Effect of internal excitations of polarons on magneto‐optical absorption and cyclotron resonance in polar crystals

The energy spectrum of Fröhlich polarons in a magnetic field is investigated theoretically. The constants of the motion of the system are studied and a “Feynman model” serves as B guide in this study. A crucial difference between the energy spectra of polarons and rigid particles is predicted at rather strong coupling strengthsor large magnetic fields. It is due to the internal structure of the polaron system. When the electron‐LO‐phonon coupling constant becomes large enough to allow internal excitations, a resonance may appear in the one‐phonon continuum. This resonance is split by Zeeman effect at small field values and the components of the multiplet tend asymptotically to the Landau levels with increasing field strengths. The stability of one component at least is enhanced by the magnetic field, so that magnetic fields could be used to reveal internal excitations of polarons, in cyclotron resonance or magneto‐optical absorption experiments. Recent experimental results on magneto‐absorption by excitons are discussed in the light of the present work.