Kd. Rector et Md. Fayer, VIBRATIONAL DEPHASING MECHANISMS IN LIQUIDS AND GLASSES - VIBRATIONALECHO EXPERIMENTS, The Journal of chemical physics, 108(5), 1998, pp. 1794-1803
Picosecond vibrational echo studies of the asymmetric stretching mode
(2010 cm(-1)) of (acetylacetonato)dicarbonylrhodium(I) [Rh(CO)(2)acac]
in liquid and glassy dibutyl phthalate (DBP) (3.5 K to 250 K) are rep
orted and compared to previous measurements of a similar mode of tungs
ten hexacarbonyl [W(CO)(6)]. The Rh(CO)(2)acac pure dephasing shows a
T-1 dependence on temperature at very low temperature with a change to
an exponentially activated process (Delta E congruent to 400 cm(-1))
above similar to 20 K. There is no change in the functional form of th
e temperature dependence in passing from the glass to the liquid. It i
s proposed that the T-1 dependence arises from coupling of the vibrati
on to the glass's tunneling two level systems. The activated process a
rises from coupling of the high-frequency CO stretch to the 405 cm(-1)
Rh-C stretch. Excitation of the Rh-C stretch produces changes in the
back donation of electron density from the rhodium d(pi) orbital to th
e CO pi antibonding orbital, shifting the CO. stretching transition f
requency and causing dephasing. In contrast, W(CO)(6) displays a T-2 d
ependence below T-g in DBP and two ether solvents. Above T-g, there is
a distinct change in the functional form of the temperature dependenc
e. In 2-methylpentane, a Vogel-Tammann-Fulcher-type temperature depend
ence is observed above T-g. It is proposed that the triple degeneracy
of the T-1a mode of W(CO)(6) is broken in the glassy and liquid solven
ts. The closely spaced levels that result give rise to unique dephasin
g mechanisms not available in the nondegenerate Rh(CO)(2)acac system.
(C) 1998 American Institute of Physics.