T. Reinot et al., ELECTRONIC DEPHASING OF APT IN GLASSY FILMS OF WATER FROM 5 TO 100 K - IMPLICATIONS FOR H-BONDING LIQUIDS, The Journal of chemical physics, 104(3), 1996, pp. 793-804
Nonphotochemical hole burning was used to characterize thelinear elect
ron-phonon coupling and measure the temperature dependence of the pure
electronic dephasing of Al-phthalocyanine tetrasulphonate (APT) in un
annealed and annealed hyperquenched glassy films of water (HGW). Below
about 10 K, the dephasing is dominated by coupling to the intrinsic t
wo-level systems (TLS(int)) of HGW. This dephasing is a factor of 5 X
faster for unannealed HGW due to its higher TLS(int) number density. F
or annealed HGW the pure electronic dephasing time (i.e., that dephasi
ng associated with the zero-phonon line), T-2, is 6.4 ns at 5 K, the
slowest dephasing time yet reported for a molecular glassy system at t
his temperature. At higher temperatures, dephasing due to exchange cou
pling with pseudolocalized modes at 50 and 180 cm(-1), which correlate
well with the transverse and longitudinal acoustic modes of water, be
comes dominant. The exchange coupling mechanism is based on diagonal q
uadratic electron-phonon coupling. At 100 K, for example, the pure ele
ctronic dephasing times (T-2) are close to 1 ps in value for both typ
es of film. Whereas the transverse acoustic mode is Franck-Condon acti
ve (S similar to 0.5), the 180 cm(-1) mode is silent (S<0.02). The det
ermination of the electron-phonon coupling parameters and static inhom
ogeneous broadening (400 cm(-1)) of APT's origin band allowed for simu
lation of the burn temperature dependence of the overall hole profile
(zero-phonon hole plus phonon-sideband holes). Comparison with experim
ental profiles shows that the hole profile theory of Hayes et al. [J.
Phys. Chem 98, 7337 (1994)] captures the main features of the temperat
ure dependence. The S value of the transverse acoustic mode is used to
quantitatively explain the burn temperature dependence of the saturat
ed intensity of the zero-phonon hole and its demise at temperatures ju
st above 100 K (due to Franck-Condon forbiddeness). In view of the ess
entially complete understanding of the electron-phonon coupling and pu
re electronic dephasing of APT in HGW attained in this work, the data
are used for extrapolation to ice (0 degrees C) and water (at temperat
ures not far above 0 degrees C) in order to connect with recent photon
echo studies of optical coherence loss of dye molecules in liquids. T
he extrapolation predicts an ''average'' T-1-dephasing time of similar
to 0.1 ps due to multiphonon (Brownian oscillator) transitions associ
ated with the transverse acoustic mode and subpicosecond pure electron
ic dephasing due to exchange coupling with the longitudinal mode. It i
s suggested that the marriage of hole burning and photon echo techniqu
es in studies bf glass forming Liquids should be a powerful approach t
o understanding optical coherence loss in liquids. (C) 1996 American I
nstitute of Physics.