Rotational Distributions and Imaging of Singlet O 2 Following Spin-Forbidden Photodissociation of O 3 .

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 spin-forbidden 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.