Theoretical Studies of Relaxation, Energy Transfer and Non-linear Spectroscopy in Molecular Systems

理论与计算化学项目支持麻省理工学院的西尔贝教授。在接下来的三年里，将为几个不同领域的研究开发理论模型。第一个涉及凝聚态分子的低温光学性质，特别是玻璃固体。光谱空穴燃烧、光子回波和单分子光谱学都表明，由于凝聚态的缓慢波动，分子的所谓均匀线形与时间有关。这些方法具有不同但经常重叠的时间尺度，因此可以找到从皮秒到周的时间依赖性的完整图像。基于量子统计力学方法的这些实验的理论解释是第一个研究领域的目标。第二个涉及溶液中分子聚集体的光学和电子性质。分子聚集体，如光合装置的光收集复合体，在过去的十年里得到了大量的实验研究。在正确光谱范围内的飞秒激光器的出现导致了研究这些聚集体中激发能量转移和振动弛豫的时间依赖性的新实验。对这些聚集体的电子态和振动态以及能量流动力学的理论理解是本阶段支持研究的中心问题。由于实验的进步，溶液中分子的二次谐波光散射重新成为测量分子非线性光学性质的重要工具。然而，从分子和溶剂性质的角度解释这些实验需要现代理论基础，这是本基金第三部分的目标。特别是，溶质分子处溶剂场波动的描述对于正确解释实验结果是必不可少的。一个分子的光谱是它的电子结构和分子所处环境的标志。环境的影响可能是深远的，并且可能使分子的光学性质的识别和理解变得困难。因此，对这些影响的研究不仅提供了分子光谱方面的信息，还提供了环境方面的信息。本研究的目的是从理论上了解环境（溶剂）对分子光学性质的影响。将要研究的分子是具有较大线性和非线性光学响应的有机分子；这些包括：用于激光研究的染料分子，叶绿素和叶绿素聚集体（天线配合物和反应中心），以及用于光开关和通信设备的候选共轭分子。分子所在的溶剂或环境包括普通液体、固体和玻璃。理论研究将与测量这些光学特性的实验学家合作进行。

The Theoretical and Computational Chemistry program is supporting Professor Silbey at MIT. During the next three years theoretical models will be developed for the study of several distinct areas. The first involves low temperature optical properties of molecules in condensed phases, in particular glassy solids. Spectral hole burning, photon echoes, and single molecule spectroscopy have all shown that the so called homogeneous line shape of a molecule is time dependent because of slow fluctuations in the condensed phase. These methods have different but often overlapping time scales, so that a complete picture of the time dependence from picoseconds to weeks can be found. The theoretical interpretations of these experiments based on a quantum statistical mechanical approach is the goal of the first area of research. The second involves optical and electronic properties of molecular aggregates in solutions. Molecular aggregates, such as the light harvesting complexes of the photosynthetic apparatus, have been under intense experimental study over the past decade. The advent of femtosecond lasers in the correct spectral range has led to new experiments studying the time dependence of excitation energy transfer and vibrational relaxation in these aggregates. The theoretical understanding of the electronic and vibronic states and the dynamics of energy flow of these aggregates is the central problem of this phase of the supported research. Because of experimental advances, second harmonic light scattering from molecules in solution has reemerged as an important tool to measure the non-linear optical properties of molecules. However, the interpretation of these experiments in terms of the molecular and solvent properties requires a modern theoretical basis, which is the goal of the third part of this grant. In particular, the description of the fluctuations of the solvent field at the solute molecule is essential to a correct interpretation of the experimental results. The optical spectrum of a molecule is a signature of its electronic structure and the environment in which the molecule finds itself. The effects of the environment can be profound and can make the identification and understanding of the optical properties of the molecule difficult. Thus, a study of these effects provides information not only on the molecular spectrum, but also on the environment. The purpose of this research is to understand theoretically the effects of the environment (solvent) on the optical properties of a molecule. The molecules that will be studied are organic molecules that have a large linear and non-linear optical response; these include: dye molecules used in laser research, chlorophyll and aggregates of chlorophyl (antenna complexes and reaction centers), and conjugated molecules that are candidates for use in optical switching and communication devices. The solvents or environments that the molecules are in include normal liquids, solids and glasses. The theoretical research will be carried out in collaboration with experimentalists who are measuring these optical properties.