Electron-spin-reversal phenomenon in optically pumped rubidium

Abstract

We have studied the optical pumping of mixtures of Rb vapor and ${\mathrm{N}}_{2}$ buffer gas by laser light tuned to the ${D}_{1}$ transition having a spectral width of $~500$ MHz. The Rb densities are of the order of ${10}^{13}$ cm${}^{\ensuremath{-}3}$, while the buffer-gas pressures range from 0.1 to 10 torr. As the frequency of the right-hand circularly polarized laser is varied across the ${D}_{1}$ absorption profile, the electron spin polarization of the Rb is found to take on negative values for small negative values of pump detuning from the absorption profile center. This occurs for ${\mathrm{N}}_{2}$ pressures below $~1$ torr; at 10 torr the electron spins consistently point in the same direction as the angular momentum of the pump light. The spin-reversal effect can be understood in terms of populations of the $F=2$ ($^{85}\mathrm{Rb}$) and $F=1$ ($^{87}\mathrm{Rb}$) states caused by small unpolarized fractions in the pump beam and its elimination in terms of pressure broadening caused by the ${\mathrm{N}}_{2}$ buffer gas. We speculate that this effect could be used for fast Rb spin modulation.