Vibrational relaxation in jet-cooled alkyl benzenes. III. Nanosecond time evolution

Spectrally resolved fluorescence time decay measurements have been completed with nanosecond resolution on a series of n-alkylbenzenes laser excited into well-localized ring distortion vibrations in the S1 electronic state. Results indicate that those early members of the series which continue to exhibit some sharp, vibrationally unrelaxed emissions do so because they are intermediate case examples with inadequate density of states to permit dynamical relaxation on a nanosecond time scale. The longer chain, statistical limit molecules show a relaxed fluorescence pattern which displays no residual nanosecond time evolution. Intramolecular vibrational relaxation thus appears to proceed to an essentially complete randomization within a time period shorter than the excitation laser pulse. © 1980 American Institute of Physics