A calculation is made of the parity-forbidden two-photon absorption cross section of the transition between the lowest Stark sublevels for a tetragonal site Cc3+ (symmetry Ca,) in a cubic fluorite crystal as well a8 of polarizat.ion characteristics of a CaF, : Ce3+ monocrystal with different orientations ( k l ) LlOO], k 11 [llO], k 11 [ l l l ] , k is t.he wave vector of the incident laser beam). The calculated cross-sectional value (a, = 3.7 x m~-~ s) and the polarization dcpendences agree with experiment.
Theoretical and experimental investigations are made of two-photon absorption spectra of trivalent rare-earth ions in crystals. Effective cross-sections, selection rules, and relative intensities of the components are found. The most intensive two-photon transitions are established. Two-photon excitation spectra of luminescence of Er3+ and Ho3+ ions in CaF, are obtained and effective excitation cross-sections are measured using a tunable dye laser (715 to 850 nm). It is shown that the spectra have a peculiar structure which differs considerably from that of the one-photon absorption spectra. The experimental results agree with theory. It is established that one of the levels in Hos+ is the level 5G, rather than sK, usually assumed.
Investigations are made of some regularities in the position of Stark components and a number of conclusions are drawn on the nearest surrounding structure rare-earth (RE) ions. Since the splitting of the J-manifold is generally defined by the overlap of RE ions and ligand orbitals, it is possible to establish a relation between the phenomenological crystal field parameters BtpP (CFP) and the structure of the nearest surrounding. It is important that the various multipolarity CFP ( t = 2, 4, 6) in the overlap model depend identically on R while their values in the CF theory are proportional to 7 / R t + l . The dependence on the ligand angular coordinates is similar for the two models.
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