Femtosecond Spectroscopy of Condensed-phase Photochemistry

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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.