Barium autoionising Rydberg series in the vicinity of a shape resonance

Abstract

The authors have measured angular distributions and branching ratios for Ba (6p32/ns)J=1 autoionising Rydberg states, using time-of-flight spectroscopy of the electrons. The experiment focused on the energy range between the Ba+ 6p1/2 and 6p3/2 fine structure levels. Strongly energy-dependent angular distributions have been found, which could not be explained by conventional multichannel quantum defect theory (MQDT) treatments. The authors conclude, by comparison with their own ab initio Hartree-Fock calculations for the autoionisation process, that the angular distributions are affected by a shape resonance in 5d excited neutral barium.