Comparison of Absorption Spectrum of Uranium Tetrachloride with that of Praseodymium Trichloride and of Uranium Trichloride with that of Neodymium Trichloride

Abstract

The absorption spectra of the following substances were taken at 77°K: crystals of anhydrous uranium tetrachloride, praseodymium trichloride, uranium trichloride, neodymium trichloride, uranium tetrafluoride, and a solution of uranium tetrachloride in a solvent consisting of propyl alcohol, propene, and propane. The spectra of the ``iso‐electronic'' ions U<sup>+4</sup> and Pr<sup>+3</sup> exhibited a match in wave number of the centers of gravity of the separate absorption groups. The correspondence is especially evident between the spectrum of uranium tetrachloride and that of praseodymium chloride. The uranium tetrafluoride furnished a spectrum resembling the chloride but the separations of the absorption regions were not so clear. These correspondences between U<sup>+4</sup> and Pr<sup>+3</sup> lead to term assignments for the basic and activated electronic states of U<sup>+4</sup> since those of Pr<sup>+3</sup> are known. Thus, the two least stable electrons of U<sup>+4</sup> in the basic state and to a good approximation in the excited states lies within the 5f shell. The interaction of the fields of the environment of the ions in the case of U<sup>+4</sup> is greater than in the case of Pr<sup>+3</sup> and is especially marked in their <sup>1</sup>I<sub>6</sub> states. The greater interaction is in agreement with the greater effects of different ionic environments on the spectra of U<sup>+4</sup>, the greater intensity of the ``forbidden'' transitions and other physical properties. The correlation of the spectra of U⁺³ and Nd⁺³ is less distinct even though the same approach may well be valid in view of their similar color. The areas of absorption are not as distinctly separate and may even overlap. A new method has been developed for obtaining the absorption spectra of very small anisotropic crystals.