In this thesis we describe the results of two sets of experiments on spin-polarized atomic hydrogen at temperatures below 1 K using a 115 GHz heterodyne ESR spectrometer. First, we have made measurements of the rates of the two-body recombination processes in spin-polarized atomic hydrogen gas and spin-polarized atomic deuterium gas in a 41 kG field. In the second set of experiments, we examined the ESR absorption line associated with the spin-polarized atomic hydrogen atoms adsorbed on surfaces of liquid helium. In hydrogen, a comparision of the measured recombination rates in 41 kG with the corresponding rates measured in different magnetic fields showed that an additional recombination process is activate for the conditions of these experiments. We demonstrate that this is due to a resonant recombination process via the (v,J) = (14,4) level of molecular hydrogen. In spin-polarized deuterium, we found that the recombination rate are much larger than the analogous rates in hydrogen, and therefore that atomic deuterium gas is much less stable than the hydrogen system under comparable conditions. We report the first observation and study of doubly spin-polarized deuterium, a gas in which both the deuteron and electron spins are aligned. At temperatures near 0.1 K, the absorption line of doubly spin-polarized H atoms changes drastically as a peak appears on both sides of the peak associated with the bulk atoms. We have shown that these new peaks are due to the atoms which are adsorbed onto the surface of the liquid helium film that coats the walls of the microwave cavity. The geometry dependent non-zero average magnetic dipolar field due to aligned spins on a planar surface results in a shift of the position of the ESR absorption lines. The lineshapes of the side peaks are very unusual and are best described as ramp-like. Even after extensive attempts to explain this lineshape, we still do not know what mechanism is responsible.
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.
Spin-polarized atomic hydrogen (H↓) gas at low temperatures (<0.5 K) has been studied using Electron Spin Resonance (ESR) techniques which allow direct measurements of the densities of H atoms in each of the two lower hyperfine states (a and b). It has been demonstrated that ESR can be made to yield accurate and detailed information about the decay of H↓ densities in time. Crucial to the analysis of the data is the ability to confine the sample of H↓, gas inside the liquid ⁴He coated microwave cavity where the magnetic field and temperature are well known. Measurements of K[sub aa] and K[sub ab], the two surface recombination rate constants, and their ratio γ=K[sub aa]/K[sub ab] have been made and from these results the binding energy of H on liquid ⁴He has been extracted. Also reported are the results of the first detailed study of the one-body surface relaxation rate which we find is due to microscopic magnetic impurities in the cell walls. Theoretical calculations of the two-body dipole-dipole relaxation rate of H↓ in the gas and on the surface are also presented.
We have studied spin-polarized deuterium, ${\mathrm{D}}_{\ensuremath{\downarrow}}$, in the temperature range 0.3 to 0.7 K in a field of 40 kG using electron-spin resonance at 114 GHz. The intrinsic rates for recombination of two D atoms to form
D2 on the surface of $l\ensuremath{-}^{4}\mathrm{He}$ have been measured for both the ortho and para channels. The two rates are approximately equal and when extrapolated to zero field are 1600 times greater than the rates for
H2 formation. Doubly spin-polarized deuterium, ${\mathrm{D}}_{\ensuremath{\downarrow},↡}$, has been observed for the first time, with nuclear-spin purities > 0.98 and sample lifetimes as long as 1 h.
We have studied relaxation and recombination processes in compressed, doubly polarized atomic hydrogen at temperatures from 0.13 to 0.60 K and magnetic fields from 3 to 9 T. The gas and surface dipole three-body recombination rate constants at a field of 7.6 T are measured to be, respectively,
Lg=8.9(8)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}39}$
cm6 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and
Ls=1.2(4)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}24}$
cm4 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. They decrease slowly with field and
Ls exhibits no significant angular dependence. The three-body recombination rate due to hyperfine mixing has also been measured. Electronic and nuclear relaxation rates have been measured; the b-c electronic relaxation rate constant in the gas is
Gbc=1.03(7)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$
cm3 ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$exp(-
Ebc/
kBT). The temperature and field dependence of the nuclear relaxation rate in the gas are observed to be in excellent agreement with recent theoretical calculations. Three-body surface recombination-rate measurements using $^{3}\mathrm{\ensuremath{-}}^{4}$He surfaces indicate that as little as one monolayer of
3He on the liquid
4He surface appreciably decreases the adsorption energy of atomic hydrogen. Densities achieved include 4.5\ifmmode\times\else\texttimes\fi{}
1018 atoms${\mathrm{?}}^{\mathrm{\ensuremath{-}}3}$ at 0.55 K (pure
4He walls), and 1.4\ifmmode\times\else\texttimes\fi{}
1018 atoms${\mathrm{?}}^{\mathrm{\ensuremath{-}}3}$ at 0.19 K (8 at. ? sup 3---He).
We study three-body recombination in the gas of spin-polarized atomic hydrogen adsorbed on the surface of superfluid helium at temperatures from
45to130mK. The two-dimensional gas is thermally compressed to densities up to $\ensuremath{\approx}4\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ using the ``cold spot'' method which makes the three-body process the dominant decay channel in the system. We measure the loss rate and surface density of atoms directly and independently and observe the former to be proportional to the third power of the latter. The result for the surface three-body recombination rate constant at
4.6T, ${L}_{s}=2.0(7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}25}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{4}∕\mathrm{s}$, significantly reduces the discrepancy between the theory and earlier measurements where the surface density was inferred from the adsorption isotherm. We also measured the three-body rate constant on a 0.1% $^{3}\mathrm{He}\text{\ensuremath{-}}^{4}\mathrm{He}$ film and found ${L}_{s}=1.3(4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}24}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{4}∕\mathrm{s}$. This larger value is attributed to an increased delocalization of adsorbed hydrogen atoms in the direction normal to the surface.
Electron-spin resonance at 114 GHz is used to measure the three-body recombination rate (
L3) and the two-body surface relaxation rate (
G2s) for doubly-spin-polarized H between 100 and 200 mK. Our results for
L3 confirm and extend those of Cline, Greytak, and Kleppner. We find that
G2s is much smaller than previously reported and in reasonable agreement with theory. Around 100 mK, we also observe sidebands due to surface atoms whose frequency is shifted by dipole-dipole interactions.