Electronic spectrum of the confined auride ion

Abstract

Several models of the confinement of the auride ion are discussed in the context of the colour of the tetramethylammonium auride crystals. These models correspond to either neutral or charged nearest neighbourhood of the auride ion as represented by a cluster of helium and He-like atoms, respectively. The models involving neutral helium clusters account for what is referred to as the confinement effect and simulate the Pauli repulsion between the auride ion and its surrounding. The charged clusters supply additional attractive potential which is shown to be indispensable for the interpretation of the large blue shift of the first absorption band of the auride ion. It is also found that the Pauli repulsion and the effect of the increase of the attractive potential due to the charged neighbourhood of the auride ion are not additive. The confinement models discussed in this paper are used to calculate the energy of the first electronic transition in the confined ion. The calculations have been carried out in the TDHF (RPA) and CASSCF/CASPT2 approximations with relativistic effects accounted for by using the Douglas-Kroll approximation. The charged confinement models are found to increase the importance of both the electron correlation and relativistic contributions to the energy of the first electronic transition in the encapsulated auride ion. The computed data are used to discuss the observed spectral transparency of the tetramethylammonium auride crystals.