F-A1 : La2+ and F-A2 : Sc2+ color center lasers and color image sensitization at the low coordination (001)surface of AgBr by ab initio calculations

The twofold potentials of F-A1:La2+ and F-A2:Sc2+ color centers at the low coordination (0 0 1) surface of AgBr play important roles in providing tunable laser oscillation and color image sensitization. Double well-potentials at these sites are investigated by using ab initio methods of molecular electronic structure calculations. F-A1:La2+ and F-A2:Sc2+ defect containing clusters were embedded in the simulated Coulomb field that closely approximates the Madelung field of the host surface, and the ions that are the nearest neighbors to the F-site are allowed to relax to equilibrium. As far as color center lasers are concerned, the calculated Stokes shifted optical transition bands suggest that the F-A2:Sc2+ center is more laser active than the F-A1:La2+ center. An attempt has been made to explain this result on the basis of size of the impurity cation and optical-optical conversion efficiencies. All relaxed excited states of the defect containing surfaces are deep below the lower edge of the conduction band of the ground state defect-free surface implying that both of F-A1:La2+ and F-A2:Sc2+ centers are suitable laser defects. The probabilities of orientational destruction of the two centers, which are attributed to the assumed relaxed excited state saddle-point ion configurations along the <1 1 0> axis, have been found to be approximately identical. Equivalent recording sensitivities are expected from both centers. The F-A1:La2+ and F-A2:Sc2+ defect formation energies are calculated, and the Glasner-Tompkins empirical relation was generalized to include the dopant cations. As far as color image formation is concerned, F-A1:La2+ and F-A2:Sc2+ color centers increase the sensitizing capabilities of primary dyes and the effect of supersensitizers by lowering the bottom of conduction band of the silver bromide surface. The supersensitizer increases the sensitizing capability of a primary dye in the excited state by increasing the relative yield of quantum efficiency. On the basis of quasi-Fermi levels, the difference in the sensitizing capabilities between the examined dyes in the excited states are determined. (C) 2003 Elsevier B.V. All rights reserved.