A study of O2(a 1Δg) with photoelectron spectroscopy using synchrotron radiation

Abstract

The atmospherically important species O2(a 1Δg) has been studied by photoelectron spectroscopy using vacuum ultraviolet radiation from a synchrotron as the photon source. Constant-ionic-state (CIS) spectra, recorded for vibrational levels of O2+(X 2Πg) v+=0,1,2,3 accessed from O2(a 1Δg) v″=0, exhibit intense signals in the photon energy region 14.0–15.5 eV which are shown to arise from autoionization from a Rydberg state with an O2+(C 2Φu) core. On the basis of the results obtained and earlier evidence derived from vacuum ultraviolet absorption spectroscopy, this state is assigned as a (C 2Φu,3sσg) 1Φu Rydberg state. Photoelectron spectra recorded for O2(a 1Δg) at positions of strong resonances have allowed extended vibrational structure to be obtained in the first photoelectron band. The relative vibrational component intensities in the resonant photoelectron spectra are in good agreement with computed relative intensities obtained via Franck–Condon calculations, confirming the vibrational numbering of the resonances in the 1Φu state. Competition between autoionization and predissociation in the 1Φu Rydberg state is discussed on the basis of the results obtained. Weaker structure is observed in CIS spectra recorded in the photon energy regions 12.5–13.5 and 15.0–20.0 eV. Suggestions are made for the nature of the highly excited states of O2 associated with this structure, based on available ionization energies and spectroscopic constants of known ionic states accessible from O2(a 1Δg). For example, two broad bands centered at ≈16.4 and ≈17.75 eV are assigned to excitation to Rydberg states arising from the configurations (D 2Δg,3pπu) and (D 2Δg,4pπu), respectively.