Symmetry Adaptation in Two-Photon Spectroscopy
  Symmetry adaptation techniques are applied to the determination of the intensity of two-photon transitions for transition ions in finite symmetry environments. We treat the case of intra-configurational transitions with some details and briefly report some results on inter-configurational transitions. In particular, for intra-configurational transitions, we describe a model which takes into account the following ingredients: symmetry, second- plus third- order mechanisms, S-, L- and J-mixings. 
M. Kibler