Internal energy dependence of the photodissociation dynamics of O 3 - 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 (1 Δg ). The O2 - product of the second photodissociation pathway undergoes autodetachment for O2 - (2 Πg , 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 - .