Relaxation dynamics of NH stretching vibrations of 2-aminopyridine and its dimer in a supersonic beam.

Picosecond time-resolved IR-UV pump-probe spectroscopy has been carried out to elucidate vibrational energy relaxation (VER) of the NH stretching vibrations of 2-aminopyridine monomer (2AP) and dimer [(2AP)(2)] in supersonic beams. In bare 2AP, intramolecular vibrational energy redistribution (IVR) of the NH vibrations is described by the two-bath mode model, in which the initial vibrational energy flows to the doorway states rapidly (6.5 ps) and then dissipates into the dense bath states with a time constant of approximately 20 ps. No clear difference was observed in the IVR lifetime between the symmetric and asymmetric NH(2) stretch modes. In (2AP)(2), IVR and vibrational predissociation (VP) were involved in VER. It was found that the rate constants of both IVR and VP of the hydrogen-bonded NH stretching vibration are larger than those of the free NH.