There are two possibilities which could happen during
the nonelastic collision. The energy of the photon could
increase or decrease. If the energy of the photon decreases,
the energy of the molecule increases and the molecule
moves on higher energetic state. The energy of the photon
has decreased so the frequency of the photon decreases. It
corresponds with so called Stokes region in the Raman
spectrum (see Fig. 4, h is the Planck's constant). It is the
red shift to lower frequencies or higher wavelengths [1].