THE ROLE OF OXIDATION STATES IN LASER LIGHT GENERATION AND COLOR IMAGE FORMATION (II) ANAB INITIOSTUDY OF FA:Hg+AND FA:Hg++COLOR CENTERS AT THE LOW COORDINATION (100) AND (110) SURFACES OFAgBrCRYSTAL

Abstract

The oxidation states of mercury in FA: Hg+and FA: Hg++color centers at the low coordination (100) and (110) surfaces of AgBr play important roles in laser light generation and color image formation. Two simultaneous potentials at these surfaces were 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 oxidation state of mercury, 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 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 FA: Hg+and FA: Hg++centers are suitable laser defects. The dependence of the orientational destruction, recording sensitivity and exciton (energy) transfer on the oxidation state of mercury and the coordination number of the surface ion on 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 Hg++surfaces of AgBr are significantly more sensitive than the corresponding surfaces. On the basis of quasi Fermi levels, the difference in the sensitizing capabilities between the examined dyes in the excited states is determined.