Theoretical modeling of laser light generation and color image formation: FA1:Tl+ and FA2:Ga+ color centers at the low coordination (100) and (110) surfaces of AgCl and AgBr

Abstract The FA1:Tl+ and FA2:Ga+ color centers at the low coordination (1 0 0) and (1 1 0) surfaces of AgCl and AgBr play important roles in laser light generation and color image formation. Double-well 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 FA-defect site were allowed to relax to equilibrium. The calculated Stokes shifts suggest that laser light generation is sensitive to the simultaneous effects of the vibrational coupling mode, the impurity cation, the coordination number of the surface ion, the lattice anion, 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 and optical–optical conversion efficiency. 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 the FA1:Tl+ and FA2:Ga+-centers are suitable laser defects. The dependence of orientational destruction, recording sensitivity and exciton (energy) transfer on the impurity cation, the coordination number of the surface ion, and the type of the lattice anion is clarified. The Glasner–Tompkins empirical rule is generalized to include 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 (1 1 0) surfaces of AgBr and AgCl were more sensitive than the corresponding (1 0 0) surfaces, and AgBr thin film was found to be more sensitive than that of AgCl. On the basis of quasi-Fermi levels, the difference in the sensitizing capabilities between the examined dyes in the excited states was determined.