Multifrequency electron paramagnetic resonance and electron-nuclear double-resonance studies of photo-hole processes in AgBr and AgCl emulsion grains

By using a combination of multifrequency EPR spectroscopy, ENDOR spectrosco py and calculations of structure and energy, the reactivities of photo-gene rated holes in microcrystalline AgBr and AgCl dispersions (photographic emu lsions) have been followed in detail. Progress has been facilitated by the use of both gelatin and polyvinyl alcohol (PVA) as peptizers. The initial t rapped hole centres produced by band-gap excitation have been identified. I n AgBr, this species is [(Br-4)(3-).V], a neutral complex formed from hole trapping by the four nearest neighbours of a surface Ag+ vacancy (=V). [(Br -4)(3-).V] reacts with gelatin to produce a transient organic radical at th e grain's surface. It does not, however, react with PVA. The formation of t he oxidized gelatin radical might involve atomic bromine as an intermediate . In AgCl, the well-known self-trapped hole centre (AgCl6)(4-) is me initia l hole species. The hole diffuses by an electron exchange process until it is trapped by a silver ion on the grain's surface or within its penultimate layer of lattice ions. It is subsequently released from this Ag2+ site to be retrapped at a centre containing four equivalent Cl- ions. The precise i dentity of this defect has yet to be determined, but its decay also results in the oxidation of gelatin.