Some features of the optical manifestation of relaxation

It is shown that, in general, the interaction between relaxing quantum systems and radiation is described by non-Markov transport equations for the density matrix $\rho^p$ and the relaxation matrix depends on the properties of the radiation. The usual Markov-type equations for $\rho^p$ are obtained from these equations if $\gamma\tau_c\ll1$ and $(\epsilon^2+4\vert V\vert^2)^{1/2}\tau_c\ll1$ ($\gamma$ is the width of the investigated transition in a quantum system, rc is the correlation time of the perturbations causing relaxation, e is the frequency detuning, and $\vert V\vert$ is the energy of the interaction between the system and the radiation field). When the second inequality is disobeyed, the relaxation process deviates strongly from the Markov type. In the case of small values of $\vert V\vert$, this results in a non-Lorentzian profile of the absorption lines characterized by a steep descent in the wings. In the range of high values of $\vert V\vert$ when 2$\vert V\vert\tau_c\gtrsim1$, the whole relaxation process becomes greatly modified: the dependence of the relaxation coefficients on $\vert V\vert$ becomes strong and a coupling appears between the relaxations of the diagonal and the nondiagonal elements of the density matrix. This is manifested, in particular, in the saturation effect: the term describing saturation in the expression for the polarizability changes considerably and the expression for the line width loses the component due to the adiabatic (elastic) perturbation.