Polarization and Relaxation Processes inHe3Gas

Abstract

Polarization of ${\mathrm{He}}^3$ when used as a buffer gas for optically pumped rubidium vapor has been investigated experimentally and theoretically. Experiment shows that the coupling between a rubidium and ${\mathrm{He}}^3$ atom is scalar in form (i.e., I·S) and is about three orders of magnitude greater than would be expected from the direct magnetic interaction of the dipoles associated with each atom. A theoretical explanation of this effect, which depends upon the overlap of the Rb valence electron with the electrons of the ${\mathrm{He}}^3$ atom is presented and is in agreement with experiment. Experiments concerning the optical pumping of metastable ${\mathrm{He}}^3$ atoms in order to produce ground-state polarization are described. Polarizations of 60% in 1-Torr (1 STP mm) samples at room temperature and 56% in 3-Torr samples at 77°K have been achieved. The process of metastable pumping works only at these lower pressures. Mercury liquid is shown experimentally, however, to present a poor surface for ${\mathrm{He}}^3$ relaxation. This result opens the possibility of achieving high polarization at high pressure using metastable pumping and gas compression. Various wall coatings of ${\mathrm{He}}^3$ samples are found to have only small effects upon relaxation rates. A 3.1-amagat (1 STP atm) sample in Pyrex but of high purity is found to have a relaxation time of 2×${10}^4$ sec. This value sets present limits upon the experimental ${\mathrm{He}}^3$-${\mathrm{He}}^3$ relaxation cross section. A new effect which involves relaxation processes of the ${\mathrm{He}}^3$ polarization in a magnetic field with a gradient is investigated both experimentally and theoretically. The results of both theory and experiment are in close agreement with each other. An effect involving enhanced polarization of either sign of spin temperature of a high-pressure sample of ${\mathrm{He}}^3$ in which a gas discharge is struck, and which is in a 10-kG field, is described experimentally.