Electron-Optical Phonon Interaction for Paramagnetic Ions in Crystalline Fields

Abstract

The theory of vibronic transitions in rare‐earth and actinide series crystalline salts is applied to ions and ion complexes interacting with the vibrational modes of the entire crystal. Selection rules and expected fine structure are presented making use of space group representation theory. The results are applied to the visible spectra of three U⁴⁺ ion crystalline salts which are analyzed in higher dispersion than in previous work, and including [N(C₂H₅)₄]₂UCl₆ for the first time. The similarity of the absorption bands associated with each electronic level provides additional evidence for the vibronic nature of the spectrum. Electronic level positions given earlier are confirmed. The broad features of the spectrum such as the strongest vibronic bands can be described in terms of the vibronic states of a single ion complex. These bands, however, have structure at 77°K in thin crystals and a finer structure more evident at 4.2°K, which we explain for some levels as due to the splitting of the vibrational optical branches when the wave vector k is nonzero, and for other levels the latter in combination with a splitting of vibronic bands resulting from electron‐vibration interaction. The electron‐vibration interaction applied to a single ion complex does not lead to structure in a vibronic spectral line. It is experimentally demonstrated that pure electronic electric dipole transitions forbidden to appear when the U⁴⁺ ion is at a center of inversion symmetry can appear due to the presence of impurities which destroy the inversion symmetry. The splitting of some of the pure electronic levels expected as one goes from O_h to D_3d to D_2h symmetry seems not to occur except possibly in one or two cases. More precise temperature shift measurements are presented including the [N(C₂H₅)₄]₂UCl₆ salt for the first time. The possible existence of some metastable optical branches at 4.2°K is indicated.