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<p> Single crystals of sodalite have been studied by means of electron paramagnetic resonance (E. P.R.) both at 0.8 cm. and 3 cm. wavelength. The existence of at least four different E. P. R. spectra was established of which one was attributed to the manganese impurity and a possible model for two other spectra is proposed. It was found that two of the observed spectra disappear upon heat treatment although the bleaching of the characteristic blue colour of sodalite crystals does not appear to be related to any of the spectra studied. </P> / Thesis / Master of Science (MSc)

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This thesis is concerned with the design and realisation of components for a new state of the art 94GHz Electron Spin Resonance (ESR) spectrometer capable of operating in both pulsed and CW modes. The complete spectrometer is designed to provide phase coherent 1kW peak power sub-nanosecond π/2 pulses having variable duration and repetition rate. The mm-wave response of a paramagnetic sample to these pulses is detected with a superheterodyne detector. Such a system would offer a step change in performance, promising unprecedented resolution and sensitivity. These aims should be compared with the performance of commercial (Bruker) instruments capable of delivering 200mW 30ns π/2 pulses. For this type of system, both the long term (thermal) and short term (phase) stability of oscillators and sources employed are extremely important. Consideration of phase noise, frequency, tunability and power output shows that multiplied sources offer substantial benefits compared to fundamental sources. A delay line discriminator method of phase noise measurement, suitable for use with the low frequency oscillators is described and implemented. This is extended to 94GHz using a down convertor with a quasi-optically stabilised Gunn oscillator. These tools are used to select an optimum oscillator-multiplier combination to produce a low noise 94GHz source. Anew method of pulse generation, which has produced +23dBm peak power 250ps rectangular and 115ps Gaussian envelope phase coherent pulses, is described. These are believed to be the shortest phase coherent pulses at 94GHz available. This system will be used to provide ns pulses suitable for amplification to 1kW using a Klystron amplifier. A heterodyne detector has been constructed which employs the same oscillator/multiplier techniques identified above to produce the required local oscillator signal. It is demonstrated that by careful consideration of multiplication factors a system employing one variable and one fixed oscillator allows all the signals required in the spectrometer to maintain phase coherence. It is demonstrated that the complete demodulator responds to pulses on a ns time scale and has a noise temperature of 737K.

Call number: LD2668 .T4 1985 N64 / Master of Science

Magnetic resonance force microscopy (MRFM) is a new measurement technique combining scanning probe microscopy (SPM) and MR spectroscopy, offering the potential of high resolution chemical specific imaging. MRFM is based on the principle of force detection of magnetic resonance (FDMR) in which the magnetisation of a sample in a magnetic field is coupled to an atomic force microscopy cantilever via a field gradient. Magnetic resonance is used to modulate the sample magnetisation at the cantilever resonant frequency and the resulting oscillating force on the cantilever leads to oscillations which may be detected optically. The high sensitivity of force detection offers the potential for single electron spin sensitivity. This thesis describes instrumental aspects of ESR based FDMR experiments and presents the first results at high fields (3.3T). High fields are advantageous for sensitivity and spectral resolution. However, they pose significant technical challenges. FDMR measurements on the organic conductor (fluoranthene)2PF6 were carried out in experiments based around an existing quasi-optical high field ESR spectrometer. Further measurements on (FA)2PF6 and DPPH are presented together with progress towards the construction of a high field MRFM system, based on a commercial SPM instrument. Experiments were performed with both magnet-on-cantilever and sample-on-cantilever configurations with the former the favoured method for potential imaging applications. Signal detection uses a novel fibre-optic interferometer. Cantilever magnets of low conductivity ferrite appear to be more promising for high Q measurements than the metallic magnets favoured by most other groups. Experiment sensitivities are estimated at around 4.4 x 10⁸ polarised electron spins, comparable to conventional commercial ESR spectrometers. Experimental consistency was difficult, especially regarding the positioning of probe and sample, an area in which refinement is essential for repeatable and sensitive experiments. The potential for imaging is attractive and the prospect of single spin detection is discussed.

The addition of benzyltrimethylsilane to maleic anhydride mediated by TiO2 photocatalysis was initially investigated. The affect of changing the catalyst, the radical trap loading and the substitution of the benzyltrimethylsilane molecule was assessed. Cyclisation precursors based on benzyltrimethylsilane were prepared, but were found not to cyclise via TiO2 photocatalysis. A number of other systems were assessed for their ability to cyclise under TiO2 photocatalysis; tertiary amines, aminomethyltrimethylsilanes, phenoxymethyltrimethylsilanes and phenoxyacetic acids. Phenoxymethyltrimethylsilane and phenoxyacetic acid were found to add effectively to maleic anhydride under TiO2 photocatalysis conditions, however they were unreactive with regards to cyclisation. EPR spectroscopy has been employed to characterise further the reaction of benzylsilanes with maleic anhydride under TiO2 photocatalytic conditions. A number of EPR active species were observed; trapped holes and electrons, which reside within the TiO2 catalyst. In addition, methyl and benzyl radicals were observed and were found to originate from the oxidation of the benzylsilanes by trapped holes in the TiO2 catalyst. However, no radical species were observed from the maleic anhydride. These observations had the following consequences for the currently proposed reaction mechanism for the addition of benzyltrimethylsilane with maleic anhydride under TiO2 photocatalysis.  The observation of the benzyl radical definitely proved that the reactive intermediate was indeed the proposed benzyl radical  The absence of any maleic anhydride EPR active species cast doubt on the role of maleic anhydride as an electron trap. Moreover when maleic anhydride is removed from the reaction system, interstitial Ti3+ species is absent from the EPR spectra, indicating that maleic anhydride is in fact acting as a hole trap.