Stimulated Raman Spectroscopy

When the scattered photon is lower in energy, or frequency, it is said to be Stokes shifted. The shift in frequency of the Stokes is related to the characteristic of the medium. This is illustrated in Fig. 2. Calling this material characteristic frequency , the Stokes frequency is then . The Raman Stokes scattering results in an excitation from the ground state of the medium to an excited state mediated by a two photon inelastic scattering. If the material is already in the excited state (e.g. by thermal excitation), then the scattered photon can be higher in energy and is called anti-Stokes photon (Fig. 2). The probability of the anti-Stokes scattering is smaller than that of Stokes by a Boltzmann factor ⁄ .