ELECTRONIC DEPHASING OF APT IN GLASSY FILMS OF WATER FROM 5 TO 100 K - IMPLICATIONS FOR H-BONDING LIQUIDS

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.