The role of type II FB (I):Tl+ defect in laser light generation and color image formation at the low coordination surface sites of AgBr: Ab initio calculations

Abstract

The role of type II FB (I):Tl+ color center at low coordination surface sites of AgBr thin films in providing tunable laser activity and photographic sensitization is examined. The double-well potential at this site is investigated using ab initio molecular electronic structure calculations. Clusters of variable sizes were embedded in simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces, and ions that are the nearest neighbors to the FB defect site were allowed to relax to equilibrium in each case. The calculated Stokes shifted optical transition bands suggest that laser activity is relatively weak and fades quickly as the coordination number of the surface ions decreases from 5 (flat) to 4 (edge) to 3 (corner). An attempt has been made to explain these results in terms of Madelung potentials and optical–optical conversion efficiencies. 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 indicating that type II FB(I):Tl+ is suitable laser defect. The probability of orientational destruction of the two centers, attributed to the assumed saddle point ion configurations along the 〈1 1 0〉 axis, was found to decrease as the coordination number of the surface ions decreases. The possibility of exciton (energy) transfer between sites of different coordination numbers was clarified. The Glasner–Tompkins empirical relation was generalized to include type II FB (I):Tl+ doped surfaces. As far as photographic sensitization is concerned, a supersensitizer increases the sensitizing capabilities of the two examined dye molecules by increasing the relative yield of quantum efficiency Φ. FA sensitizes the low coordination surface sites of the defect free AgBr by lowering the bottoms of the conduction bands. The difference in the sensitizing capabilities between the two examined dyes was estimated by calculating the quasi Fermi levels.