Farrow Darcie A, Smith Eric R, Qian Wei, Jonas David M
Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, USA.
J Chem Phys. 2008 Nov 7;129(17):174509. doi: 10.1063/1.2982160.
By analogy to the Raman depolarization ratio, vibrational quantum beats in pump-probe experiments depend on the relative pump and probe laser beam polarizations in a way that reflects vibrational symmetry. The polarization signatures differ from those in spontaneous Raman scattering because the order of field-matter interactions is different. Since pump-probe experiments are sensitive to vibrations on excited electronic states, the polarization anisotropy of vibrational quantum beats can also reflect electronic relaxation processes. Diagrammatic treatments, which expand use of the symmetry of the two-photon tensor to treat signal pathways with vibrational and vibronic coherences, are applied to find the polarization anisotropy of vibrational and vibronic quantum beats in pump-probe experiments for different stages of electronic relaxation in square symmetric molecules. Asymmetric vibrational quantum beats can be distinguished from asymmetric vibronic quantum beats by a pi phase jump near the center of the electronic spectrum and their disappearance in the impulsive limit. Beyond identification of vibrational symmetry, the vibrational quantum beat anisotropy can be used to determine if components of a doubly degenerate electronic state are unrelaxed, dephased, population exchanged, or completely equilibrated.
类似于拉曼退偏比，泵浦 - 探测实验中的振动量子拍频取决于泵浦光和探测光的相对激光束偏振，其方式反映了振动对称性。偏振特征与自发拉曼散射中的不同，因为场 - 物质相互作用的顺序不同。由于泵浦 - 探测实验对激发电子态上的振动敏感，振动量子拍频的偏振各向异性也可以反映电子弛豫过程。利用双光子张量的对称性来处理具有振动和振动 - 电子相干性的信号路径的图解方法，被用于确定方形对称分子中电子弛豫不同阶段的泵浦 - 探测实验中振动和振动 - 电子量子拍频的偏振各向异性。通过电子光谱中心附近的π相位跃变及其在脉冲极限下的消失，可以区分不对称振动量子拍频和不对称振动 - 电子量子拍频。除了识别振动对称性之外，振动量子拍频各向异性还可用于确定双重简并电子态的组分是否未弛豫、去相位、发生了布居交换或完全平衡。
