Preliminary Analysis of U4+ Ion Spectra in Crystals

Abstract

An analysis of a number of features of the absorption spectra of U⁴⁺ ions in crystals is given. Absorption spectra of crystals of the compounds Cs₂UCl₆, and [N(CH₃)₄]₂ UCl₆, and to a lesser extent [N(C₂H₅)₄]₂ UCl₆ have been studied at room temperature, liquid N₂ and liquid He temperatures. The visible spectrum is essentially entirely vibronic. Pure electronic transitions are absent or very weak in thick crystals in agreement with a model of electric dipole transitions within the 5 f² shell at an equilibrium site of inversion symmetry. We believe this to be the first demonstrated case of an essentially purely vibronic spectrum reported for rare earth or actinide salts. The three ``ungerade'' frequencies of the UCl₆⁼ complex are coupled to the electronic frequencies and are determined by their appearance with the same frequency of superposition in all three salts as well as by being the most intense of the vibronic lines in every group in which they appear. Other lattice modes and internal frequencies of the tetramethylammonium ion are also found to be superimposed. The weak pure electronic transitions are fairly sharp, an indication that there is still considerable shielding in the 5 fⁿ electronic states even though this shielding is less than in 4 fⁿ states. The independence of superimposed vibration frequencies on 5 f² electronic states indicates the fact that the 5 f states do not contribute much to the binding. The similarity in the spectra of the three salts studied is due to the fact that the nearest‐neighbor Cl^— ions are the principal origin of the crystalline field in contrast to the rare earths, wherein this type of similarity is due to the fact that the crystalline field is small and each group of lines represents a multiplet level of the free ion. Evidence is presented that the crystalline field splitting is larger than hitherto believed and is the same order of magnitude as the spin‐orbit interaction. Methods of attacking the problem of the experimental classification and calculation of energy levels in the situation in which three major interaction are of comparable size are discussed.