Raymond S. Eachus

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.

Radiation-produced electron and hole centres in oxygen-containing BaFBr. I. EPR and ODEPR studies

An O- centre formed upon hole trapping by O2- in X-irradiated BaFBr is long-lived at room temperature and has been detected by EPR below 30 K. It can also be produced in low concentrations by exposure of BaFBr to sub-bandgap (200-300 nm) radiation. The formation of F(Br-) centres at a charge-compensating bromide ion vacancies is a corollary of the photoionization of O2-. High concentrations of oxide have been successfully introduced into this material by doping with anhydrous BaO, including Ba17O for unambiguous identification of the O- defect. Oxide addition and the formation of O- produces a series of optical bands that have been assigned using a combination of optical and magneto-optical techniques. The influence of the material preparation conditions on the concentration of O2- is discussed and possible mechanisms for the formation of O- are proposed.

Photo-stimulated emission of X-irradiated BaFBr:Eu

Abstract The photo-stimulated luminescence (PSL) of Eu2+-doped BaFBr is excited in the two F centre absorption bands produced by X-irradiation at room temperature. The PSL intensity decreases with decreasing temperature by about a factor of 10. After bleaching at low temperatures the PSL is nearly zero, it can be regenerated by warming the crystal to room temperature. The results are discussed with a new model of the PSL process.

Electron nuclear double resonance study of pure and aquated rhodium complexes in silver chloride emulsions

Detailed information has been obtained on the structure of pure and aquated complexes in AgCl using multifrequency EPR and powder ENDOR spectroscopy. Because of the anisotropy of the centre g matrix it was possible to selectively excite centres with a specific angle between the z axis of the g matrix and the magnetic field axis. As a result, almost single-crystalline-like ENDOR spectra were obtained and the angular dependence of the ENDOR spectra could be obtained by scanning the magnetic field over the range of the EPR spectra. Three different centres were detected that are distinguished by different thermal stabilities, different parameters of the g matrix and the hyperfine matrices of the central Rh nucleus and ligand H, Cl and Ag nuclei. A pure centre without ligands could only be generated in an emulsion where great care was taken to prevent the aquation of the dopant ion in the dopant solution and during the precipitation. This centre had the highest thermal stability. In an emulsion prepared conventionally two centres were found that are associated with one and two ligands bound to the ion in the plane perpendicular to the g matrix z axis. Using the results of the EPR and ENDOR analysis in combination with the results of a total energy calculation two models are proposed that contain the positions of the ligands and the charge compensating vacancies.

The possibility of a split interstitial silver ion in AgCl

Hartree-Fock calculations treating the normal and split interstitial geometries for the interstitial silver ion at low temperature in AgCl have been performed. Several different embedded cluster models having different sizes, basis sets, and treatments of lattice polarization and electron correlation were studied. We found that the split interstitial geometry was favoured by up to a few tenths of an electronvolt, with the greatest relative stability occurring with the most extensive basis sets and with treatment of electron correlation. This species has a calculated bond length of 2.36 AA and is not a deep electron trap in AgCl.

Optically detected magnetic resonance of crystalline AgCl

Abstract The intrinsic ODMR spectrum of AgCl is due to holes trapped at tetragonally distorted Ag + sites, recombining with electrons trapped at unidentified substitutional sites which are similarly distorted. Bromide ions induced characteristic changes in the spectrum.

The structure of the shallow electron centre in silver chloride

The results of an ENDOR study of electrons shallowly trapped at complexes doped in AgCl emulsions reveal that the dopant replaces a lattice unit without an additional lowering of the local symmetry and without the association of charge-compensating cationic vacancies. Effects of the grain structure seem to interfere in the interpretation of the results from -enriched -doped emulsions, for which an ensemble of slightly different electron centres is believed to be observed.

Investigation of the storage phosphor BaFBr:Eu by multiple magnetic resonance techniques

Abstract Spatial correlation between the activator ion Eu 2+ and electron and hole traps generated by X-irradiation is thought to play a decisive role in the photostimulated emission process of the storage phosphor BaFBr:Eu 2+ . We report the first direct experimental evidence for correlations among F centers, hole centers and Eu 2+ obtained by measuring cross-relaxation effects in optically detected electron paramagnetic resonance. BaFBr treated with the known procedures for removing oxygen still retains 50 ppm or more of O 2− . This ion is critically involved in the formation of F and hole centers.

Radiation-produced electron and hole centres in oxygen-containing BaFBr. II: An ENDOR study of OF-

For pt.I see ibid., vol.3, p.9327 (1991). Oxygen contamination introduces hole-trapping centres into barium fluorohalides. In the preceding paper, an O- centre was reported as the product of the reaction between out-of-plane Br2- VK centres and oxide impurities. This paper describes an ENDOR study of this centre which identifies it as OF-, an oxygen ion substituted at a fluoride site. No evidence has been found for the presence of a neighbouring defect (anion vacancy or interstitial cation) remaining from its charge-compensating role for the proposed precursor OF2-.

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.