Optical-Optical Double Resonance Spectroscopy of the Quasi-Linear S₂ State of CHF and CDF: II. Predissociation and Mode-Specific Dynamics


In this work, we report on our full results of the dynamics of the quasi-linear, predissociated S2 state of the prototypical halocarbene, CHF, and its deuterated isotopomer CDF using optical-optical double resonance spectroscopy through the S1 state. Homogeneous linewidths were determined for a total of 51 S2 state vibrational levels with angular momenta in the range ℓ = 0 − 3 for CHF, and 76 levels for CDF. Progressions involving all three fundamental vibrations were observed. The linewidth data reveal pronounced mode specificity for both CHF and CDF, where pure bending states have the largest linewidths. For CDF, the linewidths are uniformly narrower. Calculated (CCSD(T)/aug-cc-pVQZ//MP2/aug-cc-pVQZ) stationary points on the CHF potential energy surface show that two dissociative pathways are available at the energies accessed in this experiment: dissociation on the triplet surface, over a barrier, to form C(3P) + HF, and dissociation to ground state CF + H products. The former is excluded as a primary channel based on the small spin-orbit coupling in this system. A 27-state dynamically weighted full-valence complete active space self-consistent field calculation was performed with maximal weight focused on the S2 state, which was then used as a reference for Davidson-corrected multireference configuration interaction calculations MRCI(+Q) of the three lowest A′ and two lowest A″ states. These calculations reveal the presence of multiple conical intersections in the singlet manifold. Consistent with our experimental results, the most important of these involves the repulsive S3 state, which conically intersects with S2 at linearity.



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