Electron Spin Relaxation in Metal—Ammonia Solutions

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Abstract
The pulsed magnetic resonance technique was employed to measure the electron spin relaxation times T1 and T2 in solutions of sodium and potassium in liquid ammonia. Measurements were carried out over a range of temperatures from —50°C to +30°C, and for the concentration range of 10—4 to 10—2 mole metal per mole NH3. It was found that T1 and T2 are equal at all measured concentrations and temperatures, these times ranging from 1 to 3 μsec. For dilute solutions, the data are shown to be quantitatively consistent with the assumption that the relaxation is owing to the contact hyperfine interaction of the electrons with the N14 nuclei of ammonia. This interaction is modulated by the relative motion of the electrons and the ammonia molecules, the spin relaxation time being proportional to the modulation rate, as is typical in cases of extreme motional narrowing. An electron tunneling mechanism is proposed as a possible source of the modulation. That the electron-nitrogen hyperfine interaction is largely responsible for the spin relaxation in dilute solutions is verified by measurements on a sample prepared from isotopically substituted ammonia (N15H3). A qualitative explanation of the variation of the electron spin relaxation time with concentration is proposed.