Abstract
The excited-state dynamics of o-hydroxyacetophenone (OHAP) and its related molecules have been studied in the gas phase using femtosecond time-resolved multiphoton ionization spectroscopy. Following femtosecond excitation to their S1 state, the OHAPs were found to exhibit a biexponential decay behavior coupled with a rapidly damped low-frequency oscillation. The hydroxy deuterated OHAP showed a significant isotope effect, indicating that the oscillation is related to the H-atom motion. The combination of experimental and theoretical results suggested that, although the initial motion in the excited-state intramolecular proton transfer occurs in less than 100 fs, a low-frequency vibration motion involving both O…O and O-H stretches may allow the system to oscillation between the two tautomeric forms.