Non-Markovian nature of relaxation processes and its spectroscopic manifestations

An analysis is made of some manifestations of the non-Markovian nature of relaxation processes observed in the response of quantum systems to external radiation. It is shown that allowance for this nature enhances the saturation effect and the non-Lorentzian nature of spectral line wings, and gives rise to additional absorption (luminescence) lines due to transitions involving changes in the states of several particles.