Spectral diffusion in glasses under high pressure: A study by time-resolved hole-burning

We have studied optical dephasing and spectral diffusion of the S1←S0 0–0 transition of bacteriochlorophyll-a (BChl-a) in the glass 2-methyltetrahydrofuran (MTHF) at ambient (Δp=0) and high pressure (Δp=3.6 GPa) between 1.2 and 4.2 K by time-resolved hole-burning. The “effective” homogeneous linewidth Γhom′ follows a power law dependence on temperature, Γhom′=Γ0′+aT1.3±0.1, where Γ0′=Γ0+Γ0ET+Γ0ET→SD(td) is the residual linewidth and a=aPD+aSD(td)+aET→SD(td) is the coupling constant. The separate contributions to Γ0′ and a are the fluorescence decay rate Γ0=(2πτfl)−1, the “downhill” energy-transfer rate Γ0ET, the coupling constants due to “pure” dephasing aPD and “normal” spectral diffusion aSD(td), and two terms related to “extra” spectral diffusion induced by energy transfer, Γ0ET→SD(td) and aET→SD(td). We have quantitatively analyzed these contributions at ambient and high pressure. The results show that “normal” SD, “extra” SD, and ET→SD are strongly influenced by pressure. We have interpreted our findings in terms of a change in the number of two-level-systems, the low-frequency modes characteristic for the glassy state.