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

Abstract

Extensive new data on the even-parity autoionized spectrum of Ba has been provided by two-step optogalvanic spectroscopy. The spectral range below the 5d3/2 ionization limit presents a complicated scheme of interacting levels, which is analyzed using the multichannel quantum defect theory (MQDT). The analysis concerns the 5d3/2nd and 5d3/2ns series converging on the 5d3/2 limit and their perturbers belonging to 5d5/2nd and 5d5/2ns series converging on the higher 5d5/2 limit. This is a favourable case for MQDT since the number of perturbing levels is very large: for each 5d5/2nl perturbing series, nine levels are located below the 5d3/2 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 5dnd-5dns mixing. The analyses only concern the position of autoionized levels. In fact the 5dnl autoionized lines observed in this range are relatively narrow and we can neglect the coupling of 5dnl levels with 6sϵl 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 5dnl levels are bound, i.e. below the 6s limit and the MQDT treatment of their mixing with the 6snl 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.