Time-resolved linear dichroism (TRLD) spectroscopy is a powerful tool for studying the relationships between molecular structure, electronic configuration, and reactivity. In a TRLD experiment, the absorption of a beam of linearly polarized light is used to produce an oriented sample of excited states by a process known as photoselection. When this oriented sample is probed with a second beam of linearly polarized light, the difference between the absorbance measured parallel and perpendicular to the excitation beam is defined as the linear dichroism. The initial magnitude of the linear dichroism is related to the absolute angle between the excitation and probe transition moment vectors and the time decay of the linear dichroism is related to the rotational diffusion of the excited states. Detailed descriptions of photophysical processes are possible when these two types of information are analyzed simultaneously. Examples of this approach are presented for the charge separation dynamics of excited charge transfer complexes and the anisotropic rotational dynamics of polycyclic aromatic hydrocarbons.
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