Svelto is also mentioning the photon-phonon interaction as generating homogeneous broadening and therefore a Lorentzian line shape too. An inhomogeneous broadening (such as those caused by the Doppler effect and thermal effect) is giving a Gaussian line shape. However, in spectroscopy, the most observed case is that of an intermediate profile, given by the convolution of the resonance relative probability and the broadening function, because the natural band can be modified by different mechanisms .