Propagation and Beam Geometry Effects on Two-Dimensional Fourier Transform Spectra of Multilevel Systems
Propagation and Beam Geometry Effects on Two-Dimensional Fourier Transform Spectra of Multilevel Systems
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DOI：
10.1021/jp904504z
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发表时间：
2009-11-26
影响因子：
2.9
通讯作者：
Jonas, David M.
中科院分区：
文献类型：
--
作者：
Cho, Byungmoon;Yetzbacher, Michael K.;Jonas, David M.
在指数弛豫（光学布洛赫模型）极限下，用响应函数模拟了四能级二维（213）傅里叶变换弛豫谱。在这项研究中的参数选择耦合羰基伸缩振动模型。只要相干性持续存在，真实的2D光谱中的每个峰就具有部分混合的吸收/色散（“相位扭曲”）形状，因为非线性信号相对于前两个脉冲的互换是不对称的。二维弛豫谱中的这种不对称性是由单激发态之间的相干性和双激发态的红移引起的。单激发态之间的相干性导致二维光谱的振荡和在量子拍频处的相关光谱分辨泵浦-探测（SRPP）瞬态。将2D峰值的相位扭曲性质投影到检测频率轴上，SRPP峰值在不处于最大或最小振幅时也关于其最大值不对称。采用三维傅里叶变换（3DFT）方法模拟了具有交叉峰的多级二维谱的吸收/色散和束流几何畸变。可以通过考虑它们对有助于2D光谱中的峰值的各个相干路径的影响来理解失真。光束几何畸变解释了先前由Khalil等人（J.Chem.Phys.2004，121，362）实验观察到的一些不相等的交叉峰幅度。提出了一种减少光束几何畸变的二维光谱表示法。如果校正光束几何畸变的变换与校正吸收/色散传播畸变的变换相结合（J.Chem.Phys.2007，126，044511），则在所有相干性被破坏之后，恢复的2D光谱与理想的2D光谱匹配。在相干性的存在下，新的表示减少了失真的2D光谱的误差的一个因素为4实际的2D-IR实验条件。
Four-level two-dimensional (213) Fourier transform relaxation spectra are simulated with response functions for a chromophore pair in the exponential relaxation (optical Bloch model) limit. The parameters in this study are chosen to model coupled carbonyl stretching vibrations. As long as coherence persists, every peak in the real 2D spectra has a partially mixed absorptive/dispersive ("phase-twisted") shape because the nonlinear signals are not symmetric with respect to interchange of the first two pulses. This asymmetry in 2D relaxation spectra arises from coherence between singly excited states and a red shift of the doubly excited state. Coherence between the singly excited states causes oscillation of the 2D spectra and the associated spectrally resolved pump-probe (SRPP) transients at the quantum beat frequency. Projecting the phase-twisted nature of the 2D peaks onto the detection frequency axis, the SRPP peaks are also asymmetric about their maximum when not at maximum or minimum amplitude. Three-dimensional Fourier transform (3DFT) methods are used to simulate absorption/dispersion and beam geometry distortions of the multilevel 2D spectra with cross peaks. The distortions can be understood by consideration of their effects on individual coherence pathways that contribute to peaks in the 2D spectra. The beam geometry distortion explains some unequal cross peak amplitudes previously observed experimentally by Khalil et al. (J. Chem. Phys. 2004, 121, 362). A representation of 2D spectra that reduces beam geometry distortion is presented. If the transformation to correct for beam geometry distortion is combined with the transformations that correct absorptive/dispersive propagation distortions (J. Chem. Phys. 2007, 126, 044511), the recovered 2D spectrum matches the ideal 2D spectrum after all coherence is destroyed. In the presence of coherence, the new representation reduces the error in the distorted 2D spectrum by a factor of 4 for practical 2D-IR experimental conditions.