Vibrational population relaxation of the -N=C=N- antisymmetric stretching mode of carbodiimide studied by the infrared transient grating method

Vibrational dynamics of the carbodiimide (-N=C=N-) antisymmetric stretching mode of dicyclohexylcarbodiimide (DCCD) in carbon tetrachloride and N,N-dimethylformamide have been investigated by the transient grating method. The magic angle transient grating experiment reveals that the population relaxation of the nu = 1 state occurs nonexponentially and is reproduced well by a biexponential function with time constants of 200-300 fs and 1.4-1.5 ps with almost equal amplitudes. A time scale of the orientation relaxation is longer than a few picoseconds. Frequency-resolved transient grating measurement has shown that this nonexponential behavior is interpreted by a model of rapid population distribution to other modes and irreversible relaxation from these modes.