Fundamental Studies of Relationships between Experimental Nonlinear Coherent Vibrational Spectroscopies
Fundamental Studies of Relationships between Experimental Nonlinear Coherent Vibrational Spectroscopies
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实验非线性相干振动光谱之间关系的基础研究
DOI：
10.1021/acs.jpclett.9b01280
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发表时间：
2019
期刊：
影响因子：
--
通讯作者：
Wright, John C.
中科院分区：
文献类型：
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作者：
Wright, John C.
这一贡献检查创建的振动相干性和输出信号，并表明，二维红外（2D-IR）激发的效率是有限的，当激发脉冲远短于相干退相时间。它不处理非线性电子光谱，其中失相时间比典型的激发脉冲宽度快得多，因此电子相干振幅已经达到稳态。使用一个简单的封闭形式的解决方案的刘维尔方程，这个观点推导出的相干性和输出信号的振幅使用不同的非线性振动方法的现实条件。1结果允许直接比较2D-IR光谱、2− 4受激光子回波（SPE）、5泵浦-探测、6瞬态吸收（TA）、7瞬态光栅（TG）、三重振动增强光谱（TRIVE）、8双重振动增强（DOVE）光谱、9− 11三重共振和频（TRSF）光谱、12− 14相干反斯托克斯拉曼光谱（汽车）、15受激拉曼光谱（SRS），16和飞秒SRS（FSRS），17以及控制这些方法的能力和局限性的实验折衷。特别是，这个观点侧重于理解控制这些方法如何用于测量振动光谱中重要的各种模式的因素。该观点（1）确定了为什么2D-IR限于最强的振动跃迁而FSRS覆盖整个振动区域，（2）估计完全相干和部分相干2D-IR的相对信号水平，（3）将非线性红外光谱与涉及拉曼跃迁的光谱进行比较，（4）确定占空比和非共振背景在确定检测限中的重要性，（5）比较了外差和零差检测，（6）讨论了非线性方法学对单分子振动光谱学的扩展。确定控制这些测量的因素可以为设计实验和开发优化振动光谱应用的实验系统提供指导。理论非线性振动光谱是基于创建相干态，形成薛定谔猫叠加态，其中光子场和分子量子态同时存在于多个状态。虽然CMDS通常不使用猫态描述，但猫态描述在认识CMDS的基本属性方面特别重要。CMDS相干态的猫态特性表明，分子的量子态与激发光场和输出光场是纠缠的。对分子态或光子场的测量使猫态坍缩，并定义其余分子和光子场的状态。测量可以通过识别猫态的坍缩、激发场强度的增加或减少或由输出相干性产生的新光子场的存在而产生的状态来进行。不同测量策略的示例分别是2D-IR和受激光子回波、逆拉曼和受激拉曼或汽车和TRSF。它们都是同一个四波混频过程的不同方面。此外，CMDS猫态的性质清楚地说明了为什么相干性的测量是状态的指纹和原始叠加态的状态之间的直接耦合。它还清楚地说明了为什么光谱指纹不受种群松弛效应的影响，因为任何种群松弛都会破坏猫态。
This contribution examines the creation of vibrational coherences and the output signal and shows that the efficiency of two-dimensional infrared (2D-IR) excitations is limited when the excitation pulses are much shorter than the coherence dephasing times. It does not treat nonlinear electronic spectroscopies where the dephasing times are much faster than typical excitation pulse widths so the electronic coherence amplitudes already reach the steady state. Using a simple closed form solution of the Liouville equation, this Viewpoint derives the coherence and output signal amplitudes using realistic conditions for different nonlinear vibrational methods. 1 The results allow direct comparisons between 2D-IR spectroscopy, 2− 4 stimulated photon echo (SPE), 5 pump− probe, 6 transient absorption (TA), 7 transient grating (TG), triply vibrationally enhanced spectroscopy (TRIVE), 8 doubly vibrationally enhanced (DOVE) spectroscopy, 9− 11 triply resonant sum frequency (TRSF) spectroscopy, 12− 14 coherent anti-Stokes Raman spectroscopy (CARS), 15 stimulated Raman spectroscopy (SRS), 16 and femtosecond SRS (FSRS), 17 and the experimental compromises that control the capabilities and limitations of these methods. In particular, this Viewpoint focuses on understanding the factors that control how these methodologies can be used for measuring the wide range of modes that are important in vibrational spectroscopy. This Viewpoint (1) identifies why 2D-IR is constrained to the strongest vibrational transitions while FSRS covers the entire vibrational region,(2) estimates the relative signal levels for fully coherent and partially coherent 2D-IR,(3) compares nonlinear infrared spectroscopies with those involving Raman transitions,(4) identifies the importance of the duty cycle and nonresonant background in determining detection limits,(5) compares heterodyne and homodyne detection, and (6) discusses the extension of nonlinear methodologies to single-molecule vibrational spectroscopy. Identifying the factors that control these measurements can provide guidelines for designing experiments and developing experimental systems that optimize vibrational spectroscopy applications. Theory. Nonlinear vibrational spectroscopies are based on creating coherences that form a Schrödinger cat superposition state where the photon fields and molecular quantum states exist in multiple states simultaneously. Although CMDS is not usually described using cat states, the cat state description is particularly important in recognizing the fundamental properties of CMDS. The cat state character of CMDS coherences makes it clear that the quantum states of a molecule and the excitation and output photon fields are entangled. A measurement of a molecular state or a photon field collapses the cat state and defines the states of the remaining molecular and photon fields. Measurements can be made by identifying the state resulting from the collapse of the cat state, the increase or decrease in the excitation field intensities, or the presence of a new photon field created by the output coherence. Examples of the different measurement strategies are 2D-IR and stimulated photon echo, inverse Raman and stimulated Raman, or CARS and TRSF, respectively. They are all different aspects of the same four-wave mixing process. Moreover, the nature of the CMDS cat state makes it clear why the measurement of a coherence is a fingerprint of the states and the direct coupling between the states of the original superposition state. It also makes it clear why the spectral fingerprint is immune to population relaxation effects because any population relaxation destroys the cat state.