Hyper Raman Scattering
Hyper Raman scattering is a modified version of Raman scattering, where the scattered light occurs at frequencies somewhat lower than twice the frequency of the pump light. This means that two pump photons are converted into one photon of Raman scattered light and one phonon. This effect is usually fairly weak, but it has aspects which make it interesting for Raman spectroscopy. In particular, hyper-Raman spectra can provide vibrational information on molecules where ordinary Raman scattering is suppressed due to symmetry issues (silent modes, non Raman-active modes). The scattering rate can be substantially enhanced near optical surfaces.