An attempt has been made to examine FA(Gaþ; Inþ; Tlþ) 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 FA(Tlþ) center was found to be the most laser active in
agreement with the experimental observation that the optical emissions of FA(Inþ) and FA(Gaþ) centers were strongly quenched.
The disappearance of the anisotropy and np splitting observed in the absorption of FA(Gaþ; Inþ; Tlþ) centers were monotonically
increasing functions of the size of the impurity cation. The FA(Gaþ; Inþ; Tlþ) defect formation energies followed the order
FAðGaþÞ > FAðInþÞ > FAðTlþÞ. 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 FA(Inþ; Tlþ) 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 FA(Inþ; Tlþ) surface imperfections. 2001 Elsevier Science B.V. All rights reserved. |
