No description available.

Thesis (Ph. D.)--Virginia Polytechnic Institute, 1970. / Also available via the Internet.

Thesis (M.S.)--University of Michigan, 1967.

Brief theoretical descriptions of the magneto-optical Faraday effect, the influence of paramagnetic resonance on the Faraday effect and the use of this effect for measurements of saturation spectra and spin-lattice relaxation times are presented. A detailed description of an apparatus and of experimental procedures is given, with the help of which the Faraday effect can be used to study paramagnetic resonance spectra and spin-lattice relaxation times at liquid helium temperatures. Measurements on concentrated neodymium ethylsulfate at liquid helium temperatures are reported. The paramagnetic resonance spectrum was resolved as a main triplet with an extensive hyperfine structure. The positions of the lines were found to be in agreement with a spin Hamiltonian derived earlier from conventional absorption spectra obtained at higher temperatures. The intensity distribution was found to become increasingly asymmetric with decreasing temperature below 4.2°K. This asymmetry is explained qualitatively for the main triplet in terms of existing theories. Measurements of saturation versus microwave power at constant magnetic field and bath temperature gave results not readily explainable in terms of existing theories. The existence of a spatial gradient in the spin-temperature was observed in the case of partial saturation of the resonance. Measurements on the spin-lattice relaxation time t gave the following results: t is independent of the magnetic field from 800 ∅ to 2600 ∅ at a temperature of 4.2°K, but strongly field-dependent at temperatures ≤ 2°K, indicating the importance of cross-relaxation processes at very low temperatures. Some experiments are suggested which may provide information necessary for the complete understanding of the complex processes involved in paramagnetic resonance phenomena at low temperatures. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

A 9 K Mc electron spin resonance spectrometer has been constructed and has been used in conjunction with a specially designed liquid helium dewar. Provision was made for condensing free radicals from the gas phase and for generating radicals by the irradiation of low temperature deposits. Experiments were performed in which monomeric NO₂ was trapped at 4°K in a series of different matrices. An attempt is made to explain the observed spectral lines in terms of magnetic interactions due to nuclear spins and electrostatic interactions due to neighbouring matrix particles. The equation which gives the electron spin resonance spectral lines as a function of magnetic field is [formula omitted] Thus an esr spectrum is characterized by two numbers, the hyperfine splitting constant A and the g value. When a free radical is trapped in an inert matrix, shifts occur in the hyperfine splitting constant and in the g value. Adrian developed a semiquantitative theory to explain these effects for hydrogen atoms trapped in inert matrices. This theory considers two important interactions between the free radical and matrix particle. The van der Waals interaction leads to a negative shift in A, while overlap effects lead to a positive shift in A and a negative shift in g value. Electron spin resonance spectra were obtained for nitrogen dioxide trapped in argon, methane and nitrogen at 4°K. Attempts were made to employ the ideas of Adrian's theory to explain the esr spectra obtained from these systems. Partial success was achieved in explaining the form of the spectra obtained. The divergences between the predictions of the theory and the experimental results leads to a questioning of the assumptions made and the approximations used in developing the theory. / Science, Faculty of / Chemistry, Department of / Graduate

