It is found that a sufficiently strong electric field can substantially decrease the relaxation rate of high-frequency polarization of a gas corresponding to the resonance dipole-dipole interaction of its molecules. The corresponding decrease in the integrated cross section for intermolecular Weisskopf collisions is calculated using the two-level approximation for the description of quantum dynamics of internal (vibrational) degrees of freedom of molecules, whereas their translational and rotational motions are treated classically. It is shown that the effect is caused by the Stark change of dipole moments of molecules and the ordering of their orientations in a constant electric field. As a result, for a certain magnitude of this field, the contributions of constant and high-frequency dipole moments of a molecule to the change in its density matrix upon a collision are counter-balanced to a considerable extent. The conditions required for the experimental observation of the effect are formulated.
A spin system with unperturbed Hamiltonian ${\mathcal{H}}_{0}=\frac{1}{2}\ensuremath{\Sigma}\ensuremath{\Sigma}{B}_{\mathrm{jk}}{{\ensuremath{\sigma}}_{x}}^{j}{{\ensuremath{\sigma}}_{x}}^{k}\ensuremath{-}\ensuremath{\gamma}H\ensuremath{\Sigma}{{\ensuremath{\sigma}}_{x}}^{j}$ relaxing via the spin-lattice coupling $G=\frac{1}{2}\ensuremath{\Sigma}\ensuremath{\Sigma}{C}_{\mathrm{jk}}{{\ensuremath{\sigma}}_{x}}^{j}({{\ensuremath{\sigma}}_{y}}^{k}+{{\ensuremath{\sigma}}_{z}}^{k})$ is studied by means of the general density-matrix theory of magnetic relaxation. By making some assumptions about the magnitude and time constants of the lattice correlation functions
〈Cij(t)Ckl(0)〉, a master equation is obtained. It agrees at high temperatures with a master equation previously suggested by Glauber for the one-dimensional nearest-neighbor case. At high temperatures the magnetic moment relaxes with a single relaxation time, and the spin pair-correlation functions satisfy a closed set of equations. At low temperatures, however, the equations for the magnetization and the correlation functions are coupled to higher-order moments.
The polarization density matrix of resonantly scattered radiation from an atomic system has been obtained in terms of the polarization density matrix of the incident radiation and a four-time correlation function of the interaction operators between radiation and matter. From this we can obtain information not only about the intensity but also the polarization of the scattered radiation as they are affected by the relaxation processes. We have been able to compute the four-time correlation function and hence the polarization density matrix of the scattered radiation within the framework of some stochastic models developed earlier. The results are valid for very general kind and arbitrary rate of relaxation.