Die optische Absorption der U₂-Zentren in Alkalihalogeniden des NaCl-Typs

Abstract

The UV absorption of the U₂ center in nine alkalihalides at 20 °K is presented. In the bromides and iodides the U₂ band consists of three components. This splitting is interpreted as a combined effect of spin-orbit coupling and dynamic lattice distortion. The energy of the strongest U₂ band is calculated by using the effective charge of the H atom in the U₂ ground state as a parameter. This comes out to be −0.17 e in chlorides, −0.20 e in bromides, and −0.25 e in iodides.

In mixed crystals of KBr+Δ KJ U₂ centers of the general kind HBr _n J_4−n (n=0, ..., 4) are investigated. They form three classes of U₂ bands, which are distinguished by thep-orbital in the excited state. This can bep _BrBr,p _BrJ,p _JJ depending on what halide ions the defect electron is located in the excited state. Dichroic bleaching of each band with polarized light allows to orient the complexes. This demonstrates that by optical stimulation the H atom can diffuse through the interstitial lattice. By warming up to 70 °K HBr₄ changes to HBr₃J which becomes unstable above 80 °K.