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

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To study how highly magnetized liquids behave in NMR experiments, we have performed low-field NMR on laser-polarized liquid 129 Xe, with nuclear polarization of up to 6%. Contrary to conventional NMR results, we find that instabilities develop after a large pulse, leading to an unexpectedly abrupt decay of the signal. In contrast, the NMR spectrum after a small pulse collapses to a series of unusually sharp lines ͑spectral cluster-ing͒, whose widths correspond to precession lifetimes longer than a half minute. We discuss the key role of long-range dipolar coupling in such spin dynamics.

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