The IUVS beamline is devoted to the study of inelastic scattering with ultraviolet radiation, in a time-space domain not accessible at present by other facilities. In the inelastic scattering process, an incident photon, with energy Ei, momentum, Ki and polarization pi; interacts with the matter. In this interaction the photons can be diffused, with energy Es, momentum Ks and polarization ps. Here q=Ks-Ki is called "exchanged moment" and is of considerable importance in the study of inelastic diffusion. We can define the “elastic scattering” when the incident photon energy is the same of the diffused one while we refer to “inelastic scattering” when there is an energy variation.It is important to point out that q is directly dependent on the time and energy as well as from the speed of light in the sample (and therefore on its refractive index).