LIF Spectroscopy of p-Fluorophenol···Water Complex: Hydrogen Bond Vibrations, Fermi Resonance, and Vibrational Relaxation in the Excited State.

Laser-induced fluorescence (FE) and vibrationally resolved dispersed fluorescence (DF) spectra of a 1:1 water complex of p-fluorophenol (pFP) have been measured in a supersonic jet expansion. The hydrogen bond stretching fundamental (σ0 1 ) of the complex appears in the FE spectrum as a doublet with band maxima at 155 and 161 cm-1 . Emission spectra recorded upon excitations of the two components reveal that a Fermi resonance between σ1 and a combination involving a low-frequency intramolecular mode of pFP (mode 11) and a bending mode of water at the hydrogen bonded interface (mode ρ1 ) is responsible for the observed splitting. The DF spectra of the Franck-Condon active 6a0 1 band (00 0 + 427 cm-1 ) of pFP reveals signatures of hydrogen bond induced vibrational energy relaxation (VER) predominantly from the bright (6a0 1 ) to a dark (9b1 ) level in S1 . The relative intensities of the emission bands from the locally excited and relaxed levels indicate that VER for excitation up to this level occurs at a time scale similar to the fluorescence decay time of the complex. However, complete VER at a much faster time scale occurs for excitation beyond 822 cm-1 above S1 origin.