Dj. Myers et al., TEMPERATURE-DEPENDENCE OF VIBRATIONAL LIFETIMES AT THE CRITICAL DENSITY IN SUPERCRITICAL MIXTURES, The Journal of chemical physics, 107(23), 1997, pp. 9741-9748
Experimental measurements are reported for the temperature dependence
of the vibrational lifetime, T-1, of the asymmetric CO stretching mode
of tungsten hexacarbonyl in supercritical ethane at constant density
from just above the critical temperature to substantially higher tempe
ratures. T-1 is found initially to increase with temperature along an
isochore (reaching a maximum at about 70 degrees above the critical po
int of ethane), and then subsequently to decrease. Using a recent clas
sical theory of vibrational relaxation, we attempt to rationalize the
T-1 data. This behavior can be semiquantitatively reproduced by the th
eory if quantum corrections to the classical rate expressions are assu
med to be temperature independent in the limit when the transition ene
rgy is much greater than thermal energy. In this case, the theory indi
cates that the initial increase in T-1 with temperature arises because
of a competition between properties of the solvent which are changing
rapidly as the temperature is raised above the critical temperature.
At sufficiently high temperature, properties of the solvent vary slowl
y with temperature, and the explicit temperature dependence of the vib
rational relaxation dominates, producing a decrease in T-1 with increa
sing temperature. The predictions of the theory are also examined when
other postulated forms of the quantum correction factors are used, an
d the implications of these results for theoretical approaches to vibr
ational relaxation are discussed. (C) 1997 American Institute of Physi
cs. [S0021-9606(97)50447-8].