  The magnetic-field dependence of the energy spectrum of GaAs doped with
nitrogen impurities is investigated. Our theoretical model is based on the
phenomenological Band Anticrossing Model (BAC) which we extended in order to
include magnetic field and electron - phonon interaction. Due to the highly
localized nature of the nitrogen state, we find that the energy levels are very
different from those of pure GaAs. The polaron correction results in a lower
cyclotron resonance energy as compared to pure GaAs. The magneto-absorption
spectrum exhibits series of asymmetric peaks close to the cyclotron energy.
