Abstract. The structure of the excited 2 3 S and 2 3 P triplet states of 3 He and 4 He in an applied magnetic field B is studied using different approximations of the atomic Hamiltonian. All optical transitions (line positions and intensities) of the 1083 nm 2 3 S-2 3 P transition are computed as a function of B. The effect of metastability exchange collisions between atoms in the ground state and in the 2 3 S metastable state is studied, and rate equations are derived, for the populations these states in the general case of an isotopic mixture in an arbitrary field B. It is shown that the usual spin-temperature description remains valid. A simple optical pumping model based on these rate equations is used to study the B-dependence of the population couplings which result from the exchange collisions. Simple spectroscopy measurements are performed using a single-frequency laser diode on the 1083 nm transition. The accuracy of frequency scans and of measurements of transition intensities is studied. Systematic experimental verifications are made for B=0 to 1.5 T. Optical pumping effects resulting from hyperfine decoupling in high field are observed to be in good agreement with the predictions of the simple model. Based on adequately chosen absorption measurements at 1083 nm, a general optical method to measure the nuclear polarisation of the atoms in the ground state in an arbitrary field is described. It is demonstrated at B ∼0

Abstract. Metastability exchange optical pumping (MEOP) is experimentally investigated in3 He at 4.7 T, at room temperature and for gas pressures ranging from 1 to 267 mbar. The 2 3 S-2 3 P transition at 1083 nm is used for optical pumping and for detection of the laser-induced orientation of 3 He atoms in the rf discharge plasma. The collisional broadening rate is measured (12.0 ± 0.4 MHz mbar −1 FHWM) and taken into account for accurate absorption-based measurements of both nuclear polarization in the ground state and atom number density in the metastable 2 3 S state. The results lay the ground for a comprehensive assessment of the efficiency of MEOP, by comparison with achievements at lower field (1 mT-2 T) over an extended range of operating conditions. Stronger hyperfine decoupling in the optically pumped 2 3 S state is observed to systematically lead to slower build-up of 3 He orientation in the ground state, as expected. The nuclear polarizations obtained at 4.7 T still decrease at high pressure but in a less dramatic way than observed at 2 T in the same sealed glass cells. To date, thanks to the linear increase in gas density, they correspond to the highest nuclear magnetizations achieved by MEOP in pure 3 He gas. The improved efficiency puts less demanding requirements for compression stages in polarized gas production systems and makes high-field MEOP particularly attractive for magnetic resonance imaging of the lungs, for instance.

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