Symmetry of Excitons inCu2O

The anisotropy of the absorption of the $1s$ exciton line in ${\mathrm{Cu}}_2$O which is observed by quadrupole radiation shows that this exciton is of type ${{\Gamma }_{25}}^{+}$. The details of the anisotropic absorption properties of the Zeeman and Stark effect are predicted and compared with experiment. The anisotropy of the indirect absorption edges allows the symmetry of the phonon involved to be determined. From this symmetry identification it is possible to speculate about the nature of the valence and conduction bands in this substance. The results appear to be in agreement with a spin-orbit split valence band of Cu $3d$ functions and a conduction band of Cu $4s$ functions. Estimates of the ortho-para exciton splitting and Stark strain splitting on this model are of the right order of magnitude but smaller than those observed.