Rotational Distributions and Imaging of Singlet O₂ Following Spin-Forbidden Photodissociation of O₃

Abstract

We report REMPI spectra and velocity-mapped ion images of the O 2 (a 1 Δ g ) and (b 1 Σ g + ) fragments arising from the spinforbidden photodissociation of O 3 near 320 and 330 nm. The O 2 (a 1 Δ g , v = 0) REMPI spectrum following a 320 nm dissociation shows enhanced peak intensity for the odd rotational states relative to the even states, which is the opposite of the trend observed by Gunthardt et al. (J. Chem. Phys. 2019, 151, 224302) for spin-allowed dissociation at 266 nm but is consistent with the couplings between the B state and 3 A′ and 3 A″ states calculated by Grebenshchikov and Rosenwaks (J. Phys. Chem. A 2010, 114, 9809−9819). There are no significant differences between the ion image angular distributions of fragments in odd and even rotational states, which indicates a cold distribution of O 3 and supports the explanation that the alternation in peak intensities results from a difference in the couplings. Quantitative analysis of the image angular distributions was limited due to the single laser polarization geometry accessible in one-color experiments. Radial distributions of the 320 nm images indicate a broad rotational distribution, evidenced in bimodal speed distributions with peaks corresponding to both high (j = 35−43) and low (j = 17−20) rotational states. The REMPI spectrum of O 2 (a 1 Δ g ) near 330 nm was collected, and while quantitative population analysis is difficult because of the perturbed resonant state, the spectrum clearly supports a broad rotational distribution as well, consistent with the images collected at 320 nm. A 2D-REMPI spectrum was collected following dissociation of O 3 near 330 nm, which showed evidence of contributions from O 2 fragments in both the a 1 Δ g and b 1 Σ g + states. The rotational distribution for the O 2 (b 1 Σ g + , v = 0) product peaks at j = 32 and is narrower than that of the O 2 (a 1 Δ g ) fragment, consistent with distributions reported by O'Keeffe et al. at longer dissociation wavelengths (J. Chem. Phys. 2002, 117, 8705−8709). At smaller radii in the 2D-REMPI spectrum, there is additional signal assigned to v = 1−4 of O 2 (b 1 Σ g + ), with rotational distributions similar to v = 0. The vibrational distribution of the O 2 (b 1 Σ g + ) fragment peaks at v = 0, with populations monotonically decreasing with increasing vibrational state. Ion image angular distributions of the O 2 (b 1 Σ g + ) fragment and the corresponding anisotropy parameters are also reported.