Spectral Diffusion in Organic Glasses: Time Dependence of Spectral Holes

Time-resolved spectral hole-burning experiments have been performed to probe the dynamics of the S₁ ← S₀ 0?0 transition of bacteriochlorophyll-a at low concentration (1 × 10^-5M) in four different glasses (2-methyltetrahydrofuran, protonated and deuterated ethanol, diethyl ether, and triethylamine) as a function of delay time t_d (from 10^-6 to 10³ s) and temperature T (1.2?4.2 K). It is shown that spectral diffusion, the broadening of the optical linewidth followed here over nine orders of magnitude in time, increases with temperature as T^1.3±0.1 and strongly depends on the glass structure. The functional dependence, however, is not influenced by the specific glass. The variation of the “effective” homogeneous linewidth (Γ‘_hom) with T and t_d is described by a function Γ‘_hom(T,t_d) derived by modifying the standard model of two-level systems (TLS). This revised TLS model, in which the distribution functions of the TLS tunneling parameters are different from those in the standard model, takes into account the common origin of the dependence of Γ‘_hom on t_d and T. It is shown that other hole-burning and photon-echo data reported in the literature can also be fitted by the same function Γ‘_hom(T,t_d). In ethanol glass, the number of TLSs and the amount of spectral diffusion appear to be independent of the probe molecule.