Resonance photoelectron spectroscopy of 5p hole states in atomic barium

Photoelectron spectra of atomic barium have been recorded at several photon energies in the range 20<or=hv<or=29 eV. The variations of the Ba ⁺5p⁶nl (nl=6s,5d,6p,7s,6d) photoelectron peak intensities were measured in the 20-21 eV autoionisation region. The results indicate that each autoionising state decays to the various states of the ion in a characteristic way. Asymmetry parameters measured for the 6s, 5d and 6p channels show strikingly different values that follow the general trend beta (6s)> beta (6p)> beta (5d). Above 21 eV, 5p ionisation begins to dominate, and the Auger spectra were used to monitor the production of the various 5p hole states. As the photon energy is scanned across the autoionising resonances, these Auger spectra indicate that excitation of 5p hole states decaying by two-step autoionisation is the dominant mode in the production of very-low-energy photoelectrons (<or=2 eV) and their corresponding high-energy Auger electrons. Calculations suggest that a heretofore unobserved 5p ⁵6p^{2 2}P₃2/5d autoionising level is responsible for the Auger distribution measured at hv=28.9 eV.