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

亚利桑那州州立大学光合作用中心开发了一个主要的研究计划，调查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。超快光学激光光谱学是这项研究工作的关键工具。六个小组，代表约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研究细菌，植物和合成天线和反应中心复合物。该研究包括设计和合成分子器件，这些器件模仿自然系统的快速和高产率，但化学上简单得多，并可能作为分子开关和逻辑门应用。此外，调查结构明确的反应中心的紫色非硫细菌是导致对初始太阳能转换事件的物理学的理解，并对蛋白质在修改电子转移的热力学和动力学参数中所起的作用的理解。高等植物光系统I反应中心的光捕获和能量捕获反应以及运动太阳杆菌的相关光合反应中心的研究也在进行中。 本仪器已经取得了很大的进步，但是随着所进行的实验变得越来越复杂，在可用的时间分辨率、灵敏度、噪声抑制、光子激发密度、可用的激发和探测波长、进行多激发脉冲实验的能力、并且简单地在于设备上每个组可用的时间量。拟议购买一台新仪器，其能力将大大扩大。与目前使用主动锁模Nd：YAG激光器同步泵浦染料激光器作为超快脉冲源的仪器不同，新系统将使用由CW氩离子激光器泵浦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重复率被放大到毫焦耳水平，并用于在400 nm和2.4 nm之间的任何波长处产生多达两个激发脉冲，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一个白色光连续，允许在一个单一的实验中在许多不同的波长同时吸光度变化测量。通过在非线性晶体中使用光学选通，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ASU的光合作用研究人员开辟许多新的研究途径。新的激发波长将允许更具体的分子激发态的制备在较高的光子密度的多个激发脉冲将允许创建特定的中间态与一个脉冲的光化学，然后可以研究与第二个激发事件。该系统的更高的时间分辨率将有助于研究先前检测到但在100 fs时间尺度上未解决的过程，并且还打开了一个窗口，以了解系统的分子振动与吸光度跃迁和光化学反应。更高的信噪比吸光度变化数据将允许更彻底的调查与大量的色素，其吸光度掩盖了小的吸光度变化与太阳能转换的主要光化学的天然光合系统。

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.