Singlet O₂ Produced by Ultraviolet Dissociation of the β-ionone-O₂ Complex

We formed the gas-phase β-ionone-O₂ complex in a supersonic expansion and then photodissociated the complex with light near 312 nm. Photodissociation resulted in the production of O₂ in the a ¹Δ_g state, which was ionized at 312 nm using (2 + 1) resonance-enhanced multiphoton ionization (REMPI). We recorded the ¹O₂ REMPI action spectrum and O₂⁺ velocity map ion image following photodissociation of the complex. From the velocity map image, we determined the total recoil kinetic energy distribution from dissociation of the complex. Fitting the REMPI spectrum showed that the ¹O₂ product has an effective rotational temperature of about 50 K, while the recoil kinetic energy distribution was well fit with a statistical Boltzmann distribution having an effective translational temperature of 289 K. Using the average translational energy from the Boltzmann fit along with the complex dissociation energy from ab initio calculations, we determined that β-ionone was formed with an average of 2.87 eV of internal energy, which was 0.49 eV higher than previous measurements for the β-ionone triplet-state energy. Our own CCSD/cc-pVDZ//(U)MP2/cc-pVDZ calculations gave a minimum triplet-state energy of 2.04 eV. However, a large structural change occurs between the minimum singlet-ground-state geometry and the minimum triplet-excited-state geometry, and as a result, the calculated vertical energy for the triplet-state β-ionone was determined to be 3.30 eV. Comparing the ab initio and experimental results indicated that following excitation, β-ionone was formed in the triplet state but with significant internal vibrational energy. As such, complex dissociation likely proceeds following internal vibrational energy redistribution, which explains the statistical recoil kinetic energy distribution.