NMR on 3He Adsorbed in Grafoil at Near Monolayer Completion: The Solid Phase
The effect of mobility of adsorbed atoms on thermal measurements such as specific heat and vapor pressure isotherms is not very direct. NMR relaxation times, however, do provide temporal information of a quite direct kind. Existing relaxation time data come principally from three studies. Rollefson,1 using graphitized carbon black as substrate, carried out cw NMR measurements at 20.5 MHz. He concluded that for fractional coverages x above 0.7 of a completed monolayer the NMR line which is narrowed by motion at 4 K, broadens as the sample is cooled, reaching the rigid lattice value for the case of x = 0.9 by about 2.5 K. Grimmer and Luszcynski,2 using pulsed NMR methods on 3He adsorbed in grafoil, found that T2 at 1 K and 4 K for a monolayer was about half as long at 20 MHz as at 10 MHz, which rules out dipole-dipole coupling as the main relaxation mechanism. Hedge, Lerner, and Daunt3 working at 5.5 MHz in grafoil found a similar change to Rollefson in the NMR line width as a function of temperature for a sample near monolayer completion (x = 0.96), but the low temperature line width was still a factor 4 lower than the rigid lattice line width, suggesting some quantum tunnelling was occurring.
KeywordsAnisotropy Graphite Carbide Helium Boulder
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