Coherent Infrared Hyper-Raman Four Wave Mixing Spectroscopy—A Probe of Vibronic Coupling and Non-Condon Effects

Ultrafast electronic relaxation is often accelerated by relaxation pathways allowed through vibronic coupling. 2D electronic spectroscopies are often employed to resolve vibronic coupling through vibronic sidebands in electronic transitions. Here, we propose an alternative method, hyper difference frequency generation (HDFG) spectroscopy, based upon coherent hyper-Raman type pathways which directly probe coupling between vibrational and vibronic states. In HDFG, an infrared pulse is resonant with a vibrational mode and a two-photon absorption is used to stimulate a hyper-Raman transition. An Albrecht-type A-B-C term expansion of the HDFG hyperpolarizability suggests HDFG is a sensitive probe of vibronic coupling and, in particular for centrosymmetric species, non-Condon behavior (i.e. B term contributions).