The Role of Oxidation States in Laser Light Generation and Color Image Formation of F_A: Co⁺², Co⁺³ Color Centers at the (001) Surface of AgBr crystal: ab initio Calculations

 The oxidation states of cobalt in FA color center at the low coordination (001) surface of AgBr play important roles in laser light generation and color image formation. These two applications are investigated simultaneously 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 surface and the nearest neighbors ions to the FA 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 cobalt and the choice of the basis set centered on the anion vacancy. All relaxed excited states of the defect-containing surfaces were deep below the lower edges of the conduction bands of the ground state defect-free surfaces, suggesting that FA: Co+2 and Co+3 color centers are suitable laser defects. The dependence of the orientational destruction, recording sensitivity andexciton (energy) transfer on the oxidation state of cobalt is clarified. The Glasner-Tompkins empirical rule is generalized to include the oxidation state of the impurity cation. 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 Co+3 surfaces of AgBr are significantly more sensitive than the corresponding Co+2 surfaces. On the basis of quasi Fermi levels, the difference in the sensitizing capabilites between the examined dyes in the excited states is determined.