Generation of tunable subpicosecond light pulses in the midinfrared between 4.5 and 11.5 Mum

Stable subpicosecond infrared pulses in the spectral region of 4.5-11.5 ,um are generated by difference-frequency mixing in AgGaS2. The system uses femtosecond pulses from a Ti:sapphire regenerative amplifier and from a tunable traveling-wave dye laser. The infrared pulses have a duration of 400 fs, an energy of more than 10 nJ, and a repetition rate of 1 kHz

The accessory bacteriochlorophyll

The primary electron transfer in reaction centers of Rhodobacter sphaeroides is studied by subpicosecond absorption spectroscopy with polarized light in the spectral range of 920-1040 nm. Here the bacteriochlorophyll anion radical has an absorption band while the other pigments of the reaction center have vanishing ground-state absorption. The transient absorption data exhibit a pronounced 0.9-ps kinetic component which shows a strong dichroism. Evaluation of the data yields an angle between the transition moments of the special pair and the species related with the 0.9-ps kinetic component of 26 +/- 8 degrees. This angle compares favorably with the value of 29 degrees expected for the reduced accessory bacteriochlorophyll. Extensive transient absorbance data are fully consistent with a stepwise electron transfer via the accessory bacteriochlorophyll