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The excited-state intramolecular proton transfer of 2-(2′-hydroxyphenyl)benzothiazole was investigated with 30 fs temporal resolution. The proton takes 60 fs to arrive at its S1 equilibrium position and the electron distribution changes on the same time scale. The molecule cannot adjust to the new structure of the H-chelate ring equally fast and starts to strongly vibrate in four low-frequency skeletal modes. This coherent wave packet motion is the cause of the experimentally observed strong signal oscillations that are only weakly damped despite the solvent environment.
Lochbrunner et al. (Thu,) studied this question.