Interpretation of Frequency Shifts Due to Electron Exchange Collisions

Experiments by Hobart and Franken in 1961 demonstrated a shift in the free-electron spin-resonance frequency due to collisions with sodium atoms whose electron spins were polarized. The results have been interpreted on the assumptions that the S-wave phase shifts were proportional to the wave number k and that higher partial waves were not important. The triplet and singlet electron-sodium scattering lengths of Salmona and Seaton were used, but no check on the calculation was made since the sodium polarization was not known. Recently Balling, Hanson, and Pipkin have measured the shift for collisions with highly polarized rubidium atoms. They have also derived complete formulas for the frequency shift and line broadening in terms of the phase shifts with all partial waves included. In the present paper, an alternate derivation of the frequency shift and line broadening formulas is given and the experimental results are discussed on the basis of the phase-shift information that is available for hydrogen, sodium, and cesium. The small size of the observed ratio of frequency shift to line broadening for rubidium appears somewhat surprising in view of the larger ratios calculated for other alkalis.