Vibrational relaxation of the free terminal hydroxyl stretch in methanol oligomers: Indirect pathway to hydrogen bond breaking

Vibrational relaxation of methanol-d (MeOD) in carbon tetrachloride has bee n investigated via ultrafast infrared pump-probe experiments. Exciting at 2 690 cm(-1), only the free O-D (where the D is not H-bonded) stretching mode is initially populated. For MeOD mole fractions less than or equal to0.025 , a 2.15 ps single exponential decay is observed. At mole fractions greater than or equal to0.0375, the signal decays (2.15 ps decay time) below zero (increased absorption) and then recovers on time scales of 22 ps and much g reater than 300 ps. The increased absorption indicates the formation of add itional free ODs caused by the breaking of H-bonds that are not directly co upled to the initially excited vibration. The two-time scale recovery of th is signal arises from geminate and nongeminate recombination. The data are fit with a set of kinetic equations that accurately reproduce the data. The results suggest that vibrational relaxation of the initially excited free OD stretch into intramolecular modes of the methanol leads to H-bond breaki ng. This contrasts studies that suggest direct relaxation of a vibrationall y excited OH stretch into an H-bond stretch is responsible for H-bond break ing. (C) 2001 American Institute of Physics.