Ran Pan and Lionel M. Raff
J. Chem. Phys. 106(4), 1382 (1997)
Abstract
l
Continuous frquency modulated (CFM) line
splittings are used to determine the energy transfer rate
coefficients for the local C-Br and C=C vibrational modes
in vinyl bromide and the C-H stretching modes in doubly-
deuterium substituted vinyl bromides. The global potential
developed by Abrash et al. is employed in all calculations.
Energy transfer rate coefficients are extracted from the fine
structure spacing of the numerically computed power
spectrum of the bond coordinates. The consistency of the
averaged individual rate coefficients is evaluated by
comparison with results obtained from local mode energy
decay curves. It is found that the total IVR rate coefficients
for all modes investigated are large relative to the
unimolecular decomposition rate. However, previous
studies show that IVR is not globally rapid so statistical
behavior of the unimolecular reaction is not expected. It is
shown that near overlapping resonances in the power
spectrum make it difficult to accurately extract CFM line
splittings. This limitation effectively precludes the use of
power spectra to investigate IVR rates for some modes.
For the specific case of vinyl bromide, it is demonstrated
that the C-Br and C=C stretching modes have sufficiently
isolated bands that IVR rates out of these modes can be
determined from the line splittings. However, the
superposition of the three C-H stretching fundamentals
makes it essentially impossible to investigate these modes
in vinyl bromide. For the case of doubly-deuterium
substituted vinyl bromides, the C-H stretching fundamental
is well isolated so that IVR relaxation rates can be easily
obtained from the power spectrum line splittings. The
consistency of the IVR rate coefficients obtained from line
splittings is investigated by calculation of these coefficients
from the envelopes of bond energy decay curves. The
differences between the two results varies from 15% for the
C=C stretch to 43% for one of the C-H stretching modes.
The average deviation is 30% which is in accord with the
accuracy of the method (±25%) previously estimated by
Agrawal et al. The effect of initial local excitation energy
on the line splittings and associated rate coefficients is
investigated for the C-Br stretching mode. The results
show that the line splitting and rate coefficients are nearly
independent of excitation energy below 0.8 eV. Above this
energy, both the line splittings and the IVR rate coefficients
increase rapidly. This is interpreted as being due to
increased intermode coupling at higher energies produced
by the greater vibrational anharmonicity. It is concluded
that CFM line splittings can be effectively used as a probe
of energy transfer rates in six-atom molecules provided the
modes under examination have reasonably isolated bands in
the power spectrum.