Like an MRI for molecules. It shows how different vibrations "talk" to each other.
Most practical nonlinear experiments (photon echoes, transient gratings, 2D spectroscopy) rely on three distinct laser pulses. Why three? Because two wouldn't be enough to separate "blurring" from "moving." Like an MRI for molecules
) as the primary tool to track the "state" of a system—populations (diagonal elements) and coherences (off-diagonal elements). Like an MRI for molecules