L. G. Rowan

Electron paramagnetic resonance forbidden transitions from hydrogen in polycrystalline diamond films

Investigation of polycrystalline diamond films by electron paramagnetic resonance at 9.5 and 35 GHZ has revealed the presence of forbidden transitions resulting from a simultaneous microwave induced flipping of unpaired electron spins and environmental nuclear spins. The spacing of the resonance lines identifies hydrogen as the atom neighboring the paramagnetic active site.

Dynamics of photohole trapping in silver chloride

Abstract EPR experiments on photoholes in both AgCl :Cu and AgCl :Pd show the existence of an activation energy barrier, of height near 1.8 meV, in the transition to the localized self-trapped state. The self-trapped hole then migrates athermally via the small-polaron band, for temperatures below 30 K, and by phonon-assisted hopping for temperatures above 35 K. In the hopping regime, the activation energy is 61 meV, suggesting that the binding energy is about 0.1 eV. From the pre-exponential factor, one estimates that the electron transfer integral is about 1% of the energy of the pertinent acoustical phonon. Experiments on AgX:Cu in which 1% of the halide is Br− show that all of the self-trapped holes ultimately migrate to sites with 2 bromide neighbors. Upon annealing, these disappear with complex kinetics and an activation energy of about 180 meV. In AgCl :Pd, Fe, the temperature-dependence of the efficiencies of production of trapped photocarrier centers is determined and correlated with the dynamics ...

Echo envelope modulation with quadrupole interaction

Abstract Echo envelope modulation is calculated which includes quadrupole interaction of the nuclear spin. The strength of the quadrupole interaction must be smaller than the electron Zeeman interaction and the interaction between the electron spin and the applied microwave field during the microwave pulses. But the interactions of the nuclear spin (Zeeman, hyperfine, and quadrupole), can all be comparable since the nuclear Hamiltonian is diagonalized exactly and no perturbation theory is used. The calculations show that the effect of the nuclear quadrupole interaction on the modulation is to create more frequency components and phase changes on the observed modulation as long as the electron-nuclear hyperfine interaction is not negligible.

Low-temperature formation of Fe3+ centres in silver chloride

Single crystals doped with the electron trap Pd2+ and a small concentration of Fe2+ were irradiated with ultraviolet light to produce the trigonal and the cubically symmetric ferric centres. These consist of an interstitial Fe3+ bound to either three or four cation vacancies, respectively. Evidence is presented that the capture of a hole by Fe2+ at low temperature results in the ejection of a neighbouring cation into a more distant interstitial site, and that this process involves an activation energy barrier. The ranges of temperature for efficient production of the two types of ferric centre in AgCl were found to correlate well with the trapping and detrapping of photocarriers at other sites, and with the temperature dependence of the jump frequency of the cation vacancy.

Electron paramagnetic resonance study of hole trapping at defects in silver halides

Abstract Hole-trapping at cation vacancies and iodide impurity in silver halides was studied by means of electron paramagnetic resonance (EPR). In AgCl, the hole was found to be preferentially self-trapped on a Ag ion next to a cation vacancy, forming a perturbed self-trapped hole center. The position of the vacancy relative to the center of the STH was determined, by comparison of computer simulations to the experimental EPR spectra, to be in either the nearest-neighbor or next-nearest-neighbor equatorial position. The thermal trap depths of the hole in these centers are estimated to be 0.31 and 0.24 eV, respectively. For lightly doped (several tens of ppm) AgBr:I, an interesting spectrum has been observed, consisting of six equidistant and approximately equi-intense hyperfine EPR lines, consistent with a trapped hole localized at an iodide ion (nuclear spin 5/2), but not with an iodide-perturbed STH.