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Observation by fluorescence detection of direct two-photon absorption within the $4f$ configuration of the Gd³⁺ ion following excitation with a continuous-wave laser is reported. Quantitative intensity measurements test assumptions behind the Judd-Ofelt theory of lanthanide transition intensities in a new way, and significant discrepancies are found. The discrepancies indicate that the radial integrals 〈4f|r|5d〉 vary markedly for different J multiplets, and that ligands significantly influence two-photon intensities.
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Dagenais et al. (1981) studied this question.
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