Myra Toffolon Olm

Electron nuclear double resonance spectroscopy

Precise information about the molecular structure, stereochemistry, and environment of paramagnetic species can be obtained by electron nuclear double resonance (ENDOR) spectroscopy. This technique has been applied in a wide range of disciplines to liquid-phase, single-crystal, and powder samples. In some cases—the study of defects in ionic single crystals, for instance—the volume and complexity of data obtained by ENDOR can hinder interpretation. Such difficulties have been overcome by the use of supplemental ENDOR techniques that simplify the assignment of ENDOR lines. The increased use of computers for the automation of instrumentation, the design of experiments, and the analysis of data has made possible the study of a wider range of problems. With these improvements, as well as with the increased sensitivity provided by optically detected ENDOR, it is now feasible to study polycrystalline and amorphous materials, such as thin-film semiconductors and biological samples in vivo.

Defects in the silver halide photographic process

The imaging efficiency of todays photographic film and paper is influenced in a variety of ways. Among them, the incorporation of dopants is widely used to increase efficiency, control contrast or improve imaging deficiencies such as reciprocity law failure. Transition metal-organic ligand dopants are attractive because the organic ligand influences the photographic properties. Experimental studies have shown that many of these dopants incorporate well into the silver halide microcrystal even with large organic ligands. In this paper, experimental and computational results are presented on a variety of Ir-OL dopants in AgCl where OL is a small nitrogen and/or sulfur-containing heterocycle. Electron paramagnetic resonance methods provide information about the electronic structure, electron trapping properties and the stability of the electron trap state. These results are complemented by ab initio studies that give information about the optimum structure, charge compensation and the possibility of electron and hole trapping.

Defects and the photographic process

Abstract The high imaging efficiency of todays color negative film depends on the unique defect properties of silver halides. We have chosen two important defect topics to highlight: defect-induced anisotropic crystal growth, and shallow electron trapping at surface defects and transition metal dopant sites. High imaging efficiency depends on the ability to concentrate photoelectrons and thereby to form a single latent image center per crystallite. This requires electron localization at a partially-charged, shallow electron trap at the surface followed by a shallow—deep transition to a metastable atom state. Subsequent ionic and electronic trapping processes that build the latent image occur only at this site. In some cases, doping the bulk of the microcrystal with extrinsic shallow electron traps can improve latent image formation efficiency. Electron concentration does not occfur at these dopant sites since they are not partially charged. Magnetic resonance and photoconductivity studies of the dynamics...

Photo-EPR studies of electron and hole trapping by [Fe(CN)6]4- complexes in silver chloride

Abstract [Fe(CN)6]4- was incorporated into silver chloride powders precipitated from aqueous solution with all six cyanide ligands intact. The most notable feature of this dopant was its amphoteric behavior, acting as either a deep hole trap or a shallow electron trap. The filled t2g orbital manifold of this complex lies below the Fermi level, which places the vacant eg levels well above the conduction band minimum. The trapping behavior of [Fe(CN)6]4- was determined by whether or not it was closely associated with a charge-compensating silver ion vacancy.

The influence of point defects on the photophysics of the silver halides doped with Re3+ complexes

Abstract An EPR study of silver halide single crystals and emulsions doped with Re3+ complexes has been completed. Deep electron trapping by the dopant is facilitated by the presence of substitutional Re3+ sites with less than their complete complement of charge compensating silver ion vacancies.