Kinetic theory of resonance and relaxation in spin systems

The kinetic equations which describe relaxation processes in EPR are analysed in the case of absence of the spin resonance frequency shift. The boundary conditions problem for integro- differential equations in dependence on the choice of initial time t0 (the lower limit of integration) is discussed in detail. It is explicitly exhibited that the asymptotic states of a system described by the non-Markovian equations as well as the asymptotic states of a system described by the Markovian ones (which are approximate in relation to the integro-differential ones) are determined by equations of the same form. The question of the Markovian approximation is considered.