Computational study of sulfur sensitizing centers on AgBr

The structure and properties of a variety of sulfur centers are calculated using an embedded cluster method with the density functional method. Models representing adsorbed silver sulfide molecules and compensated surface and bulk substitutional monomer and dimer sulfide are treated. The molecule Ag 4S2 partially incorporated into the surface is calculated to have an electr on trapping depth of several tenths of an electron volt and a lowest single t absorption in the long-wavelength range, both of which are consistent wit h reported experimental measurements of sensitized surfaces. Substitutional dimeric sulfide units incorporated into the surface or bulk and compensate d by interstitial silver ions were not found to trap electrons, but holes c ould be trapped at this center. The ions at a positive kinks containing sub stitutional sulfide were found to be displaced upon electron trapping indic ating a strong coupling of the electron to the crystal lattice. The energy levels, structure and absorption energies calculated for these and related centers are presented and discussed.