Femtosecond mid-infrared spectroscopy of condensed phase hydrogen-bonded systems as a probe of structural dynamics.

We report the first time-resolved site-specific mid-infrared study of the photo-induced excited state hydrogen transfer reaction in 2-(2'-hydroxyphenyl)benzothiazole (HBT) with 130 fs time resolution. The transient absorption of the C=O stretching band marking the keto*-S1-state appears delayed on a time scale of 30-50 fs after electronic excitation to the enol*-S1-state. Its line center subsequently shifts up by about 3-5 cm(-1) after excitation, depending on the excitation wavelength tuned between 315 and 349 nm. This effect is attributed to intramolecular vibrational energy redistribution (IVR) and vibrational energy relaxation (VER) processes. We observe for the first time the coherent effects of anharmonic coupling of low frequency modes (approximately 60 cm(-1), approximately 120 cm(-1)), on the C=O mode marking the product state. We ascribe the 120 cm(-1) mode to a Raman-active in-plane deformation mode that is coherently excited by the UV-pump pulse. We tentatively explain the coherent excitation of the infrared active 60 cm(-1) out-of-plane deformation mode by nonradiative processes within the excited enol state after electronic excitation.