Mixed frequency-/time-domain coherent multidimensional spectroscopy: research tool or potential analytical method?
Mixed frequency-/time-domain coherent multidimensional spectroscopy: research tool or potential analytical method?
复制标题
混合频域/时域相干多维光谱：研究工具还是潜在的分析方法？
DOI：
10.1021/ar900032g
复制
发表时间：
2009
影响因子：
18.3
通讯作者：
J. C. Wright
中科院分区：
文献类型：
--
作者：
Andrei V. Pakoulev;Mark A. Rickard;K. Kornau;N. Mathew;Lena A. Yurs;S. Block;J. C. Wright
相干多维光谱学（CMDS）现在是核磁共振（NMR）的光学类似物。正如NMR异核多量子相干（HMQC）方法依赖于多量子相干一样，要实现广泛应用，CMDS也需要在很宽的量子态能量范围内激发多量子相干。本综述聚焦于频域CMDS，因为这些方法将激发频率调谐到与所需量子态共振，并且能够在能量差异很大的态之间形成多量子相干。CMDS方法使用多个激发脉冲在其退相时间内激发多个量子态，因此它们的量子力学相位得以保持。由激发态对形成的相干会发射相干光束。激发脉冲的时间顺序定义了一系列相干，这些相干可以根据多量子相干CMDS的需要产生零阶、一阶、二阶或更高阶的相干。定义时间顺序和激发频率以及对输出频率进行光谱分辨，也为相干定义了一种特定的时间路径，就像NMR脉冲序列定义了一种NMR方法一样。通过这个多维参数空间的二维等高线图可以使态能量和动力学可视化。本综述使用镍和铑螯合物作为理解混合频域/时域CMDS的模型。混合频域/时域方法使用与退相时间相当的激发脉冲宽度，因此通过扫描激发频率获得多维光谱，而通过扫描时间延迟获得相干和布居数动力学。改变时间延迟会根据脉冲序列激发零阶、一阶还是二阶量子相干而改变二维激发光谱中的峰。此外，由于量子拍频的频域表现，峰会分裂。同样，改变激发频率和单色仪频率会根据频率是否与不同时间顺序路径所涉及的共振匹配而改变对激发延迟时间的依赖。改变时间延迟和频率的等高线图可以使特定光谱特征的时间变化可视化。频域方法与特定态共振，因此相干和布居数的顺序是确定的。然而，相干转移会导致在意外频率处出现输出光束。当热库诱导两个态（a和g）之间的相干演变为一种新的相干（b和g）时，就会发生相干转移。由于这两种相干具有不同的频率，并且由于相干转移发生的时间顺序不同，延迟时间依赖性会产生取决于相干频率差的调制。通过提高激发强度也可以产生更高阶的相干。二维光谱中会出现新的特征，并且会发生动态斯塔克分裂。这些效应将构成高阶多量子相干方法的基础，并且也提供了一种探测分子势能面的方法。
Coherent multidimensional spectroscopy (CMDS) is now the optical analogue of nuclear magnetic resonance (NMR). Just as NMR heteronuclear multiple-quantum coherence (HMQC) methods rely on multiple quantum coherences, achieving widespread application requires that CMDS also excites multiple quantum coherences over a wide range of quantum state energies. This Account focuses on frequency-domain CMDS because these methods tune the excitation frequencies to resonance with the desired quantum states and can form multiple quantum coherences between states with very different energies. CMDS methods use multiple excitation pulses to excite multiple quantum states within their dephasing time, so their quantum mechanical phase is maintained. Coherences formed from pairs of the excited states emit coherent beams of light. The temporal ordering of the excitation pulses defines a sequence of coherences that can result in zero, single, double, or higher order coherences as required for multiple quantum coherence CMDS. Defining the temporal ordering and the excitation frequencies and spectrally resolving the output frequency also defines a particular temporal pathway for the coherences, just as an NMR pulse sequence defines an NMR method. Two dimensional contour plots through this multidimensional parameter space allow visualization of the state energies and dynamics. This Account uses nickel and rhodium chelates as models for understanding mixed frequency-/time-domain CMDS. Mixed frequency-/time-domain methods use excitation pulse widths that are comparable to the dephasing times, so multidimensional spectra are obtained by scanning the excitation frequencies, while the coherence and population dynamics are obtained by scanning the time delays. Changing the time delays changes the peaks in the 2D excitation spectra depending upon whether the pulse sequence excites zero, single, or double quantum coherences. In addition, peaks split as a result of the frequency-domain manifestation of quantum beating. Similarly, changing the excitation and monochromator frequencies changes the dependence on the excitation delay times depending upon whether the frequencies match the resonances involved in the different time-ordered pathways. Contour plots that change a time delay and frequency visualize the temporal changes of specific spectral features. Frequency-domain methods are resonant with specific states, so the sequence of coherences and populations is defined. Coherence transfer, however, can cause output beams at unexpected frequencies. Coherence transfer occurs when the thermal bath induces a coherence between two states (a and g) to evolve to a new coherence (b and g). Since the two coherences have different frequencies and since there are different time orderings for the occurrence of coherence transfer, the delay time dependence develops modulations that depend on the coherences' frequency difference. Higher order coherences can also be generated by raising the excitation intensities. New features appear in the 2D spectra and dynamic Stark splittings occur. These effects will form the basis for the higher order multiple quantum coherence methods and also provide a method for probing molecular potential energy surfaces.