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Resonant recombination of atomic hydrogen and deuterium at low temperatures

University of British Columbia

The contents of this thesis include several experiments and a theory, the unifying theme of which is resonant recombination of atomic hydrogen (H) and deuterium (D) at low temperatures. Measurements of the interaction of H and D with the liquid helium (l-⁴He) wall coating used in low-temperature experiments are also included. Atomic hydrogen and deuterium were studied by ESR in a field of 41 kG at temperatures below 1 K, in a second generation of experiments on an existing apparatus. The binding energy of H on l-⁴He was measured, by direct observation of the physisorbed atoms, to be E[sub B]= 1.00(5) K. Extensive meaurements of recombination rate constants were made. Resonant recombination of H via the (v, J) = (14,4) level of H₂ was observed and identified. Atomic deuterium was also observed to recombine in a resonant process. At low temperatures, resonant recombination is possible due to the predissociation of weakly bound molecular levels by the intra-atomic hyperfine interaction. The theory of resonant recombination of H and D due to hyperfine predissociation is developed in detail. In order to keep the development self-contained, it is presented as appendix D of this thesis. The theory is combined with our ESR data in order to predict the magnetic-field dependence of resonant recombination of D. It is shown that observation of the threshold magnetic fields for resonant recombination will permit accurate measurement of the dissociation energy of the (v, J) = (21,0) and perhaps (21,1) levels of the D₂ molecule. An apparatus was constructed to study atomic deuterium by hyperfine magnetic resonance at temperatures near 1 K. The D atoms were produced from D₂ of 99.65% isotopic purity by an RF discharge in a bulb coated with a film of superfluid l-⁴He. Magnetic resonance in a field of 39 G on the longitudinal β-δ transition at its minimum frequency of 309 MHz was used to observe the atoms. The experiment was designed with the capability of measuring the recombination rate as a function of magnetic field for fields up to 30 kG by moving the sample between the 309 MHz resonator and a superconducting solenoid. A number of preliminary experiments were made using this apparatus. It was discovered that the D atom density decays away exponentially in time. The decay rate was measured as a function of temperature and found to follow an Arrhenius (thermally-activated) form, with an activation energy of about 15 K. No second-order decay was seen, permitting an upper bound to be placed on the D-D recombination rate. The exponential decay of the D atom density was due to the penetration by the atoms of the l-⁴He film coating their container. Our measurement of the temperature dependence of the density decay rate gives the energy required to dissolve a D atom in l-⁴He, Es[sub s] ≃ 14 K. This is the first experimental determination of this quantity. The longitudinal relaxation time T₁ for the β-δ transition was found to be too short to be due to D-D spin-exchange. It was conjectured that the sample was contaminated with substantial amounts of H, spin-exchange in H-D collisions causing the short T₁. Measurements in an existing 1420 MHz apparatus showed directly, by magnetic resonance on the hydrogen zero-field transition, that H atoms were, in fact, present. By varying the H concentration, we were able to show that the decay of the D density by was not due to recombination of H with D. Several attempts to substantially reduce the H impurity have failed. The penetration of the l-⁴He film is undoubtedly one reason why the D is so susceptible to contamination by H, since the film is permeable to D but not to H. For use in interpreting these experiments, the effects of spin-exchange relaxation on hyperfine resonance in mixtures of H and D were calculated.

Text

2010-10-18

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http://hdl.handle.net/2429/29270

Physics

University of British Columbia

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