The problem of the interaction of radiation with arbitrary polarisation and intensity with the resonance transitions 1/2-1/2 and 1-0 is solved. Exact expressions for the orientation vectors of the ground and excited states for the transition 1/2-1/2 and for the orientation vector and alignment tensor of the transition 1-0 have been obtained. In the case of longitudinal propagation the system of equations for the absorption of an elliptically polarised wave is obtained. It is shown that, irrespective of the initial state, the polarisation at the transition 1-0 tends either to the linear or to the circular form as it propagates depending on the times of collisional relaxation of orientation and alignment and on the magnetic field value. In the case of transverse propagation the dependence of the saturation intensity of a linearly polarised wave on the magnetic field value and on relaxation widths is obtained.
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