An ultrafast time-resolved anisotropy study of bacteriochlorophyll a in pyridine

The transient absorption anisotropy spectrum of bacteriochlorophyll a (BChl a) in pyridine was measured in the wavelength interval 550-850 nm, 1 ps af ter optical excitation with a 792-nm femtosecond light pulse. In the wavele ngth region of Q(y) absorption and stimulated emission (775-825 nm), the an isotropy,vas found to be close to the theoretically expected value (0.4) fo r a two-level system, In the wavelength region 650-750 nm, where the transi ent absorption signal is dominated by elicited state absorption, the anisot ropy is reduced to similar to 0.18. Anisotropy kinetics were measured at se veral wavelengths and found to be constant within the time window 0-5 ps, s howing that no internal dynamics of the BChl a molecule change the anisotro py on the time scale of tens of picoseconds. (C) 2000 Federation of Europea n Biochemical Societies.