Intramolecular
vibrational energy redistribution (IVR) of the NH
2 symmetric and asymmetric
stretching vibrations of jet-cooled aniline has been investigated by picosecond
time-resolved IR-UV pump-probe spectroscopy. A picosecond IR laser pulse excited
the NH
2 symmetric or asymmetric stretching vibration of aniline in
the electronic ground state and the subsequent time evolutions of
the excited level as well as redistributed levels were observed
by a picosecond UV pulse. The IVR lifetimes for symmetric
and asymmetric stretches were obtained to be 18 and 34 ps,
respectively. In addition, we obtained the direct evidence that IVR
proceeds via two-step bath states; that is, the NH
2 stretch
energy first flows into the doorway state and the energy
is further dissipated into dense bath states. The rate constants
of the second step were estimated to be comparable to
or slower than those of the first step IVR. The
relaxation behavior was compared with that of IVR of the
OH stretching vibration of phenol [Y. Yamada, T. Ebata, M.
Kayano, and M. Mikami J. Chem. Phys.
120, 7400 (2004)].
We found that the second step IVR process of aniline
is much slower than that of phenol, suggesting a large
difference of the "doorway state
the dense bath states" anharmonic coupling
strength between the two molecules. We also observed IVR of
the CH stretching vibrations, which showed much faster IVR behavior
than that of the NH
2 stretches. The fast relaxation is
described by the interference effect, which is caused by the
coherent excitation of the quasistationary states.