NMR Study of the Orientational Behavior of Hydrogen Adsorbed on Graphite

Abstract

NMR has been used to study the orientational behavior of registered lattices of ortho-${\mathrm{H}}_2$ and para-${\mathrm{D}}_2$ molecules adsorbed on graphite in the range 1.3-4.2 K. The results show a weak crystal field (0.56 K for $o$-${\mathrm{H}}_2$ and 2.4 K for $p$-${\mathrm{D}}_2$). In addition, $p$-${\mathrm{D}}_2$ molecules experience a temperature-dependent molecular field arising from electrostatic quadrupole-quadrupole interactions. No orientational ordering occurs down to 1.3 K, a factor of 2 below the mean-field transition temperature.