| dc.description.abstract | Experiments have been performed that provide evidence for a Λ-doublet preference in the O2(a1Δg) fragments resulting from the photodissociation of ozone in the Hartley band. In a recent study, the odd/even population alternation in the O2(a1Δg) rotational distribution first observed at 300 K was reproduced in a jet-cooled molecular beam. The population alternation, which favors the even rotational states, was found to be temperature dependent. The measured temperature dependence of the alternation, decreasing alternation with increasing temperature, is inconsistent with a parity-selective curve crossing origin as previously proposed. In this work, additional measurements were made to determine the origin of the population alternation. Using improved experimental techniques, the 2+1 resonance enhanced multiphoton ionization (REMPI) spectrum of the nascent O2(a1Δg) fragments following the 266 nm photolysis of 60 K ozone was remeasured. A quantitative O2(a1Δg) rotational distribution was extracted from the new spectrum using improved analysis techniques. Previous REMPI spectra have been refit using the new analysis. Additionally, ion imaging was used to measure the temperature dependent vector correlations of O2(a1Δg) fragments in odd and even rotational states. Consistent with previous results, the v-j and μ-j correlations were more depolarized for the odd rotational states than for the even rotational states, while the μ-v correlation was found to be independent of rotational state parity. A classical model based on a Λ-doublet preference was developed which qualitatively reproduces the experimental trends in the odd/even population alternation and vector correlations. According to this model the odd/even population alternation is a result of preferential formation of the A′ Λ-doublet of O2(a1Δg). Initial excitation in the Hartley band is to the B state which has A′ symmetry. Singlet products result from diabatic dissociation on the B state, so conservation of symmetry dictates preferential formation of the A′ Λ-doublet of O2(a1Δg). Due to nuclear exchange symmetry restrictions, the A′ Λ-doublet can only occupy even rotational states while the A′′ Λ-doublet can only occupy odd rotational states. Parent out-of-plane rotation facilitates the formation of the A′′ Λ-doublet, explaining the relative increase in odd state population with increasing temperature as well as the depolarization of the odd state v-j and μ-j vector correlations. | en |
