  Finite symmetry adaptation techniques are applied to the determination of the
intensity strength of two-photon transitions for ions with one partly-filled
shell nl in crystalline environments of symmetry G. We treat the case of intra-
configurational transitions as well as the case of inter-configurational
transitions. In both cases, the Wigner-Racah algebra of the chain O(3) > G
allows to extract the polarization dependence from the intensity. The reported
results are valid for any strength of the crystalline field.
