Nuclear-magnetic-resonance study of methane adsorbed on graphite

We have measured the spin-spin ($T_2$) and spin-lattice ($T_1$) relaxation times of methane adsorbed on Grafoil using pulsed nuclear-magnetic-resonance techniques. Data were taken between 45 and 105 K and between 0.20 and 1.1 monolayers at frequencies of 4.586 and 2.107 MHz. In addition, proton relaxation times were measured with mixtures of 50% C${\mathrm{D}}_4$+50% C${\mathrm{H}}_4$. It was determined that relaxation was via intramolecular dipolar interactions. Activation energies computed from the $T_1$ data provide evidence for both coverage-driven (at 0.87 monolayers) and temperature-driven commensurate-incommensurate phase transitions. A frequency-independent jump in $T_1$ at lower coverages indicates first-order melting to the liquid at approximately 57 K. Higher-temperature data find supporting evidence for a hypercritical fluid (two-dimensional gas) above a critical temperature of about 75 K.