Two-photon spectroscopy of Rydberg states of molecular oxygen

Abstract

Two-photon resonant, three-photon ionization spectrum of O2X 3Σ−g, v″=0 is presented in the energy region between the 5s–4d complex and the O+2X 2Π1/2g, v+=0 ionization threshold. The spectrum exhibits a number of new transitions to ns and nd Rydberg states with n=5-9 and v′=0 and 1 as well as transitions to the previously unobserved 5s–4d, v′=3 levels. For a given value of n and v′, the observed bands split into two groups separated by approximately the O+2X 2Π1/2g−2Π3/2g spin–orbit splitting. All of the bands show at least some sharp rotational structure, although in most cases this structure is limited to a few lines. In addition, the direct two-photon ionization spectrum of O2X 3Σ−g, v″=0 is presented in the energy region between the O+2X 2Π1/2g, v+=0 and 1 thresholds; the spectrum shows transitions to Rydberg states with n=8-12 and v′=1. These states are analogous to the v′=0 states observed below the ionization threshold, and they decay by vibrational autoionization into the v+=0 continuum. Taken together, the v′=0 and 1 Rydberg states observed here provide a significant addition to the knowledge of Rydberg series converging to the ground electronic state of O+2.