Tracing Ultrafast Photo-Dissociation Pathways in Phenol: Transient X-ray Absorption Signatures

The photo-induced dissociation dynamics of phenol have attracted long interest of researchers, as this is fundamental for understanding the non-radiative decay mechanisms of bio-molecules with more complex substitutions at the aromatic ring. There is a controversy in the photochemical mechanisms that give rise to the fast and slow components in the photofragment translational kinetic energy distributions. Herein, we demonstrate that the transient X-ray absorption spectroscopy (TXAS) at the O K-edge is effective for tracking the ultrafast electronic and nuclear dynamics in photochemical processes, by multi-configurational simulations along different reaction pathways in the potential energy surfaces (PESs) along the O-H distance ROH. PESs were generated for the lowest 3 singlet valence states (S₀--S₂) and 30 O1s core-excited states (e₁--e₃₀), and TXAS spectra in three nonradiative reaction pathways were simulated showing distinct spectral signatures. The direct dissociation into the H atom and the first excited-state (~A) of phenoxyl (PhO·) residue (path 1) associates with a strong peak observed at 527-528 eV and a weak peak at 540-541 eV which vanishes rapidly along the dissociation coordinate. In comparison, the channel of dissociation into the ground state of PhO· (path 2) leads to a medium peak at 528-529 eV, and two weak peaks at 531-534 and 535-536 eV, respectively. Concerning the recrossing pathway (path 3), all peaks show obvious shifts when the photo-excited phenol decays along the S₀-state PES. The mechanisms underlying the photodissociation of phenol thus can be sensitively encoded in the O1s TXAS signals. The early-stage dynamics of photo-excited phenol were also investigated to explore the reaction pathways in S₁ and direct internal conversion into the high internal energy S₀ state, followed by decay with the increase of ROH.