Observation of the Parity-Dependent Nonadiabatic Photodissociation of trans-HONO.

Quantum state-resolved studies of molecular photodissociation provide an effective view of how electronic and nuclear motion conspire to break chemical bonds. Here, we use parity-resolved photofragment imaging of trans-HONO photodissociation to show how electronic symmetry actively steers intramolecular vibrational energy redistribution (IVR) at the moment of bond cleavage. By performing Λ-doublet-resolved detection of OH-(X 2Π3/2, υ = 0, J = 3/2, 5/2, 7/2) with velocity-map ion imaging, we simultaneously determine the vibrational state distributions and angular distributions of the NO-(X 2Π, υ) cofragment. The parity-resolved measurements reveal a striking correlation: one parity class of OH is strongly associated with highly excited NO-(υ = 2), whereas the opposite parity favors NO-(υ = 1). Analysis of the total kinetic energy release, together with ab initio potential energy and spin-orbit coupling calculations, shows that these propensities fingerprint two competing pathways on electronically distinct excited-state surfaces. A prompt, nearly adiabatic dissociation on an A″ surface preserves electronic symmetry and channels energy into a specific NO stretch, whereas nonadiabatic transfer to an A' surface enables electron-mediated IVR via the out-of-plane mode of trans-HONO. The J dependence of the parity-vibrational correlation further reveals near-resonant coupling between the N=O stretch and a combination band. Our results demonstrate that fragment parity can map electronic symmetry onto product energy flow, offering a general strategy for disentangling electronically mediated and near-resonant vibrational dynamics in complex photochemical reactions.