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

We report REMPI spectra and velocity-mapped ion images of the O₂(a¹Δ) and (b¹Σ⁺) fragments arising from the spin-forbidden photodissociation of O₃ near 320 and 330 nm. The O₂(a¹Δ, 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 ³A' and ³A″ 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 O₃ 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 O₂(a¹Δ) 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 O₃ near 330 nm, which showed evidence of contributions from O₂ fragments in both the a¹Δ and b¹Σ⁺ states. The rotational distribution for the O₂(b¹Σ⁺, v = 0) product peaks at j = 32 and is narrower than that of the O₂(a¹Δ) 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 O₂(b¹Σ⁺), with rotational distributions similar to v = 0. The vibrational distribution of the O₂(b¹Σ⁺) fragment peaks at v = 0, with populations monotonically decreasing with increasing vibrational state. Ion image angular distributions of the O₂(b¹Σ⁺) fragment and the corresponding anisotropy parameters are also reported.