Gupta Ruchir, Verma Sachin Dev
Spectroscopy and Dynamics Visualization Laboratory, Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Bhauri, Bhopal 462066, Madhya Pradesh, India.
ACS Meas Sci Au. 2024 Feb 1;4(2):153-162. doi: 10.1021/acsmeasuresciau.3c00048. eCollection 2024 Apr 17.
Relaxation rate dispersion, i.e., nonexponential or multicomponent kinetics, is observed in complex systems when measuring relaxation kinetics. Often, the origin of rate dispersion is associated with the heterogeneity in the system. However, both homogeneous (where all molecules experience the same rate but inherently nonexponential) and heterogeneous (where all molecules experience different rates) systems can exhibit rate dispersion. A multidimensional correlation analysis method has been demonstrated to detect and quantify rate dispersion observed in molecular rotation, diffusion, solvation, and reaction kinetics. One-dimensional (1D) autocorrelation function detects rate dispersion and measures its extent. Two-dimensional (2D) autocorrelation function measures the origin of rate dispersion and distinguishes homogeneous from heterogeneous. In a heterogeneous system, implicitly there exist subensembles of molecules experiencing different rates. A three-dimensional (3D) autocorrelation function measures subensemble exchange if present and reveals if the system possesses static or dynamic heterogeneity. This perspective discusses the principles, applications, and potential and also presents a future outlook of two-dimensional fluctuation correlation spectroscopy (2D-FlucCS). The method is applicable to any experiment or simulation where a time series of fluctuation in an observable (emission, scattering, current, etc.) around a mean value can be obtained in steady state (equilibrium or nonequilibrium), provided the system is ergodic.
在测量复杂系统的弛豫动力学时，会观察到弛豫速率色散，即非指数或多组分动力学。通常，速率色散的起源与系统中的异质性有关。然而，均匀系统（其中所有分子经历相同的速率，但本质上是非指数的）和非均匀系统（其中所有分子经历不同的速率）都可能表现出速率色散。一种多维相关分析方法已被证明可用于检测和量化在分子旋转、扩散、溶剂化和反应动力学中观察到的速率色散。一维（1D）自相关函数可检测速率色散并测量其程度。二维（2D）自相关函数可测量速率色散的起源，并区分均匀和非均匀情况。在非均匀系统中，隐含地存在经历不同速率的分子子集合。三维（3D）自相关函数可测量子集合交换（如果存在），并揭示系统是否具有静态或动态异质性。本文讨论了二维涨落相关光谱学（2D-FlucCS）的原理、应用、潜力，并展望了其未来发展。该方法适用于任何实验或模拟，只要在稳态（平衡或非平衡）下能够获得围绕平均值的可观测量（发射、散射、电流等）的涨落时间序列，前提是系统是遍历的。
