M. Rall

Low temperature radio frequency shielding of exfoliated graphite

Abstract The results of experiments using exfoliated graphite as a low temperature substrate for NMR studies of physisorbed molecules at ultra-high frequencies (100–300 MHz) are reported. The radio frequency shielding was observed to be anomalously high at very high frequencies for the graphite samples studied and precluded the use of this material for NMR studies requiring high sensitivity in this frequency range at liquid helium temperatures. It is shown that this behaviour can be understood in terms of the in- and out-of-plane conductivities and the capacitive coupling of the microcrystalline structure of the graphite.

NMR studies of molecular hydrogen in zeolite

We report new pulsed NMR studies of the supercooling of molecular hydrogen constrained to the interior of the molecular cages of zeolite. Measurements of the supercooling for the ultra-small structure of zeolite-5A in comparison with earlier studies show that the substantial supercooling observed does not depend strongly on the characteristic dimensions and is interpreted as a class-II growth attributed to imperfect wetting. Studies of the supercooling of HD and detailed analyses of the orientational ordering of molecular H2 show that the abrupt transitions observed in the pulsed NMR studies are associated with the supercooled liquid/solid transitions and are not associated with orientational ordering.

Vacancy-induced nuclear spin-lattice relaxation in solid H2

Abstract We report new results for the study of vacancy motion in solid hydrogen deduced from measurements of the nuclear spin-lattice relaxation of HD impurities. The temperature dependence below 11K is compared with Ebner and Sungs model for vacancy quantum tunneling.

Pressure dependence of tunneling of isotopic impurities in solid parahydrogen

Abstract Pulsed NMR measurements have been performed at low temperatures on solid hydrogen mixtures containing 1.1%HD and 0.5% ortho-H 2 impurities for pressures between 0.1 and 136 bar. The pressure dependence of the transverse spin relaxation time T 2 is analyzed in terms of a model in which the HD particles move through the parahydrogen lattice by quantum tunneling. The Gruneisen constant for the tunneling frequency τ −1 c , Γ τ c =∂ln τ c ∂ lnV , is found to be −31±3, using the theory of Kubo and Tomita to estimate τ c from T 2 .

Multistage separator column for ortho-hydrogen and para-deuterium production

Abstract A simple, seven-stage desorption column of activated alumina with independent thermal control of each stage has been used to provide highly enriched samples of ortho-hydrogen (97%) and para-deuterium (92%).

Imperfect wetting of hydrogen in zeolite

We have considered the theoretical dependencies of the amount of supercooling of liquid hydrogen as a function of pore size for constrained geometries in order to compare the different mechanisms for supercooling that can be observed: notably the inhibition of nucleation in small geometries and the use of surfaces for which there is imperfect wetting. Analysis of the dependence of the observed supercooling reported elsewhere for molecular hydrogen and hydrogen deutende on the pore size in zeolites supports the view that imperfect wetting occurs for small pores that have rough surfaces and dimension less than approximately 10Å.

NMR studies of vacancies in solid hydrogen

Abstract We report measurements of the vacancy-induced nuclear spin-lattice relaxation in solid hydrogen. The results are used to deduce (1) the characteristic motional frequency of the vacancies and (2) the formation energy of the vacancies at low densities.