ON THE NATURE OF ENERGY-TRANSFER AT LOW-TEMPERATURES IN THE BCHL A PIGMENT-PROTEIN COMPLEX OF GREEN SULFUR BACTERIA

The technique of accumulated photon echoes was used to study optical d ephasing of the lowest Q(Y) transitions of the bacteriochlorophyll a c omplex of the green sulfur bacteria Prosthecochloris aestuarii. Variat ions in the photon echo decay as a function of wavelength are interpre ted in terms of (phonon-assisted) downward relaxation within the Q(Y) manifold as a mechanism of energy transfer at the lowest temperatures. A temperature dependence study revealed contributions to the total de phasing both from transitions within so-called two level systems (TLS) of the host as well as activation of transitions to higher electronic levels within the Q(Y) manifold at temperatures above 5 K. Conclusion s are drawn about the nature of energy transfer at low temperatures.