Experimental Verification of the Overhauser Nuclear Polarization Effect

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 ${\mathrm{Li}}^{7}$, ${\mathrm{Na}}^{23}$, and ${\mathrm{H}}^{1}$ (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.