Photoionization and photoelectron spectroscopy of O2 with coherent vacuum ultraviolet radiation

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Abstract
The photoionization of O2 near its ionization limit has been studied with coherent vacuum ultraviolet radiation produced by third harmonic generation in free jet expansions of the rare gases. High resolution (∼2 cm−1) photoionization spectra were obtained in the ionization threshold region from 103 to 98 nm which includes three vibrational levels of the H 3Πu (3sσ) Rydberg state. The H, v=0 photoionization spectrum was assigned by simulating the H 3Πu←X 3∑+g Rydberg excitation, yielding spectroscopic constants as well as the overall autoionization lifetime. The v=1 and 2 levels have distinctly different rotational band contours which reflect perturbations with bound and dissociative levels of nearby ‘‘dark’’ states. The photoionization dynamics were probed further through measurements of photoelectron angular distributions for the v+=0 and 1 vibrational levels of O+2. In addition to strong variations in the asymmetry parameter (β) across the H state autoionization resonances, spectrally narrow variations in β were found in the surrounding continuum. These latter results suggest the presence of weak resonance features imbedded in the background continuum which nonetheless strongly influence the photoelectron ejection dynamics.

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