NMR spectral clustering and instabilities in polarised liquid He-3

We report results of pulsed NMR experiments performed in low magnetic field (1.3 mT) on thin films of liquid He-3. A high nuclear polarisation is obta ined by laser optical pumping in the vapour prior to fast liquifaction. In these highly magnetised anisotropic samples, classical dipolar couplings ha ve a strong influence on spin dynamics. Very narrow NMR spectra, resulting from precession lifetimes of several seconds, can be obtained in situations where inhomogeneous broadening would be expected for independently precess ing spins (when a strong gradient is applied). Conversely, precession lifet imes as short as 10 ms are found to result from large tipping angles in hig hly polarised samples (even in a very uniform held). These results are comp ared to previous findings in liquid helium mixtures, and to recent predicti ons on spectral clustering and instabilities induced by dipolar field inhom ogeneities. (C) 2000 Elsevier Science B.V. All rights reserved.