd -5 dns mixing. The analyses only concern the position of autoionized levels. In fact the 5 dnl autoionized lines observed in this range are relatively narrow and we can neglect the coupling of 5 dnl levels with 6 s l continua. The MQDT treatments presented here allow us to improve the identification of levels and to describe accurately the behaviour of perturbed series. The lower 5 dnl levels are bound, i.e. below the 6 s limit and the MQDT treatment of their mixing with the 6 snl principal Rydberg series has been previously published. The comparison of the MQDT parameters fitted to the bound or autoionized spectra confirms the large energy dependence of the parameters. This point is discussed and shows the difficulties of extrapolation procedures. For the first time the MQDT parameters are closely related to semi-empirical values of Slater integrals.