CARS spectroscopy of O2(1Δg) from the Hartley band photodissociation of O3: Dynamics of the dissociation

Abstract

Rotationally and vibrationally resolved CARS spectra of the O2(1Δ g ) photofragment produced by the photodissociation of O3 at 17 wavelengths between 230 and 311 nm are reported. The spectra are taken under collision‐free conditions, therefore, they reveal the nascent rotational and vibrational state distributions of the O2(1Δ g ) photofragment. At all photolysis wavelengths studied the vibrational distribution peaks very sharply at v=0, although all energetically allowed vibrational states are observed. The rotational state distributions are narrow, and peak typically at high J. The rotational distribution shifts to lower J as the photolysis wavelength increases. These observations imply vibrationally adiabatic, rotationally impulsive energy release in the dissociation. The shape and width of the rotational distributions can be completely accounted for by the spread in the O3 thermal rotation and zero‐point vibration contributions to the O2(1Δ g ) photofragment angular momentum. The most striking observation about the O2(1Δ g ) photofragment quantum state distribution is an apparent propensity for even‐J states. Experiments with 1 8O enriched ozone indicate that this propensity is observed only for 1 6O1 6O, not for 1 8O1 6O, and by implication not for 1 7O1 6O. We show that this is the consequence of a selective depletion of only odd‐J rotational states of 1 6O1 6O(1Δ g ) by a curve crossing to O2(3Σ g ), but an equal depletion of both even‐J and odd‐J rotational states of 1 8O1 6O and 1 7O1 6O(1Δ g ) by the curve crossing. The odd‐J selectivity for 1 6O1 6O is a consequence of the restriction of 3Σ g to only odd‐J states, due to the requirement of even nuclear exchange symmetry for this homonuclear species with spin‐zero nuclei. As a result of the different curve crossing behavior, the quantum yield for 3Σ g is twice as great for 1 8O1 6O and 1 7O1 6O as it is for 1 6O1 6O, and this imposes a mass‐independent isotopic fractionation in the photodissociation: the O2(1Δ g ) fragments are depleted of 1 7O and 1 8O, while the O2(3Σ g ) fragments are enriched in these isotopes.