Franck–Condon overlap integrals between the 3sσ 3Πg Rydberg and the 1 3Πg valence states of O2

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Abstract
The (2+1) resonance-enhanced multiphoton ionization spectrum of the 3sσ 3Πg←←3Σ−g transitions in O2 shows diffuse structure for the 0–0, 1–0, 3–0, and 4–0 vibrational bands with linewidths ranging from about 150 cm−1. The spectrum of the 2–0 band, however, is rotationally resolved with a fullwidth at half maximum of 6 cm−1. The 3sσ 3Πg Rydberg state predissociates via the dissociative 1 3Πg valence state. The observed variations in the linewidths of different vibrations in the two-photon transition are a consequence of the variations in the Franck–Condon overlap integrals between the bound and dissociative states. We have constructed an empirical representation of the 3Πg valence potential by calculating trial-and-error Franck–Condon (FC) overlap between the Rydberg and valence states. The theoretical potential curve of the 3Πg valence state is varied systematically to obtain FC densities compatible with the experimentally determined linewidths for the two-photon transition in 16O2, 17O2, 18O2 and 18O 17O. Our study stresses the importance of using isotopic data in order to obtain an unambiguous and accurate representation of the state.