The UV photodissociation of phenol, a key nonradiative prototype, yields hydrogen atoms with a bimodal kinetic energy release (KER). While the high-KE component is often associated with direct dynamics, the mechanistic origin and branching of the low-KE component–spanning direct tunneling, statistical decay, and recrossing pathways–remain contested. To resolve this, we provide a definitive, atomically resolved roadmap by simulating oxygen K-edge transient X-ray absorption spectroscopy (TXAS) along the complete O–H dissociation coordinate. Our calculations predict distinct spectral fingerprints for the three branching pathways that pass through the second conical intersection: dissociation to electronically excited phenoxyl (Path 1), to ground-state phenoxyl (Path 2), and a nonadiabatic recrossing pathway (Path 3). Signatures are also assigned to predissociation dynamics upon UV excitation and to tunneling through the first conical intersection, as well as to both bound-side (Path 4) and dissociative-side (Path 3) channels of statistical ground-state decay. Systematic natural transition orbital (NTO) analysis along the reaction path decodes the evolving electronic character underlying these spectral features, delivering unprecedented orbital-resolved insight into the dissociation mechanism. This complete set of ab initio TXAS references establishes an unambiguous spectral framework, providing the essential theoretical foundation for future time-resolved X-ray experiments to disentangle these competing ultrafast bond-cleavage mechanisms.
Ge et al. (Wed,) studied this question.