MAGNETOOPTICAL PROPERTIES OF THE UV ABSORPTION-BANDS OF V-K CENTERS IN ALKALI-HALIDES

A new UV absorption band below the original UV band has been discovere d through the magnetic circular dichroism of the V-K centre absorption (MCDA) in seven alkali halide crystals. This new band is attributed t o the transition Sigma(mu) --> Pi(mu) which becomes dipole allowed as a result of a loss of inversion symmetry. We attribute this lowered sy mmetry to the slow axial oscillation along the V-K axis which results in a momentarily 'frozen' geometry of the species at the classical tur ning point. An approximate estimation of the coupling between the even and odd parity molecular orbitals of Sigma and Pi symmetry is made in KBr. From this it appears that the coupling between the even and odd parity states is not negligible. Observed experimental trends in the s even crystals are explained from this model.