Internal energy dependence of the photodissociation dynamics of O3- using cryogenic photoelectron-photofragment coincidence spectroscopy.

Photoelectron-photofragment coincidence (PPC) spectra of ozonide, O3-, were measured at 388 nm (Ehν = 3.20 eV) using a newly constructed cryogenic octopole accumulation trap coupled to a PPC spectrometer. The photoelectron spectra reveal three processes consisting of a stable photodetachment channel, and two distinct photodissociation pathways yielding (1) O2 + O- or (2) O + O2-. The first photodissociation pathway is observed in the PPC spectra by photodetachment of the O- product by a second photon, and produces electronically excited O2(1Δg). The O2- product of the second photodissociation pathway undergoes autodetachment for O2-(2Πg, v″ > 4), a process greatly enhanced by vibrational excitation of the precursor O3-. Cooling anions thermalized at 300 K to <17 K in a cryogenic octopole accumulation trap essentially turns off this autodetachment pathway. The product kinetic energy distribution in coincidence with the autodetached electrons from O2-(v″ = 4) exhibits resolved features consistent with bend (ν2), asymmetric stretch (ν3) and a stretching combination band (ν1 + ν3) in the intermediate electronic state, illustrating the insights that can be gained from kinematically complete measurements. These results are discussed in the context of the low-lying excited states of O3-.