Longitudinal Spin Relaxation of Optically Pumped Rubidium Atoms in Solid Parahydrogen

We have grown crystals of solid parahydrogen using a single closed-cycle cryostat. We have doped the crystals with rubidium atoms at densities on the order of ${10}^{17}{\mathrm{cm}}^{-3}$ and used optical pumping to polarize the spin state of the implanted atoms. The optical spectrum of the rubidium atoms shows larger broadening than previous work in which the rubidium was implanted in solid argon or neon. However, the optical pumping behavior is significantly improved, with both a larger optical pumping signal and a longer longitudinal relaxation time. The spin relaxation time shows a strong dependence on orthohydrogen impurity levels in the crystal, as well as the applied magnetic field. Current performance is comparable to state-of-the-art solid state systems at comparable spin densities, with potential for improvement at higher parahydrogen purities.

Figure 1

Spectra of rubidium-doped parahydrogen crystals, before and after exposure to broadband light, as described in the text. Both spectra show enormous broadening when compared to gas-phase Rb atoms, which have two strong, narrow absorption lines at 780 and 795 nm. The transmission $T$ of the crystal is determined by comparing spectra before and after crystal deposition: $T\equiv e^{-\mathrm{OD}}$.

Figure 2

Measurement of optical pumping for a transverse field of $\sim 0.5\mathrm{G}$ and a longitudinal field of 80 G. The crystal was optically pumped at $t=0$. The transmission signals are normalized to a level of 1 before optically pumping. This parahydrogen crystal had a Rb density of roughly $2\times 10^{16}{\mathrm{cm}}^{-3}$ and a peak OD of 1.5.

Figure 3

Spin polarization signal, as described in the text. The longitudinal field data are fit to a double exponential. Experimental conditions were the same as in Fig. 2.

Figure 4

Longitudinal relaxation times as a function of the applied longitudinal magnetic field.

Figure 5

Inverse relaxation time as a function of the orthohydrogen fraction. The orthohydrogen fraction is calculated assuming the hydrogen has reached thermal equilibrium in the ortho-para converter, as discussed in the Supplemental Material [20]. The inset shows the same data, plotted as relaxation time vs the ortho-para converter temperature.