Two-Photon Spectroscopy of Alkali Halides

Symmetry considerations show that "p-like" excitons in cubic crystals can in general give rise to two-photon transitions to states of A1g, Eg, T1g, or T2g symmetry and having a polarization dependence characteristic of any combination of these. A discussion is given of the physical significance of the fact that the two-photon absorption coefficient is independent of the orientation of the beam polarizations with respect to crystal axes in Fr\"ohlich and Stagginnus's observations on RbI and in Mahan's theoretical treatment of the alkali halides. It is noted that Matsuoka's data on TlCl cannot be accounted for in terms of k=0 transitions of any symmetry type.