Unraveling the mysteries of metastable O-4*

A recent report from our laboratory described 1+1 resonant photoionization spectra of an energetic, metastable O-4 species produced in a dc discharge [Bevsek et al., Faraday Discuss. 108, 131 (1998)]. Although a definitive as signment of the spectra was lacking, the long-predicted covalent forms of O -4, either cyclic (D-2d) or "pinwheel'' (D-3h), were adduced as possible ca ndidates. We here present rotationally resolved photoionization spectra, ph otoelectron spectra, and ab initio calculations providing strong evidence f or the identity of this species as a novel complex between a ground state O -2 molecule and one in the c((1)Sigma(u)(-)) state, which is excited via an allowed transition to the 1((1)Pi(g)) valence state. The latter then coupl es to the d((1)Pi(g)) Rydberg state, shifted in energy owing to the presenc e of the adjacent O-2 molecule, from which it then ionizes. The results und erscore the potential importance of the fully allowed but overlooked 1((1)P i(g)) <--c((1)Sigma(u)(-)) electronic transition in O-2 in the near ultravi olet, and provide a simple interpretation of puzzling results in an earlier study of electron transfer to O-4(+) [H. Helm and C. W. Walter, J. Chem. P hys. 98, 5444 (1993)]. (C) 1999 American Institute of Physics. [S0021-9606( 99)01613-X].