Ab initio study of the stokes shift of the ns-np transition of Tl+ and In+ in KCl, Jahn-Teller effect in the nsnp configuration

Detailed lattice relaxation studies of the excited nsnp states of TI+ and In+ in KCl have shown that the calculated Stokes shift of the 6s-6p transition of TI+ is most sensitive to the accuracy of the lattice relaxation around the substituted ion. This is a result of the strong cancellation between the influence of the spin-orbit interaction and the lattice relaxation on the 6s6p level positions. The results were calculated with a HF-LCAO embedded cluster method as well as with a DFT based supercell approach. Unexpectedly the supercell method completely fails for thallium in predicting the Stokes shift. Using the embedded cluster method, a value of 0.706 eV was found for the shift for thallium and 1.24 eV for indium. The experimental values are 0.88 and 1.44 eV respectively. It was also clearly established that the temperature dependence of the decay time of the main emission line of thallium is directly related to the Jahn-Teller distortion by which the trapping (3)A(1u) level approaches the emitting T-3(1u) level. For indium this does not play a role because of the much smaller spin-orbit interaction.