Spin Resonance of Free Electrons Polarized by Exchange Collisions

An experiment is described in which thermal electrons, t₄400^K, become polarized in detectable numbers by undergoing exchange collisions with oriented sodium atoms during which the atom orientation is transferred to the electrons. The collisions establish interrelated equilibrium values for the atom and electron polarizations which depend upon the balance between the polarizing agency acting upon the atoms (optical pumping) and the disorienting relaxation effects acting both on atoms and electrons. When now the electrons are furthermore artificially disoriented by gyromagnetic spin resonance, an additional reduction of the atom polarization ensues which is detected by an optical monitoring technique, thereby allowing a determination of the free-electron spin g factor, gₒ. Since it was experimentally convenient, at this stage only the ratio g₉ (Na) {gₒ}=1. 0000260. 00003 was determined, showing no significant difference between gₒ and g₉ (Na), the g factor of the ^2S₁₂ sodium ground state. From the experimental strength and width of the electron disorientation signal a lower limit was obtained for the sodium exchange cross section with thermal electrons: Q>2. 310^-14 cm^2. This may be compared with a theoretical exchange cross section, Q=2. 310^-14 cm^2, which is derived under the assumption that the 3s^2S₀ state of the Na^-ion has essentially zero binding energy, thereby causing strong singlet scattering while the triplet scattering is negligible in comparison. Spin-orbit coupling during collisions of the electrons with the atoms of the inert argon buffer employed to slow down wall diffusion is discussed as the chief cause for the shortness of the observed free-electron spin relaxation time, T₄610^-5 sec.