THE POLARIZATION-ANGULAR STRUCTURE AND ELLIPTIC DICHROISM OF THE CROSS-SECTIONS FOR 3-PHOTON BOUND-BOUND TRANSITIONS IN ATOMS

Using the electric dipole approximation, we present, in invariant form , the cross section of an arbitrary three-photon transition between th e discrete states of an atom with total angular momenta J(i) and J(f). The cross section contains scalar and mixed products of the photon po larization vectors, and invariant atomic parameters dependent only on the photon frequencies. We determine the number of independent atomic parameters at fixed values of J(i) and J(f) and obtain their explicit expressions in terms of the reduced composite dipole matrix elements. The polarization dependence of the cross sections is expressed in term s of the degrees I and xi of linear and circular photon polarizations. We analyze the phenomenon of dissipation-induced circular dichroism i n three-photon processes, i.e., the difference Delta of the cross sect ions for opposite signs of the degree of circular polarization of all the photons. We study in detail the case of two identical photons and the phenomenon of elliptical dichroism, when Delta similar to l xi hol ds and dichroism occurs only when the photons are elliptically polariz ed, with 0<\xi\<1. Finally, we discuss the dissipation-induced effects of atom polarization in three-photon processes involving linearly pol arized or unpolarized photons. (C) 1997 American Institute of Physics.