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Laser-induced fluorescence excitation and dispersed fluorescence spectra of a model flexible bichromophore, 1,1-diphenylethane (DPE), have been recorded under jet-cooled conditions in the gas phase in the region near the first pair of near-degenerate excited states (S₁ and S₂). The S₁ and S₂ origin transitions have been identified at 37 397 and 37 510 cm^–1, a splitting of 113 cm^–1. This splitting is four times smaller than the excitonic splitting calculated by ab initio methods at the EOM-CCSD/cc-pVDZ level of theory (410 cm^–1), which necessarily relies on the Born-Oppenheimer approximation. Dispersed fluorescence spectra provide a state-to-state picture of the vibronic coupling.more » Furthermore, these results are compared with the results of a multimode vibronic coupling model capable of treating chromophores in asymmetric environments.« less

The unimolecular dissociation dynamics of the dimethyl-substituted Criegee intermediate (CH₃)₂COO is examined experimentally using velocity map imaging to ascertain the translational and internal energy distributions of the OH and H₂CC(CH₃)O radical products. The energy profile of key features along the reaction coordinate is also evaluated theoretically. Unimolecular decay of (CH₃)₂COO is initiated by vibrational activation in the CH stretch overtone region and the resultant OH X²Π_3/2 (v = 0) products are state-selectively ionized and imaged. Analysis reveals an isotropic spatial distribution, indicative of a 3 ps lower limit for the timescale of dissociation, and a broad and unstructured total kineticmore » energy release distribution. The energy released to products is partitioned principally as internal excitation of the H₂CC(CH₃)O fragments with modest translational excitation of the fragments and a small degree of OH rotational excitation. The total kinetic energy release distribution observed for (CH₃)₂COO is compared with that predicted for statistical partitioning over product quantum states, and contrasted with recent experimental and quasi-classical trajectory results for syn-CH₃CHOO [N. M. Kidwell et al., Nat. Chem. 8, 509 (2016)].« less