FEMTOSECOND PUMP-PROBE SPECTROSCOPY OF BACTERIOCHLOROPHYLL A MONOMERSIN SOLUTION

One- and two-color absorption difference profiles were obtained for BC hl 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 absorpt ion difference spectrum produces nonexponential photobleaching/stimula ted emission (PB/SE) decay kinetics in 800-nm one-color experiments. N onexponential 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 co mponent with lifetime in the hundreds of fs. Much of the latter compon ent is due to vibrational relaxation and/or intramolecular vibrational redistribution, rather than solvent dielectric relaxation. Measuremen ts of the pump-probe anisotropy indicate that the electronic transitio n moment for the broad Q(y) excited state absorption band that overlap s 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 coher ent oscillations are observed at early times. Our results bear directl y on the interpretation of fs pump-probe experiments on BChl a - conta ining pigment-protein complexes.