The Photosynthesis Center at Arizona State University has developed a major research program investigating fast energy and electron transfer during solar energy conversion in both natural and artificial systems. Ultrafast optical laser spectroscopy is a key tool in this research effort. Six groups, representing about 40 faculty, postdocs and students, are actively using an existing femtosecond time resolution transient absorbance spectrometer to study bacterial, plant and synthetic antenna and reaction center complexes. This research includes designing and synthesizing molecular devices which mimic the fast rates and high yields of the natural systems but are much simpler chemically and may have applications as molecular switches and logic gates. Also, investigation of the structurally well defined reaction centers of the purple nonsulfur bacteria is leading towards an understanding of the photophysics of the initial solar energy conversion event and towards an understanding of the role played by the protein in modifying the thermodynamic and kinetic parameters of electron transfer. Research is also underway into the light harvesting and energy trapping reactions of the Photosystem I reaction center of higher plants as well as in a related photosynthetic reaction center from Heliobacillus mobilis. A great deal of progress has been made with the present instrument, but as the experiments performed have become more and more sophisticated, limitations have been reached in available time resolution, sensitivity, noise suppression, photon excitation densities, available excitation and probe wavelengths, capabilities to perform multiple excitation pulse experiments, and simply in the amount of time available for each group on the apparatus. The purchase of a new instrument is proposed with greatly expanded capabilities. Unlike the present instrument which uses an actively mode-locked Nd:YAG laser synchronously pumping a dye laser as the source of ultrafast pu lses, the new system will employ a self-mode-locked titanium sapphire laser pumped by a CW argon ion laser. The pulses from this source will be amplified in a solid-state regenerative amplifier to the millijoule level at kilohertz repetition rates and used to generate up to two excitation pulses at any wavelengths between 400 nm and 2.4 rnicrons with pulse durations of roughly 50 fs. In addition, part of the regeneratively amplified pulse will be used to generate a white light continuum, allowing simultaneous absorbance change measurements at many different wavelengths in a single experiment. By using optical gating in a nonlinear crystal, the instrument will also have the capability of performing femtosecond time resolution emission decay studies. Purchase of the proposed femtosecond system will open up a number of new research avenues for photosynthesis investigators at ASU. New excitation wavelengths will allow more specific preparation of molecular excited states Multiple excitation pulses at higher photon densities will allow the creation of specific intermediate states with one pulse whose photochemistry can then be investigated with a second excitation event. The higher time resolution of the system will both help in the investigation of processes previously detected but not resolved on the 100 fs timescale and also open up a window into the molecular vibrations of the system coupled to the absorbance transitions and photochemistry. Higher signal-to-noise absorbance change data will allow more thorough investigation of natural photosynthetic systems with large numbers of pigments whose absorbance masks the small absorbance changes associated with the primary photochemistry of solar energy conversion.
