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OCTOBER 15, I960 R(H ||c) is small because electrons with k ± p and because co c tends to zero as k ± approaches j3.For CdS Thomas et al. 5 have estimated an upper bound of the order of a few hundredths of an ev for the displacement of any energy minima below E(0) in the conduction band. In the toroidal model this energy displacement equals A/3 2 . Setting A/3 2 = 0.04 ev, one obtains A« 6 at T = 77°K. Thus, one might hope to observe a large anisotropy in the Hall effect, if the model is correct.It should be emphasized that the model results from an approximation. Extension of the 5»p perturbation calculation to higher orders will lower the symmetry of the surfaces to one of sixfold rather than continuous rotational invariance. Whether or not the corrections are important will depend upon the size of /3 and the proximity of other bands. For A/3 2 = 0.04 ev and A = (2m)" 1 , 0 is typically of the order of one-tenth the shortest distance in k space between the point 5=0 and any of the reduced zone boundaries parallel to the k z axis.Added note. The toroidal model is a direct consequence of the spin-orbit interaction. The nature of £(S) in the limit of zero spin-orbit coupling is described in the earlier work of Rashba, 11 Although almost complete polarization of alkali metal nuclei can be produced by optical pumping utilizing a buffer gas and a filter to remove the D 2 resonance light, 1 * 2 the number of atoms polarized is necessarily very small and is limited by optical opacity of the sample vapor. The usual range of pressure is 10" 7 to 10~3 mm for successful detection and production of optical polarization. The Overhauser nuclear polarization effect 3 involving dipolar interactions 4 " 8 between such an optically polarized atom and the nucleus of a suitable buffer gas should provide a mechanism for the transfer of polarization to a gas at considerably higher pressure than that of the alkali vapor. In other words, the Overhauser effect should work not by the dipole-coupled re-who has recently given independent consideration to the effects of including the interaction. 12 1 R. laxation of a saturated paramagnetic impurity toward a polarized equilibrium, but by the relaxation of an optically polarized impurity toward a nearly depolarized equilibrium. We have observed this effect in He 3 gas used as the buffer gas for the optical pumping of natural rubidium vapor.The nuclear magnetic relaxation time in a liquid or a monatomic gas depends inversely on both the square of the magnetic moment of the nucleus and the square of the moment of other spins causing the relaxation. If He 3 is contaminated with "dissolved" Rb atoms having a moment 10 3 times as great as the He 3 , a concentration of more than 1 part in 10 6 would dominate the relaxation. The dipole interaction terms in-

The experimental verification of Overhauser's proposal for polarizing nuclear spins is described. The effect on the nuclear magnetic resonance of saturating the electron spin resonance in several appropriate systems was observed at low fields and room temperatures. The systems investigated were: metallic Li, metallic Na, and Na dissolved in anhydrous liquid ammonia. The nuclear resonances of Li 7 , Na 23 , and H 1 (in the ammonia) were observed at 50 kc/sec in fields of 30.3 gauss, 44.2 gauss, and 11.7 gauss, respectively, and the electron spin resonances were saturated with the corresponding applied frequencies of 84, 124, and 33 Mc/sec. The detailed predictions of Overhauser were confirmed as far as nuclear polarization is concerned, although other relaxation processes in Li reduce the polarization, and the difficulty in completely saturating the metallic Na electron resonance leads to a partial effect. Only in fairly concentrated solutions of Na in ammonia was a substantially complete effect observed in which the proton nuclear polarization increased by the ratio of the electron gyromagnetic ratio to the nuclear gyromagnetic ratio. The proton line widths in the Na-ammonia solutions further verify the theory of Kaplan and Kittel.

Polarization of 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 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 He 3 atom is presented and is in agreement with experiment. Experiments concerning the optical pumping of metastable 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 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 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 2X10 4 sec. This value sets present limits upon the experimental He 3 -He 3 relaxation cross section. A new effect which involves relaxation processes of the 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 He 3 in which a gas discharge is struck, and which is in a 10-kG field, is described experimentally.

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