A paramagnetic resonance spectrometer operating at 3.2 cm has been built for use at liquid helium temperatures. This spectrometer employs a high Q transmission type cavity resonator which is equipped with windows on the broad faces to pass light through. The pole pieces of the electromagnet have axially centered holes for passing light through, too. This enables magneto-optical effects and paramagnetic resonances to be studied simultaneously. A high power klystron, and a high Q cavity resonator permit experiments with microwave power saturation. The influence of the paramagnetic resonance absorption saturation on the paramagnetic Faraday effect has been studied. The paramagnetic Faraday effect (i.e. the rotation of the plane of polarisation of light passing through a paramagnetic crystal in the direction of an external magnetic field) is proportional to the magnetic moment of the crystal. When the crystal is exposed to microwave radiation of the correct frequency to cause appreciable saturation of this resonance, the magnetic moment is reduced, and a reduction of the rotation angle should be noticed. This effect was predicted by A. Kastler 1951, and a theory of this effect has been developed by W. Opechowski 1953. This effect was found in Nd(C₂H₅SO₄)₃∙.9H₂O at 1.4°K. In this salt the magnetic moment is proportional to the difference of the populations of the two levels M(s)= ± ½ of the Nd⁺⁺⁺ ion, having an effective ground state of S= ½. Saturation levels of about 52%, 88%, and 100% were obtained and hence the rotation angle of the plane of polarisation reduced accordingly. Thus the effect predicted by A. Kastler has been established. From the known power levels of the applied microwave frequency radiation the electron spin-lattice relaxation time of the Nd⁺⁺⁺ ions, in the upper level was found to be about 1/20 s at 1.4°K. Guanidine aluminium sulphate hexahydrate, (CN₃H₆)Al(SO₄)₂∙6H₂O, a crystal with trigonal structure, having three Al⁺⁺⁺ ions in the unit cell, has recently been discovered to be ferroelectric. The ferroelectric direction is along the trigonal axis. The determination of the structure was not completed when this work was started. In order to complete the crystallographic data with respect to the crystalline field of the immediate neighbours of the Al⁺⁺⁺ ions, Cr⁺⁺⁺ ions were introduced into the lattice replacing Al by 2%, and then the resonance spectrum of the Cr⁺⁺⁺ ions investigated at 295°K, 195°K, 77°K, and 55°K. Two sets of axially symmetric spectra, a total of six lines were observed, set 1 being twice as intensive as set 2. The investigations show that the Cr⁺⁺⁺ ions are exposed to a trigonal field. All three ions lie on the three-fold axis, and since there are 3 ions per unit cell, two of them are magnetically equivalent. Both sets of spectra can be fitted to the spin-Hamiltonian [formula omitted] where S = 3/2. The exact Hamiltonian was diagonalized using a digital computer, and the predicted lines agree with the measured ones within 1%. It was found that the D₁ and D₂increase linearly with decreasing temperature, D₁ from (0.0576 ± 0.0005)cm⁻¹ at 295°K to (0.085 ± 0.003)cm⁻¹ at 35°K, and D₂ from (0.0730 ± 0.0003)cm⁻¹ at 295°K to (0.109 ± 0.005)cm⁻¹ at 35°K. g = 1.975 ± 0.005 and stays constant over the range of the mentioned temperatures. Further, ferroelectricity causes spontaneous distrotion of the lattice, and this way may influence the paramagnetic resonance spectrum through changes in the crystalline field. Since different parts of a (CN₃H₆)Al(SO₄)₂∙6H₂O crystal have different electrical properties, the spectrum of crystal fragments cleaved from different parts of a large crystal has been examined and no difference in the spectrum has been observed. The crystals were also polarised in a strong electric field along the trigonal axis and depolarised in a weakening alternating electric field with no effect on the spectrum. From this follows that the ferroelectric complex has very little effect on the immediate neighbours of the Cr⁺⁺⁺ ions. The paramagnetic resonance of Fe⁺⁺⁺ in orthorhombic amonium penta chlorindate hydrate, (NH₄)₂[(ln,Fe(10:1))Cl₅∙H₂O] has been investigated as this salt is interesting for adiabatic demagnetization. Noticeable signal strengths were obtained at about 25°K only. The positions of the lines as functions of the rotation angles of the crystal about all three crystalline axes separately with respect to the magnetic field have been measured. In general four sets of equally intensive spectra have been found for each rotation. From the maxima and minima of the curves of the spectra assuming the ions have axial symmetry it is possible to determine the directional cosines of the principle axes for the Fe⁺⁺⁺ ions. These are either ℓ = ± 0.1860, ℳ = ± 0.2212, ℳ = ± 0.9578 or ℓ = ± 0.7576, ℳ = ± 0.6357, ℳ = ± 0.1488 or ℓ = ± 0.6636, ℳ = ± 0.7123, ℳ = ± 0.22805 or ℓ = ± 0.3289, ℳ = ± 0.2762, ℳ = ± 0.9033 Because of the complexity of the spectra the spin-Hamiltonian has not yet been determined. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The ninety degree yield curve for the ¹²C(p,Ƴ) ¹³N reaction was examined for proton energies (Ep ) between 14 MeV and 24.4 MeV using a 99.9 % pure carbon-12 target and protons from the University of Washington FN tandem Van de Graaff. The giant dipole resonance (GDR) for the gamma transition to the ground state (Ƴ₀) was found to be centered at Ep = 20.5 MeV with a width Γ = 4 MeV and a maximum cross-section = 3 µb/sr. Intermediate structure of width Γ = 1 MeV was observed at Ep = 17.5 MeV and 23 MeV. The yield curve was compared to the ¹¹B(p,Y₀) ¹²C yield curve, and the similarities found indicated that valence nucleon transitions to the ground state play little part in the GDR of 13N. Yield curves for the transition to the first excited state (Ƴ¹) and the sum of the transitions to the second and third excited states (Ƴ2+3) are also given in the regions where they can be reliably extracted. No fine structure was observed. Measured yields of the 12.71 MeV and 15.11 MeV gamma-rays from the inelastic reaction agree well with other recent results. Proton decay widths to these states from compound nuclear states in 13N are given. Angular distributions for the (p, Ƴ₀) reaction were measured at six energies in the region of the "pygmy resonance" Ep= 10 MeV to 14 MeV, to inspect previously reported fine structure. Two narrow minima seen in the ninety degree yield are found to be minima in the integrated cross-sections, whereas the shape of the angular distribution is relatively constant. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

