Electronic dephasing of APT in glassy films of water from 5 to 100 K: Implications for H-bonding liquids

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Abstract
Nonphotochemical hole burning was used to characterize the linear electron–phonon coupling and measure the temperature dependence of the pure electronic dephasing of Al-phthalocyanine tetrasulphonate (APT) in unannealed and annealed hyperquenched glassy films of water (HGW). Below about 10 K, the dephasing is dominated by coupling to the intrinsic two-level systems (TLSint) of HGW. This dephasing is a factor of 5× faster for unannealed HGW due to its higher TLSint number density. For annealed HGW, the pure electronic dephasing time (i.e., that dephasing associated with the zero-phonon line), T2*, is 6.4 ns at 5 K, the slowest dephasing time yet reported for a molecular glassy system at this temperature. At higher temperatures, dephasing due to exchange coupling with pseudolocalized modes at 50 and 180 cm−1, which correlate well with the transverse and longitudinal acoustic modes of water, becomes dominant. The exchange coupling mechanism is based on diagonal quadratic electron–phonon coupling. At 100 K, for example, the pure electronic dephasing times (T2*) are close to 1 ps in value for both types of film. Whereas the transverse acoustic mode is Franck–Condon active (S∼0.5), the 180 cm−1 mode is silent (S<0.02). The determination of the electron–phonon coupling parameters and static inhomogeneous broadening (400 cm−1) of APT’s origin band allowed for simulation of the burn temperature dependence of the overall hole profile (zero-phonon hole plus phonon-sideband holes). Comparison with experimental profiles shows that the hole profile theory of Hayes et al. [J. Phys. Chem. 98, 7337 (1994)] captures the main features of the temperature dependence. The S value of the transverse acoustic mode is used to quantitatively explain the burn temperature dependence of the saturated intensity of the zero-phonon hole and its demise at temperatures just above 100 K (due to Franck–Condon forbiddeness). In view of the essentially complete understanding of the electron–phonon coupling and pure electronic dephasing of APT in HGW attained in this work, the data are used for extrapolation to ice (0° C) and water (at temperatures not far above 0° C) in order to connect with recent photon echo studies of optical coherence loss of dye molecules in liquids. The extrapolation predicts an ‘‘average’’ T1-dephasing time of ∼0.1 ps due to multiphonon (Brownian oscillator) transitions associated with the transverse acoustic mode and subpicosecond pure electronic dephasing due to exchange coupling with the longitudinal mode. It is suggested that the marriage of hole burning and photon echo techniques in studies of glass forming liquids should be a powerful approach to understanding optical coherence loss in liquids.
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