Time-Resolved Spectroscopy: Instrumentation and Applications

Abstract Light–matter interaction may lead to three fundamental phenomena, viz. (i) absorption, (ii) emission, and (iii) scattering. Spectroscopic techniques based on these phenomena are used to characterize the materials of interest, not only stable compounds but also short‐lived species during molecular transformations. Determining the electronic and molecular structure of the transient species is crucial in order to understand various photochemical, physical, and biological processes of fundamental and technological importance. Identifying various redox species during photosynthesis, understanding the photoinduced charge transfer process in solar cells, unraveling the photochemistry of vision that involves photoisomerization of retinal, etc. require spectroscopic techniques that can resolve various transient species in time and were practically impossible until the advent of pulsed lasers. The absorption of a photon by a molecule may initiate a cascade of dynamic molecular events, namely vibrational relaxation (VR), solvation dynamics, internal conversion (IC), intersystem crossing (ISC), electron transfer, isomerization reactions, etc., which can occur at femtosecond (fs) to picosecond (ps) timescales. Bimolecular reaction dynamics in liquids, such as H‐atom abstraction reaction, are typically diffusion controlled and therefore can be observed at nanosecond (ns) timescales. Therefore, time‐dependent (time‐resolved) spectroscopy has been an exquisite tool to follow the molecular dynamics after an initial phototrigger. Apart from studying molecular dynamics, time resolution can also be used to distinguish conformers, isomers or enantiomers, or distinguish identical compounds in different environments. It can also be applied to suppress unwanted signals occurring at different timescales, for instance fluorescence suppression in Raman experiments, or selectively detect compounds deeper inside a sample. In this article, we first give an introduction to the fundamentals of time‐resolved electronic absorption, spontaneous excited‐state resonance Raman, and coherent Raman scattering and emission techniques, spanning the range from femtosecond to microsecond timescales. Important technical aspects of time‐resolved spectroscopic equipment are also discussed. We then present some examples of cis–trans isomerization, thermal equilibrium of the two lowest triplet states, H‐atom abstraction reactions, etc. and demonstrate the molecular dynamic events after an initial phototrigger by a comprehensive kinetic analysis of the peak position, width and intensity of the marker bands in the time‐resolved electronic absorption, and Raman and emission spectra.