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Magnetic resonance studies of atomic hydrogen gas at liquid helium temperatures

University of British Columbia

Pulsed magnetic resonance studies are reported for a gas of hydrogen atoms at densities of 3-10¹³ X 10 cm⁻³ and temperatures of 4.2-77 K. The gas was produced by dissociation in a room temperature R.F. discharge, and piped through glass tubing into the cryogenic apparatus for study. The magnetic resonance transition observed is between the two lowest hyper-fine levels of the 1s atom in a magnetic field of 6481 Gauss where this splitting has its minimum value of about 765.5 MHZ. At 77 K, spin-exchange broadening of the resonance is observed. By varying the number density of hydrogen atoms, the ratio of spin-exchange broadening to atomic hydrogen density is obtained, and from this ratio the spin-exchange cross section for this transition is calculated. The cross section obtained is 60% of the theoretical value. At liquid helium temperatures, the spin-exchange cross section is shown to be at least 15 times smaller than that at liquid nitrogen temperatures, as. predicted by theory. He⁴ and H₂ buffer gases . are used to limit the diffusion broadening of the resonance, allowing the observation of small frequency shifts of the free induction signal. A model is proposed in which interactions of the hydrogen atoms with the flow tube walls cause these shifts. From the diffusion broadening of the resonance, the diffusion cross sections for H in He⁴ at 4.2 K and H in H₂ at 5-9 K are inferred to be 500 Ų and 250 Ų respectively.

Text

2010-03-19

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http://hdl.handle.net/2429/22190

Physics

University of British Columbia

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