A Femtosecond Resolution Transient Optical Spectrometer for the Study of Energy and Electron Transfer in Natural and Articial Photosynthetic Systems

亚利桑那州立大学光合作用中心开展了一项重大研究项目，研究在自然和人工系统中太阳能转换过程中的快速能量和电子转移。超快激光光谱学是这项研究的关键工具。六个小组，代表大约40名教师，博士后和学生，正在积极使用现有的飞秒时间分辨率瞬态吸收光谱仪来研究细菌，植物和合成天线和反应中心复合物。这项研究包括设计和合成分子装置，这些装置模仿自然系统的快速速率和高产率，但化学上要简单得多，可能应用于分子开关和逻辑门。此外，对紫色非硫细菌结构明确的反应中心的研究有助于理解初始太阳能转换事件的光物理学，并有助于理解蛋白质在改变电子转移的热力学和动力学参数方面所起的作用。高等植物光系统I反应中心的光收集和能量捕获反应，以及与之相关的Heliobacillus mobilis光合反应中心的研究也在进行中。目前的仪器已经取得了很大的进展，但是随着实验的进行变得越来越复杂，在可用的时间分辨率、灵敏度、噪声抑制、光子激发密度、可用的激发和探针波长、执行多个激发脉冲实验的能力以及设备上每组可用的时间数量方面已经达到了限制。建议购买一种功能大大扩展的新仪器。与目前使用主动锁模Nd:YAG激光器同步抽运染料激光器作为超快脉冲源的仪器不同，新系统将采用连续波氩离子激光器抽运的自锁模钛蓝宝石激光器。来自该源的脉冲将在固态再生放大器中以千赫兹的重复率放大到毫焦耳水平，并用于产生多达两个激发脉冲，波长在400 nm至2.4 rnicrons之间，脉冲持续时间约为50 fs。此外，再生放大脉冲的一部分将用于产生白光连续体，允许在单个实验中同时测量许多不同波长的吸光度变化。通过在非线性晶体中使用光学门控，该仪器还将具有进行飞秒时间分辨率发射衰减研究的能力。购买拟议的飞秒系统将为亚利桑那州立大学的光合作用研究者开辟许多新的研究途径。更高光子密度下的多个激发脉冲将允许用一个脉冲产生特定的中间状态，然后可以用第二个激发事件来研究其光化学。该系统的高时间分辨率将有助于研究先前检测到的过程，但在100fs时间尺度上无法解决，同时也为系统的分子振动与吸光度跃迁和光化学耦合打开了一扇窗。更高的信噪比吸收变化数据将允许对具有大量色素的自然光合系统进行更彻底的研究，这些色素的吸收掩盖了与太阳能转换的初级光化学相关的小吸收变化。

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.