RMN non-linéaire dans les liquides polarisés - Non-linear NMR in polarised liquids – DIPOL

The development of high field NMR and the continual search for optimised sensitivity have been accompanied by the observation of various new phenomena, related to the non-linear evolution of nuclear magnetisation in liquid samples. In most cases, unusual behaviours result from the intricate combination of (i) the non-linear coupling between the transverse nuclear magnetisation and the detection coil (radiation damping) and (ii) the enhanced contribution of long-range magnetic interactions, not averaged out by the Brownian motion (distant dipolar fields, DDFs). Radiation damping has become quite common due to the widespread use of improved or miniaturised probes, with high quality and high filling factors. DDF effects, difficult to observe with conventional NMR in thermally polarised systems, have recently been brought to the fore in high resolution spectroscopy. This research project aims at cooperative action for detailed investigation and better understanding of the complex non-linear NMR dynamics in highly polarised liquids. It aims at a quantitative characterisation and a comprehensive description of the spectacular effects that have been observed in dense laser polarised solutions, motivated by the growing impact of DDFs on dynamics in high field NMR, as well as in a wide range of other physical systems (such as Bose-Einstein condensates, superfluid 3He, degenerate 3He-4He mixtures, and two-dimensional atomic hydrogen gas). The joint development of new theoretical and experimental tools for efficient control of the evolution of the nuclear magnetisation is expected to lead to powerful ways to: (i) reduce the foreseen limits, set by DDF effects, to the efficiency of sophisticated pulse sequences currently at use in high resolution NMR spectroscopy, (ii) clarify recent major developments in MRI, based on DDF used as contrast agent, and (iii) take full advantage of NMR signal enhancements, recently achieved by DDF-induced transfer of nuclear polarisation from dissolved xenon to other nuclei. For more than ten years now, Paris and Saclay have independently worked on experimental and numerical investigations of DFF effects (spectral clustering, instabilities) using complementary polarised systems and approaches. Paris uses cryogenics techniques and takes advantage of the record dipolar field strengths obtained in laser polarised liquid 3He by metastability exchange optical pumping, of the wide range of diffusion coefficients reached in isotopic mixtures by temperature control or dilution in superfluid 4He, and of operation at millitesla field strengths for improved control of static field inhomogeneities. Saclay combines expertise in spin exchange optical pumping of 129Xe (using rubidium as laser pumped alkali atom) and room temperature high field spectroscopy (11.7T), taking advantage of the large chemical shift sensitivity to individually probe solute and solvent dynamics. New features of temporal evolution have recently been observed at both places. In Paris, time-reversal has been achieved and will be combined to diffusion weighting to probe growth and structure of DDF-induced magnetisation patterns. In Saclay, multiple maser emission and non-linear xenon relaxation have been observed and will be investigated using innovative high sensitivity techniques. The project also includes joint experimental and numerical work for the development of robust new pulse sequences for efficient handling of magnetisation in the presence of strong DDF and radiation damping. Finally, a new theoretical framework will be developed to better describe DDF-induced phenomena, taking into account the dynamic correlations that are overlooked in the classical description of long-range static dipolar couplings in liquids.