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 (Dp 50) and high pressure (Dp53.6 GPa) between 1.2 and 4.2 K by time-resolved hole-burning. The ‘‘effective’’ homogeneous linewidth Ghom 8 follows a power law dependence on temperature, Ghom 8 5G081aT , where G085G01G0 1G0 (td) is the residual linewidth and a5aPD 1aSD(td)1aET→SD(td) is the coupling constant. The separate contributions to G08 and a are the fluorescence decay rate G05(2ptfl) , the ‘‘downhill’’ energy-transfer rate G0 ET , 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, G0 (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. © 1999 American Institute of Physics. @S0021-9606~99!70415-0#