In this chapter, we cover in detail the connections between electronic structure theory and the experimental observables registered in linear and nonlinear optical spectroscopic setups. The main goal is to show different approaches of increasing complexity to model optical spectra and their lineshapes for both steady-state (linear) and time-resolved (pump-probe and two-Dimensional Electronic Spectroscopy, 2DES) variants, and how these models can offer a more detailed characterization of a system’s photo-response. We start with a brief introduction to Liouville space, and how the density matrix and its evolution can be used to express the different light-matter interactions occurring in linear and nonlinear optical experiments. We then introduce the sum-over-states approach and point out the connection between the density matrix and the transition energies and transition dipole moments obtained from electronic structure theory codes. Lineshape broadenings are then defined semi-classically within the cumulant expansion of Gaussian fluctuations to second-order, outlining the displaced Brownian oscillator model and the basic theory behind lineshape functions. We show different approximations in linear and nonlinear spectra, and for both steady-state and time-resolved situations, highlighting strengths and weaknesses through their application to a series of different molecular systems. We attempt to provide guidelines for specific cases where the different approaches might be appropriate, aiming to outline their range of applicability.
Elsevier
