Vibrational coherence in ultrafast excited state proton transfer

The ultrafast intramolecular proton transfer in the first excited state of 2-(2′-hydroxy-5′-methylphenyl)-benzotriazole in solution was studied in two-color pump probe experiments with 25 fs pulses. The temporally and spectrally resolved stimulated emission of the keto-type reaction product reveals for the first time coherent vibrational motions in the excited state which occur during and after proton transfer. In particular, low frequency modes at 250 and 470 cm−1 show coherent oscillations for times up to 700 fs. Our results suggest that the highly anharmonic in-plane-deformation mode at 470 cm−1 modulates the separation between proton donor and acceptor atom and plays a key role in the transfer reaction.