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

We report REMPI spectra and velocity-mapped ion images of the O(Δ) and (Σ) fragments arising from the spin-forbidden photodissociation of O near 320 and 330 nm. The O(Δ, 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. ( 2019, 151, 224302) for spin-allowed dissociation at 266 nm but is consistent with the couplings between the B state and A' and A″ states calculated by Grebenshchikov and Rosenwaks ( 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 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 ( = 35-43) and low ( = 17-20) rotational states. The REMPI spectrum of O(Δ) 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 near 330 nm, which showed evidence of contributions from O fragments in both the Δ and Σ states. The rotational distribution for the O(Σ, v = 0) product peaks at = 32 and is narrower than that of the O(Δ) fragment, consistent with distributions reported by O'Keeffe et al. at longer dissociation wavelengths ( , 117, 8705-8709). At smaller radii in the 2D-REMPI spectrum, there is additional signal assigned to v = 1-4 of O(Σ), with rotational distributions similar to v = 0. The vibrational distribution of the O(Σ) fragment peaks at v = 0, with populations monotonically decreasing with increasing vibrational state. Ion image angular distributions of the O(Σ) fragment and the corresponding anisotropy parameters are also reported.