We have measured the NMR spin-lattice relaxation time and second moment for ethylene and methane monolayers adsorbed on a graphite substrate. Coverages were in the low-density-solid region. Substantial rotational mobility (apparently isotropic) is found for both systems at temperatures below the freezing transition. Some or all of this mobility is lost in a second, rotational transition for ethylene at about 35 K and for methane at about 17 K.
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Larese et al. (1985) studied this question.
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