Nonlinear Raman spectroscopy is a versatile method to enhance the intensities of Raman scattering. It requires an intense light field that can be provided by a liquid droplet acting as a high-quality optical cavity. Here, colliding droplets were used as a novel optical cavity to enhance the intensity of Raman scattering. Specifically, multiorder stimulated Raman-scattered light was generated with significant intensity from colliding droplets of carbon tetrachloride (CCl). The intensities of the Raman bands were analyzed with a simple theory that roughly reproduced the experimental spectrum. Overall, the method facilitates Raman spectroscopy of molecules in liquids because of its high sensitivity and resolution.

Low-frequency intermolecular motions are of importance for understanding the structure of molecular liquids, which can be elucidated by Raman spectroscopy. A liquid droplet provides a field where stimulated Raman scattering (SRS) readily proceeds. Our previous study showed that multiorder SRS is more effectively generated in colliding droplets than a single droplet. Here we report that the multiorder SRS generated in colliding benzene droplets included lowfrequency bands of the liquid benzene, which is considered to represent a temporal state of liquid emerging in the course of the droplet collision. This method enables measurements of the low-frequency intermolecular motions of the novel liquid state emerging at the mixed region of the colliding droplet without suffering from background of Rayleigh scattering.