Photodissociation of triplet and singlet states of the CCO radical

Abstract

The triplet and singlet states of the ketenylidene (CCO) radical are investigated using fast radical beam photofragment translational spectroscopy, in which CCO is generated by laser photodetachment of CCO− and subsequently photodissociated, and anion photoelectron spectroscopy. In the photodissociation experiment, two bands in which the upper state of CCO predissociates are studied. Photodissociation from excitation of the à 3Π–X̃ 3Σ− band in CCO is observed from 16 666–23 529 cm−1; resonances are observed and assigned to excited vibrational levels involving all three vibrational modes. We also report the first observation of the c̃ 1Π–ã 1Δ band in CCO. Here, the ã 1Δ state of CCO is generated by laser photodetachment at higher photon energy than was used to generate the X̃ 3Σ state. The c̃ 1Π state is approximately located by photoelectron spectroscopy of CCO−, and the photodissociation experiment shows that the origin of the c̃ 1Π–ã 1Δ band occurs around 17 170 cm−1. Kinetic-energy release spectra from both bands yield accurate values for the C–CO bond dissociation energy and heat of formation of CCO: D0(C–CO)=2.24±0.02 eV (51.7±0.5 kcal/mol) and ΔHf,2980(CCO)=4.04±0.02 eV (91.1±0.5 kcal/mol). Although the translational-energy distributions resulting from excitation to the à 3Π and c̃ 1Π states are clearly nonstatistical, consideration of the potential-energy surfaces indicates that dissociation from both states occurs via radiationless transitions to the X̃ 3Σ− state.