The role of FA(I):Tl+ and FA(II):Ga+ defects in laser light generation and photographic sensitization at the low-coordination surface sites of AgBr first principles calculations

Abstract

Abstract FA(I):Tl+ and FA(II):Ga+ color centers at low coordination surface sites of AgBr thin films play important roles in providing tunable laser oscillation and photographic sensitization. Double-well potentials at these sites are 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 FA defect site were allowed to relax to equilibrium in each case. The calculated Stokes shifted optical transition bands suggest that laser generation is too sensitive to the simultaneous effects of (i) the dopant cations size (ii) the coordination number of the surface ions, and (iii) the choice of the basis set centered on the Br vacancy. An attempt has been made to explain these results in terms of Madelung potentials, electron affinities 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 FA(I):Tl+ and FA(II):Ga+ are suitable laser defects. 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 be directly proportional to the size of the dopant cation, and inversely proportional to the surface coordination number. For optical memories, a high recording sensitivity was assigned to FA(II):Ga+ relative to FA(I):Tl+ and to the flat surface relative to the edge and corner surfaces. The possibility of exciton (energy) transfer between sites of different coordination numbers was clarified. The relaxation and defect formation energies were calculated. The Glasner–Tompkins empirical relation was generalized to include the dopant cations and surface coordination. As far as photographic sensitization is concerned, the highest occupied molecular orbital of only one out of three acyclic developers was found to be high enough for electron transfer to the Fermi level of the silver bromide thin film. The FA defect formation and rotational diffusion of the central silver cluster broaden the energy gaps between the highest occupied molecular orbitals of the developer molecules and the lowest unoccupied molecular orbitals of the defect containing surfaces and make the process of electron injection less facile. About 59% of the reduction of silver ions at the flat surface of silver bromide is attributed to the host anions and FA defect formation, leaving about 41% for the reduction of photoelectrons and developer molecules. The unrelaxed rotational diffusions of the central silver clusters by 30° decrease the latter percentage to about 10%, but are severely hindered by activation energy barriers.