FEMTOSECOND PUMP-PROBE SPECTROSCOPY OF BACTERIOCHLOROPHYLL A MONOMERS IN SOLUTION

One- and two-color absorption difference profiles were obtained for BChl a in I-propanol with similar to 50-fs resolution, using a self-mode-locked Ti:sapphire laser system. Time evolution in the BChl a absorption difference spectrum produces nonexponential photobleaching/stimulated emission (PB/SE) decay kinetics in 800-nm one-color experiments. Nonexponential PB/SE rise behavior occurs for some combinations of pump and probe wavelengths in two-color experiments. Optimized parameters from triexponential fits to the absorption difference profiles depend markedly on the fitting time window; they typically include a minor component with lifetime in the hundreds of fs. Much of the latter component is due to vibrational relaxation and/or intramolecular vibrational redistribution, rather than solvent dielectric relaxation. Measurements of the pump-probe anisotropy indicate that the electronic transition moment for the broad Q(y) excited state absorption band that overlaps the Q(y) steady-state absorption spectrum makes an angle of at most 20 degrees from that of the ground --> Q(y) state transition. No coherent oscillations are observed at early times. Our results bear directly on the interpretation of fs pump-probe experiments on BChl a - containing pigment-protein complexes.