OPTICAL-PROPERTIES OF TL2-CENTERS IN ALKALI-HALIDES - II - MS-X-ALPHACALCULATIONS( HOLE)

MS-X alpha calculations at different values of the metal-ligand distan ce, R, have been performed for TlX64- units (X = Cl, Br, I) subjected to the electrostatic potential of KX lattices. The results confirm tha t the optical absorption bands peaked at 3.4 and 4.2 eV in KCI:Tl2+ ca n be associated with the t(1u)(pi) --> a(1g) and t(1u)(sigma) --> a( 1g) charge transfer (CT) transitions respectively of the TlCl64- com plex. Also, the systematic red shift experienced by such transitions o n passing from KX to RbX is related to the increase of R induced by th e host lattice change. The spin-orbit coupling in t(1u)(pi) and t(1u)( sigma) levels determines the sign of the magnetic circular dichroism o f the optical absorption (MCDA) and it is shown that for bromides and iodides the two CT transitions can exhibit a different pattern as is e xperimentally observed. Also the non-existence of MCDA signal in the t (1u)(sigma) --> a(1g) region of KCl:Tl2+ is related to a practically zero value of the spin-orbit splitting. The existence of five CT peak s for iodides is explained through the gamma(8-) component of the t(2u ) --> a(1g) CT transition, whose oscillator strength increases follow ing the ligand spin-orbit coefficient.;As t(2u) is always found to be located about 0.15 eV below t(1u)(pi), this new component can also exp lain the asymmetry observed in the MCDA spectra of chlorides and bromi des in the high energy side of the t(1u)(pi) --> a(1g) transition. In all these TlX64- units, the unpaired electron is found to be located mainly on the X ligands, the charge on them increasing along the Cl -- > Br --> I series. This is related to the corresponding decrease of th e hyperfine constant for whose core polarization effects are calculate d to be negligible. From the present results, the equilibrium Tl2+-Cl- distance would be close to 2.80 Angstrom thus implying a 10% inwards relaxation with respect to the host lattice. To our knowledge these ar e the first calculations reported on heavy 6s(1) impurities.