Coherent Vibrational Motion during the Excited-State Intramolecular Proton Transfer Reaction in o-Hydroxyacetophenone

Excited-state intramolecular proton transfer (ESIPT) reaction dynamics in o -hydroxyacetophenone (OHAP) has been investigated in a supersonic molecular beam using femtosecond time-resolved multiphoton ionization mass spectrometry. The observed transients exhibit a biexponential decay and a rapidly damped oscillation with a period of ∼600 fs. The combinations of experimental and theoretical results suggested that the oscillation is due to a coherent vibrational motion that follows the molecular transformation from the initial enol configuration to the final keto form during the ESIPT reaction.