No description available.

Hydrazoic acid has been photolysed in a krypton matrix at 4°K and the products studied by electron spin resonance spectrometry. This study showed that NH₂ radicals have been produced as a secondary product in the reaction. In addition a broad, intense resonance at g = 2 and a weak, half field resonance has been tentatively assigned to the imine (NH) radical. This suggestion is shown to be consistent with theoretical considerations. Diazomethane has also been photolysed in krypton and carbon monoxide matrices at 4°K, and the products studied by e.s.r., in an attempt to detect the methylene (CH₂) radical. The results of this study were complicated and a complete analysis was not possible. It is suggested that some of the features may be explained in terms of an overlap of the spectre from CH₃ and CH radicals. There are difficulties in such an explanation however. A study has been made on the line shapes of poly-crystalline samples of aromatic triplet state molecules. It is shown how the experimentally observed spectra of the photoexcited triplet states of axially symmetric molecules such as triphenylene and non-axially symmetric molecules such as naphthalene and phenanthrene can be explained in terms of a line shape calculated from a first order perturbation treatment. This model gives a good explanation of the observed line shapes, however better agreement with the observed field positions is obtained if a second order correction is included. Line shape calculations have also been made for molecules, such as the substituted imines, where spin-spin interaction is large. It is shown that the calculation is in agreement with the observed spectra of phenylimine and benzenesulfonylimine. In addition, it is shown how the experimentally determined value of the spin-spin interaction constant, D, can be related to the spin density on the nitrogen of the substituted imines. The spin densities calculated in this way are in good agreement with spin densities calculated on the basis of the Hückel theory. / Science, Faculty of / Chemistry, Department of / Graduate

We have determined that the temperature for desorption of gallium oxide from GaAs increases linearly with oxide thickness, for oxide layers between about 6Å and 26Å thick. The temperature for the oxide desorption ranged from 580°C to 630°C. The wafer temperature was determined from the optical band-gap measured from the diffuse reflectivity of the sample, which was polished on the front surface and textured on the back surface. Different thicknesses of oxide layers were created by varying the exposure time of the GaAs wafers to a low pressure oxygen plasma. The oxide thicknesses were determined by XPS analysis. Desorption experiments were carried out in a VG V80H MBE system under a As4 beam equivalent pressure of 1.5 x 10⁻⁵ ton. Measurement of diffuse light scattering using a HeNe laser shows an abrupt and non-reversible increase in the scattered light intensity during the oxide desorption. This suggests the surface is macroscopically roughened due to inhomogeneous desorption of the oxide. The oxide desorption was also studied by monitoring the secondary electrons produced by the high energy electrons from the RHEED gun. After the gallium oxide desorption there is a reversible, order of magnitude, increase in the number of scattered electrons produced by the incoming primary beam. We interpret this result as evidence for some form of microscopic roughening. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Optical signal processing is introduced as a tool for investigations in the field of television compression research. An optical signal processing system is designed, which performs the Fourier transform of a picture signal F[B(x,y)] and its reconstruction F⁻¹ {F [B(x,y)]} . Some basic optical filtering experiments are performed in the spatial frequency plane, and the optical analogue of the frequency sampling theorem is demonstrated. The Fourier transforms of test pattern pictures show large gaps which can be used for compression. Observation of complex spectra of continuous tone pictures is found to be impaired by noise effects. A physiological experiment is carried out, which investigates the relationship between tolerable flicker frequency and spatial frequency of a television picture. It is found that the tolerable flicker rate f decreases as the spatial frequency fx is increased, according to the empirical equation fc = fo exp(-kfx). fo and k are parameters depending on factors like contrast ratio, kind and size of picture, etc. Compression systems using the above results are found to have a limit of obtainable compression ratio of approximately 3 to 1. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

