Abstract

Understanding the role of vibrational coherence in photochemical reactions is a fundamental problem in photochemistry and biology.¹,²,⁵ Vibrational coherence may play a significant role in reactions which occur on a time scale shorter than the relevant nuclear vibrational periods. One of the fastest chemical reactions is the first step in vision,⁴,⁵,⁶ the 11-cis → all-trans isomerization of the retinal chromophore in rhodopsin (Fig. 1). We have demonstrated that this reaction is essentially complete in only 200 fs.⁴,⁵,⁶ This time is comparable to or shorter than the period of torsional vibrational modes that are expected to make up the reaction coordinate, suggesting that this reaction has approached the adiabatic limit. We report here the observation of vibrational coherence in the photoproduct of the isomerization reaction in rhodopsin. The presence of vibrational coherence in the photoproduct indicates that the ultrafast isomerization of rhodopsin is mediated by nonstationary vibrational states which couple from the excited state of the reactant to the ground state of the photoproduct in a non-adiabatic process.

© 1994 Optical Society of America