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Spin relaxation and recombination in atomic hydrogen gas at temperatures around 1 K Marsolais, Richard

Abstract

An experiment for studying a gas of atomic hydrogen at temperatures between 0.9 K and 1.3 K is described. The field dependence of the recombination rate of H atoms into H₂ molecules and the spin-exchange cross-section are investigated. Atoms are produced in situ in a low temperature discharge and are confined in a pyrex cell. The walls of this cell are covered with a film of superfluid ⁴He for preventing recombination. The detection and the study of the hydrogen atoms is done using the technique of pulsed magnetic resonance between the lowest two hyperfine levels of the 1s hydrogen atoms. The frequency is about 765.5 MHz and corresponds to a shallow minimum in the difference between the lowest two hyperfine levels, which occurs at 6481 kGauss. A simple model is also derived for explaining the field dependence of the recombination in the presence of a relatively large ⁴He density. In fact, due to the vapour pressure, the helium density will be above 10¹⁸ atoms/cm³ in the range of temperatures in which we are interested, while the density of hydrogen atoms will be about 10¹¹ -10¹² atoms/cm³. The relaxation mechanism assumed is spin-exchange. The parameters of the model are fit to the existing data. It is found in particular that the asymptotic behavior of the hyperfine populations depends sensitively on the initial populations.

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