Experimental Upper Limit for the Permanent Electric Dipole Moment ofRb85by Optical-Pumping Techniques

An optical-pumping experiment designed to detect a permanent electric dipole moment in the ground state of the ${\mathrm{Rb}}^{85}$ atom is described. It has the advantage of averaging out the motional magnetic fields that limit the more recent related atomic-beam results. The applied electric fields were necessarily smaller. Zeeman transitions in free ${\mathrm{Rb}}^{85}$ atoms in the earth's magnetic field (about 210 kc/sec) were observed in evacuated, wall-coated absorption cells under electric fields up to ±2000 V/cm. Modulation of the light beam by precessing atoms was used to operate the samples as self-excited oscillators. The difference in frequency between two ${\mathrm{Rb}}^{85}$ oscillators operating side by side in the same light beam was measured to reduce the effect of magnetic field fluctuations. This frequency difference (about 300 cps) was found to be stable to within ±0.1 cps (e.g., corresponding to magnetic-field variations between samples of 2×${10}^{-7\mathrm{}}$ G for 30-min periods, under favorable conditions), and was found to be independent of the electric field applied to one of them within that limit. It was concluded that the permanent electric dipole moment of the ${\mathrm{Rb}}^{85}$ atomic ground state, if it exists, is less than $e\times \left({10}^{-18\mathrm{}} \mathrm{cm}\right)$, where $e$ is the electronic charge. The theory of optically pumped alkali oscillators is developed for arbitrary nuclear spin in the limit of low light intensities (the high-resolution case).