The transient absorption anisotropy spectrum of bacteriochlorophyll a (BChl a) in pyridine was measured in the wavelength interval 550–850 nm, 1 ps after optical excitation with a 792-nm femtosecond light pulse. In the wavelength region of Q y absorption and stimulated emission (775–825 nm), the anisotropy was found to be close to the theoretically expected value (0.4) for a two-level system. In the wavelength region 650–750 nm, where the transient absorption signal is dominated by excited state absorption, the anisotropy is reduced to ∼0.18. Anisotropy kinetics were measured at several wavelengths and found to be constant within the time window 0–5 ps, showing that no internal dynamics of the BChl a molecule change the anisotropy on the time scale of tens of picoseconds.
