The two-fold potential of F2
+ 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 F2
+ defect sites were allowed to relax to
equilibrium in each case. As far as the tunable laser is concerned, F2
+ 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 F2
+. The excited state at
the corner has higher energy than that at the flat or at the edge surfaces. F2
+ 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 s1 silver atoms in the vapor phase was retained, it was destroyed in the crystal environment. The F2
+ 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
F2
+ defect formation, leaving ca. 42% for the reduction of photoelectrons and developer molecules. The rotational
diffusion of the central Ag2 by 901 increases the last percentage to ca. 60%, but is hindered by an energy barrier of
ca. 1.66 eV. r 2002 Elsevier Science B.V. All rights reserved. |
