Intensity of Two-Photon Absorption Transitions for Ni2+ in MgO
  The parity-allowed two-photon transitions between the ground state 3A2(T2) of the configuration 3d8 in cubical symmetry and the excited states of the same configuration are obtained via a simple model. This model is developed in a symmetry adapted framework by using second-order mechanisms and ionic wave-functions. It is applied to the recent experimental results obtained by McClure and co-workers for Ni2+ in MgO. 
J. Sztucki, M. Daoud and M. Kibler