Observation of sharp vibronic bands in the O4− `core ionʹ by mid infrared predissociation spectroscopy of O4−·Arn clusters

We report the first observation of an infrared (v0,0∼4150 cm−1) electronic band system arising from excitation of the ground state O4− ion, which we discuss in the context of the expected transition (J. Chem. Phys. 114 (2001) 3010) between the two low lying isomeric forms of this species. Surprisingly, the band displays sharp vibrational fine structure, opening the way for a detailed spectroscopic characterization of a charge–resonance stabilized dimer ion.