Two-step Laser Excitation Studies of Bound and Autoionizing States in Barium

Abstract

We report detailed experimental studies on the spectra of barium using the two-step excitation scheme to measure the absolute photoionization cross section from excited states and the optical oscillator strengths of the Rydberg transitions. In addition, the line shapes of the autoionizing resonances have been studied using various channels approached by different intermediate levels and extensive new measurements of the highly excited even parity autoionizing resonances is presented. Furthermore, the proper line shape of the 6p2 1S0 autoionizing resonance has been reported experimentally for the first time. In the first set of experiments, the photoionization cross section from the 6s6p 1,3 P1 excited states have been measured in the energy region from the first ionization threshold up to 1.69 eV excess energy. The experiments have been performed using a thermionic diode ion detector in conjunction with a Nd: YAG laser system. The absolute values of the photoionization cross section from the 6s6p 1P1 and 3P1 excited states at the first ionization threshold have been determined as 90 ± 14 Mb and 102 ± 15 Mb respectively. The studies have been extended to determine the measurements of the oscillator strengths of the 6s6p 1P1 → 6snd 1D2 (20 ≤ n ≤ 76) and 6s6p 3P1 → 6snd 3D2 (20 ≤ n ≤ 63) Rydberg transitions. The measured absolute photoionization cross-sections from the 6s6p 1P1 and 6s6p 3P1 states have been used to calibrate the f-values of the Rydberg transitions. The oscillator strength densities in the continuum corresponding to the 6s6p 3P1 excited state have also been determined. In the next set of experiments, we have investigated how the line shape and width of an autoionizing resonance varies by choosing different excitation paths and a systematic line shape analysis of the 6p7p configuration based 1P1, 3D1 and 3P1 autoionizing resonances are presented. These even parity isolated autoionizing resonances have been accessed via four different intermediate states 6snp 1P1 (6 ≤ n ≤ 8) and 5d6p 1P1. A comparison of the Fano parameters of the resonance profiles reveals that the width of an autoionizing resonance remains constant while the line profile parameter changes with the selection of different intermediate states.