Abstract.We report the results of NMR studies of the dynamics of 3 He adsorbed on hexagonal boron nitride. These studies can identify the phase transitions of the 2D films as a function of temperature. A thermally activated temperature dependence is observed for 2.6 < T < 8 K compared to a linear temperature dependence for 0.7 < T < 2.6 K. This linear dependence is consistent with that expected for thermal diffusion in a fluid for coverages of 0.4 -0.6 of a monolayer. IntroductionHelium-three monolayer and submonolayer films are of great interest because the different phases and microscopic dynamics that occur are dominated by quantum effects that lead to new phenomena in reduced dimensions at low temperatures [1,2]. In addition the films are ideal for testing our understanding of the quantum dynamics and spin ordering in two dimensions from first principles [3]. At low temperatures it had been postulated that the large quantum mechanical zero-point kinetic energy of the 3 He atoms could be sufficient to prevent the system from condensing as a 2D liquid in some scenarios [4], leading to a unique quantum gas at extremely low temperatures. Experimental studies by Bhattacharyya and Gasparini [5,6] offered evidence for such a phase but the majority of experiments [7][8][9] and the most recent theoretical treatments [10][11][12] do not support the existence of a low temperature gas phase. Recent thermodynamic measurements by Sato et al. [13,14], however, for 3 He adsorbed on bare graphite (and on 4 He plated graphite), have provided new evidence in support of a self-bound state for very low coverages at very low temperatures. We have carried out NMR studies at much higher coverages of 0.41 and 0.62 of a full monolayer and at higher temperatures to determine the microscopic dynamics of the 3 He atoms and the mechanisms of diffusion at coverages near the fluid-solid phase transition region [12,15]. At these coverages NMR studies by Crane and colleagues [16] for 3 He on BN showed evidence of a mixed coverage of fluid and solid components with the solid melting near 1.6K to form a dense fluid state with appreciable orientational order imposed by the substrate. We have carried out studies using a much higher NMR frequency which should help distinguish between the fluid and solid components.In addition to measuring the diffusion in the fluid phase, NMR studies can be used to determine the spin exchange rates in the solid phases. Below monolayer coverage, the 3 He monolayer forms a commensurate √ 3 × √ 3 solid structure that provides a unique testing ground for the multi-particle exchange model [17,18] invoked to explain the magnetic properties of bulk 3 He and 3 He films [15,19,20]. For example, coverages less than the ideal commensurate structure lead to a reduced probability of 3-particle exchange compared to the 2-particle exchange and

We report the results of measurements of the nuclear spin-lattice and spin-spin relaxation rates of hydrogen deuteride trapped in the mesoporous cages of a metal organic framework 1 (MOF) for temperatures 2.2 < T < 50 K. There is considerable interest in the use of this class of material for hydrogen storage because of the high density of adsorption. NMR studies can provide important information about the molecular interactions and dynamics inside the cages of the MOF structure. Samples were studied with filling factors of 0.5 and 1.0 molecules per cage as determined by the adsorption isotherm at 77 K. The results show strong peaks in the relaxation times at several well defined temperatures that are very different from the adsorption energy levels. These peaks are discussed in terms of the quantization of the translational degrees of freedom of the molecules inside the cages and the associated discrete energy levels. Measurements of the nuclear spin-spin relaxation times provide an important measure of the diffusivity of hydrogen through the MOF structure which is a critical parameter for the use of MOFs for storage and transport.

Measurements of nuclear spin relaxation times over a wide temperature range have been used to determine the interaction energies and molecular dynamics of light molecular gases trapped in the cages of microporous structures. The experiments are designed so that, in the cases explored, the local excitations and the corresponding heat capacities determine the observed nuclear spin-lattice relaxation times. The results indicate well-defined excitation energies for low densities of methane and hydrogen deuteride in zeolite structures. The values obtained for methane are consistent with Monte Carlo calculations of A.V. Kumar et al. The results also confirm the high mobility and diffusivity of hydrogen deuteride in zeolite structures at low temperatures as observed by neutron scattering.

We review the results of nuclear magnetic resonance studies of the molecular dynamics of the quantum gases HD and CH 4 adsorbed in the cages of microporous structures. Measurements of the variation of the nuclear spinlattice and nuclear spin-spin relaxation times with temperature provide detailed information about the translational and rotational dynamics of the adsorbed molecules over a wide temperature range. PACS: 76.60.-k Nuclear magnetic resonance and relaxation.