Vibrational Relaxation of OH and OD Stretching Vibrations of Phenol and Its Clusters Studied by IR−UV Pump−Probe Spectroscopy

The vibrational relaxation of the OH/OD stretching mode of phenol and its cluster with Ar or H 2 O has been observed directly by nanosecond IR−UV pump−probe spectroscopy. The OH/OD vibrational level was prepared by a tunable nanosecond IR light pulse, and the decay of the pumped level as well as the rise of the relaxed level were observed by two-photon ionization with a tunable UV laser. For the OH stretch level of phenol- d 0, two components were observed: a fast component with a decay time of a few nanoseconds and a slow component with a decay time longer than 1 μs. It was also found that the decay rate of the fast component coincides with that of the rise of relaxed levels. The results indicate that the intramolecular vibrational redistribution (IVR) of the OH stretching vibration of bare phenol- d 0 is classified as an intermediate-strong-coupling case. For phenol- d 1, only the slow component was identified for the decay of the OD stretching vibration, whereas the relaxed level showed a rise with a few nanoseconds, similar to the case of phenol- d 0 . In the case of phenol−Ar and phenol−H 2 O, on the other hand, the OH stretching level showed a fast single-exponential decay curve, indicating a transformation from intermediate coupling to the statistical limit upon the cluster formation.