Photolysis dynamics of m- and o-fluorophenol: Substitution effects on tunneling mechanism

The photolysis dynamics of m-fluorophenol (m-FPhOH) and o-fluorophenol (o-FPhOH) have been investigated with time-resolved velocity map imaging (TR-VMI) and time-resolved ion-yield (TR-IY) techniques. Following excitation to the origin of S₁ (ππ∗) state of m- and o-FPhOH, H atoms elimination mediated by tunneling through the potential barrier under the S₁ (ππ∗)/S₂ (πσ∗) conical intersection (CI) has been observed as a Gaussian feature signal centered at a total kinetic energy release (TKER) of ∼6000 cm^-1 for both molecules. The quantum tunneling mechanism has been identified as the main decay pathway of S₁ state for m-FPhOH, and the tunneling lifetime of 2.1 ns has been obtained from the TR-VMI measurements of H fragments. This tunneling mechanism is further confirmed by the studies on the selective O-H deuterated species, m-FPhOD, and consolidated by our theoretical calculations. However, the photolysis dynamics is quite different for the photoexcited o-FPhOH. The much lower yield of the H atoms originating from tunneling hinders the extraction of a reliable tunneling lifetime for o-FPhOH. Our theoretical calculations exhibit a broader and higher potential barrier exists beneath the S₁/S₂ CI of o-FPhOH, which increase the difficulty for tunneling. Furthermore, the special existence of intramolecular hydrogen bond in o-FPhOH is probably also the key factor that affects the tunneling rate, which would restrict the O-H stretch motion.