Femtosecond Spectroscopy of Condensed-phase Photochemistry

纳尔逊教授的研究小组得到了实验物理化学计划的支持，在飞秒时间尺度上进行了各种基本凝聚相光化学过程的时间分辨研究。沿着反应势能表面的相相干分子运动被用来启动、操纵和实时观察发生在凝聚相中的光化学过程的详细演变。目的是阐明光化学变化发生的反应势能面，并描述凝聚相对势能面和由此产生的动力学的影响。用激发机制来区分两类实验。在第一类中，飞秒脉冲的光吸收在激发态表面上启动相参分子运动（相干波包传播）。随后的可见光和红外脉冲的时间分辨吸收和时间分辨荧光被用来监测随后的动态和提取激发态表面的信息。第二类实验涉及使用脉冲受激拉曼散射（ISRS）来激发分子在基态表面上的振动运动。正在开发多脉冲技术，以显着增加ISRS技术可能的振幅。相干振动运动可以用来启动振动驱动的化学反应或增强或修改随后的光化学过程。

Professor Nelson's research group is supported by the Experimental Physical Chemistry Program in various studies of elementary condensed phase photochemical processes time-resolved on a femtosecond time scale. Phase-coherent molecular motion along reactive potential energy surfaces is being used to initiate, manipulate, and observe in real time the detailed evolution of photochemical processes occurring in condensed phases. The goals are to elucidate the reactive potential energy surfaces along which photochemical change occurs and to characterize the effects of the condensed phase upon both the potential surfaces and the resulting dynamics. Two classes of experiments are distinguished by the excitation mechanism. In the first class, optical absorption of femtosecond pulses initiates phase-coherent molecular motion (coherent wavepacket propagation) on excited potential surfaces. Time-resolved absorption of subsequent visible and infrared pulses and time-resolved fluorescence are used to monitor the subsequent dynamics and extract information about the excited-state surface. The second class of experiments involves the use of impulsive stimulated Raman scattering (ISRS) to excite molecular vibrational motion on a ground electronic state surface. Multi-pulsing techniques are being developed to increase significantly the amplitudes possible with the ISRS technique. The coherent vibrational motion might then be used either to initiate a vibrationally-driven chemical reaction or to enhance or modify a subsequent photochemical process.