Vibrational quantum interference in Raman-induced Kerr effect: momentum conservation

The pump-probe Raman-induced optical Kerr effect (RIKE) of simple molecular liquids, studied with femtosecond laser pulses, exhibit long lasting beats ascribable to vibrational quantum interference (QI). While energy conservation entails vibrational resonances in RIKE, momentum conservation boils down to wave vector-matching in the pump and probe processes, which calls for the participation of a vibrational excitation wave. The refractive index dispersion around vibrational resonances is intimately related to the focusing angle of the Pump (also probe) beam. The larger the focusing angle, the greater the excitation wave number, i.e. the more energetic the vibration in resonance; if the focusing angle is too small, energetic vibrations cannot be observed in vibrational QI, even if energy conservation is fulfilled. If the pump and probe beams are perfectly collimated, then all beams must be collinear in order to conserve momentum.