No description available.

The generation and evolution of cross-waves in a channel are investigated analytically, numerically and experimentally. The derivation of the modulation equation governing the inviscid cross-wave amplitude yields the nonlinear Schrodinger equation with a homogeneous Robin boundary condition at the wavemaker. Either of two uniformly valid scalings--cross-wave amplitude of the same order as or much larger than the wavemaker amplitude--may be used in the derivations. The differences between the two scalings are discussed. The inviscid modulation equation is augmented by a linear damping term, the coefficient of which is determined empirically from the measured neutral stability curve. The viscous modulation equation is solved numerically. The theory is compared to experiments in a channel 30.9 cm wide, for mode n = 6, for frequencies close to the cutoff frequency 7.82 Hz. Measurements include the neutral stability curve, the onset of modulation, cross-wave phase along the channel, and cross-wave amplitude as functions of wavemaker amplitude, forcing frequency and distance from the wavemaker. These measurements are in good agreement with the numerical results. The results are also observed to be sensitive to viscous effects. Additionally, both numerical calculations and experiment reveal trapped and propagating modes. The trapped mode is most easily observed at positive detuning.

This thesis considers the possibility of stochastic resonance (SR) in the following nanoscale systems: (i) hard-threshold devices; (ii) averaging structures of carbon nanotubes (CNTs); (iii) myoglobin atoms; and finally (iv) tubulin dimers. The description of SR is carried out using Kramers' rate theory in the adiabatic two-state approximation for continuous systems and using Shannon's information theoretic formalism for systems with static nonlinearities. The effective potentials are modelled by asymmetric or symmetric bistable wells in a single reaction co-ordinate. Quantum considerations have not been invoked. Hence, all results are implicitly valid in the high-temperature regime of relevance to industrial applications. It is established that information transmitted by arrays of identical CNTs is maximized by non-zero noise intensities and that the response of myoglobin and tubulin dimers to ambient molecular forces (as described by the signal-to-noise ratio or SNR) is enhanced by increasing temperature. Sample calculations are shown for solvent fluctuations, ligand interactions and dipole oscillations. These results can be used to explain: (i) the effects of temperature observed in fabrication processes for CNTs; (ii) the dynamical transition observed in myoglobin and (iii) the 8.085 MHz resonance observed in microtubules.