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

摘要

Laser-induced fluorescence (FE) and vibration ally 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 (sigma(1)(0)) 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 sigma(1) 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 rho(1)) is responsible for the observed splitting. The DF spectra of the Franck-Condon active 6a(0)(1) band (0(0)(0) + 427 cm(-1)) of pFP reveals signatures of hydrogen bond induced vibrational energy relaxation (VER) predominantly from the bright (6a(0)(1)) to a dark (9b(1)) level in Si. 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 S-1 origin.