FA(Ga+,In+,Tl+) tunable laser activity and interaction of halogen atoms (F,Cl,Br,I,At) at the (001) surface of KCl crystal: ab initio calculations

An attempt has been made to examine F_A(Ga⁺,In⁺,Tl⁺) tunable laser activity and adsorptivity of halogen atoms (F,Cl,Br,I,At) at the (0 0 1) surface of KCl crystal using an embedded cluster model, CIS and density functional theory calculations with effective core potentials. The ion clusters were embedded in a simulated Coulomb field that closely approximates the Madelung field at the host surface. The nearest neighbor ions to the defect site were then allowed to relax to equilibrium. Based on the calculated strength of electron–phonon coupling and Stokes-shifted optical transition bands, The F_A(Tl⁺) center was found to be the most laser active in agreement with the experimental observation that the optical emissions of F_A(In⁺) and F_A(Ga⁺) centers were strongly quenched. The disappearance of the anisotropy and np splitting observed in the absorption of F_A(Ga⁺,In⁺,Tl⁺) centers were monotonically increasing functions of the size of the impurity cation. The F_A(Ga⁺,In⁺,Tl⁺) defect formation energies followed the order F_A(Ga⁺)>F_A(In⁺)>F_A(Tl⁺). The Glasner–Tompkins empirical relationship between the principal optical absorption of F centers in solids and the fundamental absorption of the host crystal was generalized to include the positive ion species. As far as the adsorptivity of the halogen atoms is concerned, the F and F_A(In⁺,Tl⁺) centers were found to change the nature of adsorption from physical adsorption to chemical adsorption. The adsorption energies were monotonically increasing functions of the electronegativity of the halogen and the amount of charge transferred from the defect-free surface. The calculated adsorption energies were explainable in terms of the electron affinity, the effective nuclear charge and the electrostatic potentials at the surface. The spin pairing mechanism played the dominant role in the course of adsorbate–substrate interactions and the KCl defect-free surface can be made semiconducting by F or F_A(In⁺,Tl⁺) surface imperfections.