Collisional ${}^3\mathrm{He}$ and ${}^{129}\mathrm{Xe}$ Frequency Shifts in Rb–Noble-Gas Mixtures

The Fermi-contact interaction that characterizes collisional spin exchange of a noble gas with an alkali-metal vapor also gives rise to NMR and EPR frequency shifts of the noble-gas nucleus and the alkali-metal atom, respectively. We have measured the enhancement factor ${\kappa }_0$ that characterizes these shifts for $\mathrm{Rb}\mathrm{\text{−}}{}^{129}\mathrm{Xe}$ to be $493\pm 31$, making use of the previously measured value of ${\kappa }_0$ for $\mathrm{Rb}\mathrm{\text{−}}{}^3\mathrm{He}$. This result allows accurate ${}^{129}\mathrm{Xe}$ polarimetry with no need to reference a thermal-equilibrium NMR signal.

Figure 1

Typical ${}^3\mathrm{He}$ and ${}^{129}\mathrm{Xe}$ spectra (shown prior to apodization) from cell 155B acquired one after the other under steady-state SEOP conditions. The narrow ${}^{129}\mathrm{Xe}$ peak at 0 Hz was acquired with the laser blocked; it has been amplitude normalized to appear on the same graph. The double peak in the ${}^{129}\mathrm{Xe}$ spectra at all but the lowest temperatures represents regions of highly polarized and nearly unpolarized Rb vapor; the lines are broadened and begin to coalesce due to diffusion of ${}^{129}\mathrm{Xe}$ between these two regions. For ${}^3\mathrm{He}$, the much smaller frequency-shift dispersion and more rapid diffusion yields a single narrow peak in all cases. The respective shifts in the spectral center of mass upon reversal of the Rb magnetization were used in Eq. (3) to extract $({\kappa }_0{)}_{\mathrm{RbXe}}$.

Figure 2

Enhancement factor $({\kappa }_0{)}_{\mathrm{RbXe}}$ plotted vs temperature for (a) three low-[Xe] cells and (b) three high-[Xe] cells. The weighted average of all the low-[Xe] data points in (a) is 493, with the estimated uncertainty shown by the hatched region. We take these temperature-independent data to represent the best estimate of $({\kappa }_0{)}_{\mathrm{RbXe}}$. The identical hatched region is shown in (b) for comparison: the high-[Xe] data are consistent with the low-[Xe] data up to about $175°\mathrm{C}$; the $\approx 20\%$ dropoff at the highest temperatures is not understood.