Measurement of the 21 Ne Zeeman frequency shift due to Rb − 21 Ne collisions

Using a ${}^{21}\mathrm{Ne}{\ensuremath{-}}^{3}\mathrm{He}$ Zeeman maser, we compared the frequency shift of the ${}^{21}\mathrm{Ne}$ nuclear Zeeman resonance induced by polarized Rb vapor to the shift induced in the ${}^{3}\mathrm{He}$ nuclear Zeeman resonance. The ${}^{3}\mathrm{He}\ensuremath{-}\mathrm{Rb}$ shift has recently been measured with high precision [M.V. Romalis and G.D. Cates, Phys. Rev. A 58, 3004 (1998)], permitting the conversion of our differential measurement to an absolute value for the ${}^{21}\mathrm{Ne}\ensuremath{-}\mathrm{Rb}$ shift. We report a value of ${\ensuremath{\kappa}}_{21}=32.0\ifmmode\pm\else\textpm\fi{}2.9$ for the $\mathrm{Rb}{\ensuremath{-}}^{21}\mathrm{Ne}$ contact shift enhancement factor at a temperature of approximately $128\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ We also propose high-precision differential contact shift measurements, the absolute accuracy of which could be limited by the error in the ${}^{3}\mathrm{He}$ contact shift.