Inset: Hole burnt at Pt/A ~ 0.2 J/cm2. Bottom: b Homogeneous linewidth, selleck products Γhom, as a function of temperature T between 1.2 and 4 K in the red wing of the B850 band. Γ0 is the residual homogeneous linewidth for T → 0. Its value is consistent with a fluorescence lifetime of a few nanoseconds (J. Gallus and L. van den Aarssen, unpublished results from our laboratory) Figure 6b shows a plot of the homogeneous linewidth Γhom as a function of temperature (J. Gallus and L. van den Aarssen, unpublished results). We found small values of Γhom, between ~0.5 GHz and a few GHz at the red wing
of the B850 band, as compared to those in B800. The values in B850 are determined by ‘pure’ dephasing processes \( \left( T_2^* \right), \) i.e.
by fluctuations of the optical transition arising from coupling of the BChl a pigments to the surrounding protein. The values for B800, in contrast, are limited by T 1 processes, i.e. by energy transfer from B800 to B850 and from B800 to B800 (De Caro et al. 1994; Van der Laan et al. 1990, 1993). The temperature dependence of Γhom, in Fig. 6b, follows a T α power law, with α = 1.3 ± 0.1. Similar behaviour was found for chromophores in amorphous hosts (for reviews, see Jankowiak et al. 1993; Moerner 1988, and articles therein; Völker 1989a, 1989b), for BChl a in a triethylamine glass (Van der Laan et al. 1992) and for other photosynthetic systems, such as the B820 and B777 subunits of LH1 (Creemers and Völker selleck 2000; Creemers et al. 1999a; Störkel et al. 1998), and the PSII RC (Den Hartog et al. 1998c, 1999b; Groot et al. 1996) and CP47-RC (Den Hartog et al. 1998b) of green plants between 1.2 and 4.2 K. The dephasing times in photosynthetic systems, however, are about one to two orders of magnitude larger than in glassy systems, indicating that there is rather strong coupling between the pigments and protein. Here, optical dephasing is assumed to arise from coupling of the energy levels of the chromophore or pigment to a
distribution of TLSs of the glassy host or protein (Jankowiak and Small 1993; Putikka and Huber 1987; Völker 1989a, b). In contrast to the systems mentioned above, a crystalline-like T2±0.2 hole-width dependence was reported for the Amino acid CP43 and CP47 ‘trap’ pigments in O2-evolving PSII core complexes between 2.5 and 18 K (Hughes et al. 2005). The extrapolation value Γ0 = (2πτ fl)−1 for T → 0 in Fig. 6b is consistent with a fluorescence lifetime τ fl of BChl a of a few ns (Sundström et al. 1999). Thus, our dephasing results disprove the existence of residual exciton scattering at T → 0, which was assumed to contribute to the much broader holes reported by Wu et al. (1997c) for the red wing of the B850 band of LH2 of Rps. acidophila. Although a T 1.3 dependence of Γhom was also reported for HB experiments performed between 4.2 and 20 K (Wu et al. 1997b), the value of Γhom at 4.