Interpretation of three-photon spectra in alkali halides

A symmetry analysis is presented which determines the selection rules of three-photon absorption processes. In the case of photons of equal energy, in addition to the dipole and the polarization selection rules an additional selection rule arises due to the symmetrization of the interaction operator. Contrary to the case of two-photon transitions the polarization dependence can now contain undetermined dynamical parameters. These considerations are applied to the case of an ${\mathit{O}}_{\mathit{h}}$ point group in order to interpret recent experiments on alkali halides. The effect of a magnetic field in different directions is also studied. The theory accounts for the observed transitions and their polarization properties. Moreover, the presence of a new line in the absorption spectrum of CsI is predicted, and possible configurations in which this line could be detected are discussed.