Khurmi Champak, Berg Mark A
Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, USA.
J Chem Phys. 2008 Aug 14;129(6):064504. doi: 10.1063/1.2960589.
We have recently shown that homogeneous and heterogeneous kinetics can be distinguished by experiments that compare the evolution of the population of a state over two time intervals [E. van Veldhoven et al., ChemPhysChem 8, 1761 (2007)]. This paper elaborates on the analogy between these multiple population-period transient spectroscopy (MUPPETS) experiments and more familiar spectroscopies based on the evolution of coherences. Using a modified inverse-Laplace transform, a standard kinetics decay is re-expressed as a "rate spectrum." A nonexponential decay creates a linewidth in this spectrum. Mechanisms for line broadening in rate spectra are compared to those for line broadening in frequency-domain spectra. Homogeneous and heterogeneous kinetics are defined precisely and are shown to be the counterparts of homogeneous and inhomogeneous line broadenings in frequency-domain spectra. Homogeneous line broadening mechanisms are further divided into equilibrium and nonequilibrium mechanisms, with equilibrium mechanisms more prevalent in frequency spectra and nonequilibrium mechanisms more prevalent in rate spectra. Spectral representations of two-dimensional MUPPETS experiments are developed that are equivalent to two-dimensional coherence spectroscopies. In particular, spectra equivalent to hole-burning and to correlation spectra are defined. Frequency-domain spectra are often modeled as an inhomogeneous distribution of identical homogeneous line shapes. A parallel homogeneous-heterogeneous model for kinetics is defined. Within this model, MUPPETS has sufficient information to completely separate the homogeneous and heterogeneous contributions to a nonexponential decay, even when the homogeneous contribution is nonexponential.
我们最近表明，可以通过比较一个状态的粒子数在两个时间间隔内的演化的实验来区分均相动力学和非均相动力学[E. van Veldhoven等人，《化学物理化学》8，1761（2007）]。本文详细阐述了这些多粒子数 - 周期瞬态光谱（MUPPETS）实验与基于相干演化的更常见光谱之间的类比。使用改进的逆拉普拉斯变换，将标准动力学衰减重新表示为“速率谱”。非指数衰减会在该谱中产生线宽。将速率谱中线宽展宽的机制与频域谱中线宽展宽的机制进行了比较。精确地定义了均相和非均相动力学，并表明它们分别对应于频域谱中的均匀和非均匀线宽展宽。均匀线宽展宽机制进一步分为平衡机制和非平衡机制，平衡机制在频谱中更普遍，而非平衡机制在速率谱中更普遍。开发了二维MUPPETS实验的光谱表示，它们等同于二维相干光谱。特别地，定义了等同于烧孔光谱和相关光谱的光谱。频域谱通常被建模为相同均匀线形的非均匀分布。定义了一个用于动力学的平行均相 - 非均相模型。在这个模型中，MUPPETS有足够的信息来完全分离对非指数衰减的均相和非均相贡献，即使均相贡献是非指数的。
