Optical pumping of Rb in the presence of high-pressure ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ buffer gas

A cryogenic technique has been used to produce polarized ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ targets of up to relative density p=12 atm (≊3×${10}^{20}$ ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ atoms/${\mathrm{cm}}^3$; p=1 atm corresponds to 760 Torr or 101.3 kPa pressure at 273 K). In these targets ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ nuclei are polarized by spin-exchange collisions with optically pumped rubidium atoms. From transmission measurements at wavelengths of 790–800 nm, we have determined pressure shifts, linewidths, and line-shape asymmetries for the Rb 5${\mathit{S}}_{1/2}$→5${\mathit{P}}_{1/2}$ D1 transition. The Rb spin-destruction rate was found to exhibit a quadratic increase versus ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ pressure, which indicates the importance of Rb${\mathrm{-}}^3$He${\mathrm{-}}^3$He collision processes. The transmission results for circularly polarized light are well described by a model that predicts the dependence of the average Rb polarization on Rb density, ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ pressure, light intensity, and cell geometry. The Rb${\mathrm{-}}^3$He spin-exchange cross section, 〈${\mathrm{\sigma }}_{\mathrm{SE}}$v〉=6.1×${10}^{\mathrm{-}20}$ ${\mathrm{cm}}^3$ ${\mathrm{s}}^{\mathrm{-}1}$, was found to be independent of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ pressure up to p=12.1 atm. Maximum ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ polarizations of 72–79% were observed with cells of 17 ${\mathrm{cm}}^3$ volume that contained ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ at p=6–9 atm.