The role of oxidation states in laser light generation, color image formation and adsorbate-substrate interactions. Comparative study of F-A : Tl+1, F-B : Tl+1, F-A : Tl+3 and F-B : Tl+3 color centers at the (100) and (110) surfaces of AgBr crystal

The oxidation states of thallium in F-A and F-B color centers at the low coordination (100) and (110) surfaces of AgBr play important roles in laser light generation, color image formation, and adsorbate-substrate interactions. The color center potentials at these surfaces are investigated by using quantum mechanical ab initio methods. Quantum clusters of variable sizes were embedded in the simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces, and ions that were the nearest neighbors to the F-A and F-B defect sites were allowed to relax to equilibrium. The calculated Stokes shifts suggest that laser light generation is sensitive to the simultaneous effects of the oxidation state of thallium, the coordination number of the surface ion, and the choice of the basis set centered on the anion vacancy. An attempt has been made to explain these effects in terms of Madelung potential, electron affinity, optical-optical conversion efficiency, and adsorbate-substrate electrostatic potentials. All relaxed excited states of the defect containing surfaces were deep below the lower edges of the conduction bands of the groundstate defect-free surfaces, suggesting that the title color centers are suitable laser defects. The dependence of the orientational destruction, recording sensitivity and exciton (energy) transfer on the oxidation state of thallium and the coordination number of the surface ion is clarified. The Glasner-Tompkins empirical rule is generalized to include the oxidation state of the impurity cation and the coordination number of the surface ion. As far as color image formation is concerned, the supersensitizer was found to increase the sensitizing capabilities of two primary dyes in the excited states by increasing the relative yield of quantum efficiency. The Tl+3 surfaces of AgBr are significantly more sensitive than the corresponding Tl+1 surfaces. On the basis of quasi Fermi levels, the difference in the sensitizing capabilites between the examined dyes in the excited states is determined. The adsorbate-substrate interactions were found to be dependent on the oxidation state of the impurity cation and on the electronegativity of the adatom. (c) 2006 Elsevier B.V. All rights reserved.