Two-Step Optogalvanic Spectroscopy of Neutral Barium: Observation and Interpretation of the Even Levels Above the 6s Ionization Limit Between 5.2 and 7 eV

Abstract

Even parity levels of neutral barium above the 6s ionization limit have been investigated by using a two-step pulsed laser excitation with optogalvanic detection. From the metastable states, 5d6s, populated by a d.c. discharge in a heat-pipe vapour, we reach the 5dnl even Rydberg series with J values ranging from 0 to 5, the 6p2 1S0 state and the low-lying members (n = 7 and 8) of the 6pnp series above the 5d limits. The 5dnd configurations (18 levels for each n member) are observed from n = 8 up to n = 45 using optogalvanic spectra recorded from five intermediate 5d6p levels: 3P1,20, 3F3,40 and 1P10. These eighteen series are observed for the first time. Fine structure is resolved up to n = 25 and has been analyzed using Slater-Condon parametric calculations. In addition, the 5d3/2ns J = 1 and 5d5/2ns J = 3 series are observed up to n = 38 and 42 respectively, as well as sharp resonances interpreted as 5d3/2ng7/2 J = 2 and 5d5/2ng J = 1, 2 series up to n = 27 and n = 28 respectively. In conclusion, the energies of 636 new levels between 6s and 5d limits are given with their term and configuration identifications. Above the 5d5/2 limit, broad resonances observed from the 5d6p 1P10, 6s7p 1P10 and 3P0,1,20 intermediate levels have been identified with levels of the 6p7p and 6p8p configurations using parametric calculations based on the Slater-Condon theory.