Two-photon Raman scattering occurs when a picosecond pump beam is used to create a virtual state and a probe beam of a frequency to match the frequency of Stokes Raman scattering from the virtual state to a specific vibrational level. The Stokes emission is stimulated thus depopulating the virtual state very quickly and increasing the scattering efficiency for the chosen vibration by orders of magnitude. SRS is a non-linear process, so sufficient photons from both the pump and probe beams must be present on the molecule at the one time.