Optical properties of hole centres in alkali halides: II. MS-X calculations

Abstract

calculations at different values of the metal-ligand distance, R, have been performed for units (, Br, I) subjected to the electrostatic potential of KX lattices. The results confirm that the optical absorption bands peaked at 3.4 and 4.2 eV in KCl: can be associated with the and charge transfer (CT) transitions respectively of the complex. Also, the systematic red shift experienced by such transitions on passing from KX to RbX is related to the increase of R induced by the host lattice change. The spin-orbit coupling in and levels determines the sign of the magnetic circular dichroism of the optical absorption (MCDA) and it is shown that for bromides and iodides the two CT transitions can exhibit a different pattern as is experimentally observed. Also the non-existence of MCDA signal in the region of KCl: is related to a practically zero value of the spin-orbit splitting. The existence of five CT peaks for iodides is explained through the component of the CT transition, whose oscillator strength increases following the ligand spin-orbit coefficient. As is always found to be located about 0.15 eV below , this new component can also explain the asymmetry observed in the MCDA spectra of chlorides and bromides in the high energy side of the transition. In all these units, the unpaired electron is found to be located mainly on the X ligands, the charge on them increasing along the series. This is related to the corresponding decrease of the hyperfine constant for whose core polarization effects are calculated to be negligible. From the present results, the equilibrium distance would be close to 2.80 Å thus implying a 10% inwards relaxation with respect to the host lattice. To our knowledge these are the first calculations reported on heavy impurities.