Unraveling the mysteries of metastable O4*

A recent report from our laboratory described 1+1 resonant photoionization spectra of an energetic, metastable O4 species produced in a dc discharge [Bevsek et al., Faraday Discuss. 108, 131 (1998)]. Although a definitive assignment of the spectra was lacking, the long-predicted covalent forms of O4, either cyclic (D2d) or “pinwheel” (D3h), were adduced as possible candidates. We here present rotationally resolved photoionization spectra, photoelectron spectra, and ab initio calculations providing strong evidence for the identity of this species as a novel complex between a ground state O2 molecule and one in the c(1Σu−) state, which is excited via an allowed transition to the 1(1Πg)valence state. The latter then couples to the d(1Πg) Rydberg state, shifted in energy owing to the presence of the adjacent O2 molecule, from which it then ionizes. The results underscore the potential importance of the fully allowed but overlooked 1(1Πg)←c(1Σu−) electronic transition in O2 in the near ultraviolet, and provide a simple interpretation of puzzling results in an earlier study of electron transfer to O4+ [H. Helm and C. W. Walter, J. Chem. Phys. 98, 5444 (1993)].