Perturbed free induction decay in ultrafast mid-IR pump–probe spectroscopy

We studied theoretically and experimentally, the perturbed free induction decay phenomenon for an anharmonic vibrational system in degenerate IR pump–probe experiments. We derived the theory for the perturbed free induction decay using perturbation theory in the interaction picture for the fundamental and overtone transitions. Degenerate broadband mid-IR (MIR) pump–probe experiments on W ( CO ) 6 were carried out to measure transient absorption spectra and kinetic signals at negative time delays, respectively. The experimental measurements were compared and fitted with the presented theory, showing excellent agreement.