The performance of a spin echo spectrometer which is suitable for chemical exchange studies is described. Using the Carr-Purcell sequence of pulses, proton T₂ values were obtained from two experiments differing only in their method of eliminating accumulated error in the width of the 180° pulses. The Meiboom-Gill method of phase shifting the r.f. in the first pulse is more flexible in the range of pulse intervals. Following the theory of Bloom, Reeves and Wells, rate constants describing the exchange process in two molecules were measured from the dependence of T₂ upon the pulse interval. The values of the rate constants for the hindered internal rotation about the N-N bond in N, N-dimethylnitrosamine agree with previous high resolution studies. A similar spin echo study has been conducted in order to measure the rates of internal rotation about the C-N bond in N, N-dimethylcarbamyl chloride. The values for the entropy of activation are consistently low. The possibility of systematic errors in the spin echo method has been investigated. It is concluded that reported values of rate constants in magnetic resonance should be the result of a spin echo investigation extending over as wide a temperature range as possible in addition to a high resolution study which involves a complete theoretical line shape fit to the experimental data. / Science, Faculty of / Chemistry, Department of / Graduate

The magneto-optical Faraday effect has been used to observe the spin-bath relaxation at low temperatures in CeES and in Eu-doped CaF₂. The paramagnetic Faraday rotation ϩ is an instantaneous measure of the magnetization M and it is shown that in CeES and Eu²+ : CaF₂, ϩ∾ M at the light frequencies employed. The apparatus is the same as that previously described by Rieckoff and by Griffiths. Pulsed MW-power at X-band has been used to disturb the equilibrium between spin system and bath. In CeES, the observed relaxation time Շ is of the order of a few msec, which is several orders of magnitude longer than the theoretical estimate of T₁. This suggests a severe bottleneck in the energy transfer spin-bath. Շis found to be environment-dependent. In HeII, the relaxation is exponential. Շ is in good agreement with nonresonant relaxation measurements by Van den Broek and Van der Marel. It is explained as arising from the Kapitza boundary resistance at the CeES-HeII interface. In HeI, the relaxation is non-exponential and is slower than in He gas at the same temperature. This suggests that in this case the thermal diffusion in the helium around the crystal is the bottleneck. The same relaxation behaviour is found when the crystal is heated dielectrically with MW power far off resonance. This supports the assumption that the energy transfer spin-bath is limited by spatial diffusion. If the crystal is surrounded by a He film at a temperature below the λ-point, Շ is found to be the same as in HeII up to a well defined average MW power level. For higher powers the relaxation behaviour is similar to that of CeES immersed in HeI. In Eu²+ ; CaF₂, T₁ is expected to have the form T₁= AT + BT⁵. The observed relaxation time Շ , however, is found to be concentration dependent. All measurements have been done on the +½⇢ -½ transition with H II [100]. For the three lowest concentrations, the temperature dependence of Շ from 1.5 to 4.2°K can be fitted with the expression Շ ¯¹=CT with C = 2.75 (sec°K) ¯¹ for 0.02% Eu, C = 3.5for 0.8% and C = 5 for 0.2%, Շ is shorter and Շ ¯¹ ∾ T ² from 1.5°K to 7°K. The concentrations given correspond to the total Eu content. The Eu²+ concentration has been inferred from the magnitude of the saturation rotation. Շ(T) seems to depend on both Eu²+ and Eu³+ concentrations. It is suggested that exchange coupled pairs of Eu²+ and clusters involving Eu³+ may account for the concentration dependence of Շ. Upper limits of A = 2.5 and B = 5 x 10¯⁵ are found for T₁ by extrapolating the lowest concentrations investigated. These values are somewhat lower than both measured and calculated values found by Huang. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Electron paramagnetic resonance (EPR) signals from tuna ferricytochrome c solutions were obtained between 4.2°K and 77°K, with g-values g(1) = 1.25, g(2) = 2.25, g(3) = 3.05« The g(3) line is 380 gauss wide between 4.2°K and 50°K with Gaussian shape, but has become 700 gauss wide with Lorentzian shape at 77°K. The temperature independent shape and width are best explained by a distribution of rhombic crystal field potentials (r.m.s. deviation = 11%). The Lorentzian shape arises from a short (10(-8) sec.) spin-lattice relaxation time. EPR spectra from horse heart ferricytochrome c single crystals were analysed to obtain the orientation of the g-axes relative to the crystallographic axes. The g(3)-axis was 76° from the crystal c-axis, close to the heme normal (71.5° to c-axis) determined from the 3-dimensional X-ray structure by Dickerson. The other 2 g-axes lay approximately along the N-Fe-N directions in the heme ring. An amended version of Eisenberger and Pershan's theory was used to explain the angular variation of the broad lines (300-2000 gauss) seen in the crystals—best fit was obtained with the distribution of ligand fields from the solution study plus a 1.5° variation in g-axis orientation. The undifferentiated absorption line shapes observed at 4.2°K in both solutions and single crystals were explained by the Portis theory of rapid adiabatic passage in solids. This theory was tested with a model system of charred dextrose, and found to be valid. Using the theory the relaxation time (τ) of the cytochrome c system was found to be, from the phase lag of the EPR signal relative to the magnetic field modulation, 3.8 x 10(-6) sec. at 4.2°K. τ was obtained between 4.2°K and 18°K from the rapid passage signals, and between 50°K and 70°K from the linewidth of the spectra. The temperature dependence of τ below 20°K could arise from a combination of a T(9) Raman spin-lattice relaxation process with a temperature independent spin-spin relaxation time of order 10(-8) seconds (which might arise from dipolar interactions between neighboring iron atoms). / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Previous measurements on CdS (I) have shown a temperature shift in the single electron spin resonance (ESR) impurity band line. At 34 GHz this shift was found to be: dg= - 2.5 x 10ˉ ⁴/°K dT in going from 4.2°K to 1.7°K. This thesis presents the construction and testing of a metal temperature controlled dewar for the purpose of measuring the shift over the temperature range from 40°K to 1.7°K using an X-band (9 GHz)spectrometer. The signal was observed between 1.7°K and 4.2°K but rapidly broadened and could not be seen at higher temperatures. Since the lowest attainable temperature of the metal dewar was around 5°K, it could not be used as planned. Further studies using glass dewars proved that the g-shift at 9 GHz was too small, to be measured. The theory for the g-shift is discussed and the performance of the dewar evaluated. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

