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The products from a gaseous r.f. electric discharge in ammonia have been studied using electron spin resonance spectroscopy. A paramagnetic species from the discharge was detected and the results were consistent with it being the imine (NH) free radical. The E.S.R. Spectrum fitted the theoretical treatment and was similar to that observed for NH in a frozen matrix. No other radicals could be detected in the discharge except for the atomic species. The low pressure gas phase E.S.R. spectrum of (formula omitted) was also studied. The results showed that at low pressures the spectrum becomes exceedingly complex. A partial assignment of lines was made from the results of a simplified treatment of the theory. The work served a further purpose in developing experimental techniques for the study of unstable free radical species in the gas phase. / Science, Faculty of / Chemistry, Department of / Graduate

The purpose of this investigation has been to make a comparison of the experimental and theoretically calculated electron spin resonance spectra of nitrogen heterocyclic radical ions. Because of the recent nature of the field, and because of recent advances in theoretical aspects, the theory of ESR and of the calculation of spin density distributions is reviewed in some detail in Chapter I. In Chapter III the ESR spectra of methyl substituted pyrazine and pyridine anions prepared by chemical reduction with alkali metals are described. Huckel and McLachlan calculations have been performed using various theoretical models and it has been found that presently used simple theories of spin density distributions give a reasonably good explanation of the ESR spectra of these aromatic radical ions. The modified ESR spectra of the ion-pairs of methyl substituted pyrazine, pyridine, and benzene anions with alkali metal cations are examined in Chapter IV. Factors influencing the stability of the ion-pairs are discussed and the theory of alkali metal hyperfine interaction is considered in some detail. It is found likely that in future work attention will have to be directed at the estimation of the energy of excitation from the anion to the cation. Finally, the ESR spectra of some aromatic heterocyclic N-oxide anions generated electrolytically are examined in Chapter V and the results of Huckel and McLachlan calculations of the spin density distributions in the ions are discussed. Because of the perturbed nature of the molecules agreement with the experimental distributions is found to be not quite as satisfactory as with the pyrazine anions. The mechanism of the nitrogen hyperfine interaction in N-oxide anions is investigated and it is suggested that this interaction is overwhelmingly due to the spin density on the nitrogen itself. This last aspect of the ESR spectra of N-oxide anions awaits more complete experimental confirmation. / Science, Faculty of / Chemistry, Department of / Graduate

The electronic g-tensor in halogen-doped cadmium sulfide has been measured at 1.7°K, and its dependence upon uniaxial pressures parallel and perpendicular to the c-axis has been investigated. A sample doped with iodine to a room temperature resistivity of ≈ 0.7Ω cm exhibited an anisotropic g-tensor with gǁ = 1.785 and g⊥ = 1.767. The anisotropy in a heavily bromine-doped sample [formula omitted] was the same, but gǁ was slightly larger, 1.789. When some of the cadmium is replaced by zinc, the alloy acquires properties intermediate between those of CdS and ZnS. This was shown to apply to the g-tensor by measuring it in CdS(10%Zn):I. It was found to have a smaller anisotropy and a larger absolute value than that of CdS:I, thus having changed towards that of ZnS. The pressuredependence of the g-tensor and the change in the optical bandgap under hydrostatic pressure could be reasonably well explained in terms of two deformation potential constants for the bandgap, D₁-C₁-C₃ ≈ 6.3 eV/unit strain and D₂-C₂-C₄ ≈ 1.8 eV/unit strain. It is therefore concluded that although the bandstructure of CdS is not sufficiently well known to predict the g-tensor correctly, the interaction of the conduction band with the valence bands appears to be dominant. Further experiments to study the conduction band edge are proposed. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Paramagnetic resonances were observed in natural single crystals of anatase between 1 °K and 1230 °K and are interpreted as due to regular substitutional Fe(3+) (I) and to Fe (3+) combined with an oxygen vacancy at a nearest neighbour site (II) . The spin Hamiltonian parameter b(2)° of (I) decreases from +457 x 10(-4) cm(-1) at 1 °K almost linearly to -225 x 10(-4) cm(-1) at 1230 °K. This unusually strong temperature dependence of b(2)° and the observed temperature dependence of the orientation of the magnetic axes of spectrum (II) are both explained by assuming that the positions of the oxygen ions within the unit cell are temperature dependent. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Optical Detection of Magnetic Resonance (ODMR) has been used to observe both paramagnetic and diamagnetic resonance of photo-excited electrons and holes in GaP, ZnTe and AgBr. Paramagnetic resonance of conduction electrons in GaP has been studied and the microwave frequency and power dependence of the effect analysed. The maximum signal strength was observed to produce approximately 1% change in luminescence at 1.6 K. The g value deduced from the resonance was 2.000 ± 0.005. The resonance was homogeneously broadened giving the electron lifetime as approximately 4 nanoseconds. Paramagnetic resonance of electrons and holes has also been detected in AgBr. The background signals present in ODMR experiments have been investigated and are shown to be caused by diamagnetic or cyclotron resonance heating of photoexcited carriers. Measurements at microwave frequencies of 9.2 and 36.3 GHz have been made on GaP,ZnTe and AgBr, and cyclotron resonance of electrons and holes observed. The effective masses of light and heavy holes in GaP were found to be 0.154 ± 0.01 and 0.626 ± 0.06 respectively while the electron effective mass was 0.36 ± 0.10. The electron scattering time was shorter than that for holes by a factor of approximately three, most likely due to scattering by isoelectronic nitrogen impurities. Resonances were observed in ZnTe at effective mass values of 0.30 ± 0.20 and 0.76 ± 0.20 corresponding to electrons and heavy holes. In both GaP and ZnTe resonances due to electrons and holes appeared in different luminescence bands indicating the sensitivity of different recombination centres to heating of either carrier type. Cyclotron resonance of electrons and holes was also observed in AgBr and showed the effects of conduction and valence band non-parabolicity. A feature in the electron resonance indicated enhanced trapping of electrons with certain energies by emission of one or more LO phonons. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The Green's function method is applied to investigate the magnetic spin resonance spectra of three-dimensional and two-dimensional spin-polarized quantum gases. The Hartree-Fock approximation is employed to calculate the one-particle Green's function of the atoms, then this one-particle Green's function is used for the calculation of the vertex part of the Green's function. Such a combination yields a self-consistent result. The absorption spectra are obtained from the calculation of the susceptibility in terms of the two-particle Green's function (bubble diagram). Some general expressions for the dispersion relation, for the effective mass of a spin wave, and for the dipolar frequency shift are given in the calculation. In order to estimate the shift of the electron-spin-resonance (ESR) frequency, the effective dipole-dipole interactions among the hydrogen atoms are included in the calculation. These effective interactions are deduced from the ladder approximation, and hence are characterized by the scattering amplitude. The scattering amplitude is calculated numerically. The result shows that the theoretical value of the shift is smaller than the experimentally observed value by about 35%. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

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