F 2+ laser activity and photographic sensitization at the low coordinated surfaces of AgBr: ab initio calculations

Abstract

The two-fold potential of F 2 + center at the low coordinated surfaces of AgBr thin films in providing tunable laser activity and photographic sensitization is investigated using ab initio methods of molecular electronic structure calculations. The explicitly considered clusters were embedded in simulated Coulomb fields that closely approximate the Madelung fields of the host surfaces. The neighbor ions nearest to the F 2 + defect sites were allowed to relax to equilibrium in each case. As far as the tunable laser is concerned, F 2 + laser activity increases significantly as the bromine coordination decreases from 5 (flat) to 4 (edge) to 3 (Br − corner). The smallest calculated Stokes-shift at the flat surface suggests that emission has the same oscillator strength as absorption. All relaxed excited states (RESs) are deep below the conduction bands of the perfect ground state surfaces. More than one band are expected for edge and corner surfaces. The Br − corner of AgBr is the least probable site for RES orientational destruction of F 2 +. The excited state at the corner has higher energy than that at the flat or at the edge surfaces. F 2 + can be readily formed at the bromine corner relative to the flat and edge surfaces. The Glasner–Tompkins relation is generalized to include the reduced bromine coordination at the surfaces of the AgBr host. As far as photographic sensitization is concerned, while the alternating pattern of s 1 silver atoms in the vapor phase was retained, it was destroyed in the crystal environment. The F 2 + defect containing surface of AgBr is developable with a minor activation energy barrier to electron transfer from the developer. Defect mobility makes electron transfer from the developer more facile in the process of visible image formation. About 58% of the reduction of silver ions at the flat surface of AgBr are attributed to the host anions and F 2 + defect formation, leaving ca. 42% for the reduction of photoelectrons and developer molecules. The rotational diffusion of the central Ag 2 by 90° increases the last percentage to ca. 60%, but is hindered by an energy barrier of ca. 1.66 eV.