A cryostat has been built for cooling specimens to temperatures of the order of a hundredth of a Kelvin by thermal contact with an adiabatically demagnetized paramagnetic salt pill. The apparatus was designed for performing nuclear orientation experiments. This thesis describes the construction of the apparatus and experimental tests studying the nuclear orientation of (60)Co in an iron plate. The paramagnetic salt used was chromium potassium alum in an alum-glycerine slurry. In addition to the chrome alum pill a guard pill of manganous ammonium sulphate was used between the alum pill and the 1K helium bath. The pills were supported and thermally isolated by German silver spacers. A copper heat link was embedded in the alum-glycerine slurry and soldered to the specimen to provide thermal contact. Several heat links were used ranging from a bundle of five thousand copper wires to a copper foil "concertina" arrangement. A Ventron niobium-titanium superconducting solenoid which produced fields up to 48 kilogauss was used for the magnetic cooling. A superconducting polarizing solenoid was used to magnetically saturate the polycrystalline iron plate. Anisotropies in the gamma radiation intensity from (60)Co of 7 to 11 per cent corresponding to temperatures of 37 to 45 m K were observed. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The electronic g-tensor in antimony doped cadmium sulphide has been measured at 1.1°K. A sample doped to a room temperature resistivity of 3.3 ohm-cm exhibited an anisotropic g-tensor with g// = 1.788 and g⊥ = 1.770. A single asymmetric line was observed. The asymmetry of this line was found to decrease with decreasing incident power. Further experiments to study this line shape are indicated. / Science, Faculty of / Physics and Astronomy, Department of / Graduate