  Symmetry adaptation techniques are applied to the determination of the
intensity of two-photon absorption transitions, between Stark levels of
configurations with opposite parities, for transition ions in finite symmetry
environments. The equivalence between third-order mechanisms and second- plus
first-order mechanisms is clearly established. A compact formula is derived for
the intensity of a two-photon transition between two levels with well-defined
symmetries. A set of (selection) rules which controls the number of intensity
parameters for inter-configurational transitions is given. The formalism is
illustrated with the case of an ion with 4f configuration in tetragonal
symmetry.
