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

We have studied optical dephasing and spectral diffusion of the S-1 <-- S-0 0- 0 transition of bacteriochlorophyll- a (BChl-a) in the glass 2-methylte trahydrofuran (MTHF) at ambient (Delta p = 0) and high pressure (Delta p = 3.6 GPa) between 1.2 and 4.2 K by time-resolved hole-burning. The "effectiv e'' homogeneous linewidth Gamma'(hom) follows a power law dependence on tem perature, Gamma'(hom) = Gamma'(0) + aT(1.3+/-0.1), where Gamma'(0) = Gamma( 0) + Gamma(0)(ET) + Gamma(0)(ET-->SD) (t(d)) is the residual linewidth and a = a(PD) + a(SD) (t(d)) + a(ET-->SD) (t(d)) is the coupling constant. The separate contributions to Gamma'(0) and a are the fluorescence decay rate G amma(0) = (2 pi tau(fl))(-1), the "downhill'' energy-transfer rate Gamma(0) (ET), the coupling constants due to "pure'' dephasing a(PD) and "normal'' s pectral diffusion a(SD) (t(d)), and two terms related to "extra'' spectral diffusion induced by energy transfer, Gamma(0)(ET-->SD) (t(d)) and a(ET-->S D) (t(d)). We have quantitatively analyzed these contributions at ambient a nd high pressure. The results show that "normal'' SD, "extra'' SD, and ET-- >SD are strongly influenced by pressure. We have interpreted our findings i n terms of a change in the number of two-level-systems, the low-frequency m odes characteristic for the glassy state. (C) 1999 American Institute of Ph ysics. [S0021-9606(99)70415-0].