Polarization-dependent frequency shifts from Rb-³He collisions

We present measurements of the frequency shift of the Rb electron-paramagnetic-resonance (EPR) line in the presence of nuclear-polarized ³He gas for the temperature range of 30 to 85 °C. The frequency shift is due to the Fermi-contact interaction between the Rb valence electron and the polarized ³He nucleus. Expressions for both the frequency shift and the spin-exchange cross section are derived in terms of the strength of this contact interaction. From these expressions and the measured frequency shift, we estimate the Rb-³He spin-exchange cross section. The Rb EPR frequency shift, which is 53 kHz for a 100% polarized 10-amagat ³He sample at 50 °C, can be used to determine the absolute polarization of nuclear polarized ³He targets. From these measurements, one can also predict the shift of the ³He NMR line due to a polarized Rb vapor.