Direct comparison of amplitude and geometric measures of spectral inhomogeneity using phase-cycled 2D-IR spectroscopy
Direct comparison of amplitude and geometric measures of spectral inhomogeneity using phase-cycled 2D-IR spectroscopy
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DOI：
10.1063/5.0043961
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发表时间：
2021-05-07
影响因子：
4.4
通讯作者：
Kubarych, Kevin J.
中科院分区：
文献类型：
--
作者：
Duan, Rong;Mastron, Joseph N.;Kubarych, Kevin J.
通过从测量光谱中提取频率起伏相关函数(FFCF)，二维红外光谱提供了获得平衡动力学的途径。从实验光谱中获得FFCF的几种不同方法，如中心线斜率(CLS)、椭圆度、相位斜率和节线斜率，都取决于2D线形的几何性质，并且为了实现FFCF的测量，必然需要光谱范围。另一方面，幅度测量，如非均质指数，只依赖于信号幅度，原则上可以仅使用2D频谱中的一个点来计算。利用基于脉冲整形器的2D-IR光谱仪，结合相位循环，我们分离了用于确定非均匀指数的复相和非复相信号。同样的测量数据提供了CLS所需的吸收光谱。这两种方法都适用于两个模型分子体系：六甲基钨(WCO6)和甲基环戊二烯基锰三甲酰[Cp‘Mn(CO)(3)，MCMT]。W(CO)(6)的三个简并红外模式没有相干调制或明显的分子内振动重分布(IVR)，并被用来建立基线比较。MCMT三脚架复合体的两个波段包括带内相干和IVR，以及可能在几皮秒时间尺度上的内部扭转运动。我们发现光谱扩散本质上是相同的，但更快的非平衡动力学导致两种方法提取的FFCFs不同。在光谱复杂且能量转移可以模拟由于频率波动引起的线形变化的情况下，不均匀指数提供了一个优势。
Two-dimensional infrared (2D-IR) spectroscopy provides access to equilibrium dynamics with the extraction of the frequency-fluctuation correlation function (FFCF) from the measured spectra. Several different methods of obtaining the FFCF from experimental spectra, such as the center line slope (CLS), ellipticity, phase slope, and nodal line slope, all depend on the geometrical nature of the 2D line shape and necessarily require spectral extent in order to achieve a measure of the FFCF. Amplitude measures, on the other hand, such as the inhomogeneity index, rely only on signal amplitudes and can, in principle, be computed using just a single point in a 2D spectrum. With a pulse shaper-based 2D-IR spectrometer, in conjunction with phase cycling, we separate the rephasing and nonrephasing signals used to determine the inhomogeneity index. The same measured data provide the absorptive spectrum, needed for the CLS. Both methods are applied to two model molecular systems: tungsten hexacarbonyl (WCO6) and methylcyclopentadienyl manganese tricarbonyl [Cp ' Mn(CO)(3), MCMT]. The three degenerate IR modes of W(CO)(6) lack coherent modulation or noticeable intramolecular vibrational redistribution (IVR) and are used to establish a baseline comparison. The two bands of the MCMT tripod complex include intraband coherences and IVR as well as likely internal torsional motion on a few-picosecond time scale. We find essentially identical spectral diffusion, but faster, non-equilibrium dynamics lead to differences in the FFCFs extracted with the two methods. The inhomogeneity index offers an advantage in cases where spectra are complex and energy transfer can mimic line shape changes due to frequency fluctuations.