Résumé
In a spectroscopy experiment, radiation is used as a probe of the properties of a system. A typical experiment of spectroscopy is schematized in Fig. 5.1. The source (probe) can be X-rays, laser light (visible and infrared radiations), neutrons, electrons, … . A monochromatic radiation is obtained by using a relevant monochromator device. As long as the response of the material to the radiation is linear, the function which describes the interaction is called the response function and it can be calculated using the linear response model. This response function χ(Q, ω) relates the field associated with the source, E(Q, ω) to the response of the system, R(Q, ω)