PFI-ZEKE spectroscopy using coherent vacuum UV: O 2+ ( a4II u) ← O 2 ( X3Σ g−)

Abstract

The PFI-ZEKE spectra of the first four vibrational levels of O 2 +( a 4U u) are reported. The rotationally resolved photoelectron spectra were assigned by diagonalizing the appropriate Hamiltonian in a Hund's case a basis set. An extension of a previous model for rotational line strengths (the BOS model) to the intermediate coupling case was applied to the three vibrational bands with ν + = 1–3. As has been observed with several 2II states in previous ZEKE studies, the intensities of the spin-orbit sub-levels were not in accord with calculated line strengths. The a 4II u vibrational levels ν + = 0 and 1 were overlapped by the FranckCondon forbidden ν = 21 and 22 levels of the X 2II g state, allowing for a direct comparison between the intensities of the Franck-Condon allowed and forbidden bands in the ZEKE spectra. These data allowed us to determine an accurate ionization energy for the a 4II u state: 129892 ± 2 cm −1 (16.1046 eV).