Signatures of Herzberg-Teller coupling in three-dimensional electronic spectroscopy.

The coupling between electronic and nuclear variables is a key consideration in molecular dynamics and spectroscopy. However, simulations that include detailed vibronic coupling terms are challenging to perform, and thus a variety of approximations can be used to model and interpret experimental results. Recent work shows that these simplified models can be inadequate. It is therefore important to understand spectroscopic signals that can identify failures of those approximations. Here we use an extended response-function method to simulate coherent three-dimensional electronic spectroscopy (3D ES) and study the sensitivity of this method to the breakdown of the Franck-Condon approximation. The simulations include a coordinate-dependent transition dipole operator that produces nodes, phase shifts, and peak patterns in 3D ES that can be used to identify Herzberg-Teller coupling. Guided by the simulation results, we interpret measurements on a molecular aggregate.