Attraction between energy levels in the presence of relaxation

The excited quartet states of the ${\mathrm{Mn}}^{++}$ ion in a cubic crystal are investigated from the point of view of the attraction amongst themselves as a result of the spin-phonon interaction. The diagonal relaxation probabilities and the level shift are computed. An attraction between the quartets is established theoretically and compared with experimental data of the optical absorption in ${\mathrm{Mn}}^{++}$-doped NaCl single crystal.