Isotopic ordering in adsorbed hydrogen monolayers

Quantum ordering, mixing, and mobility of isotopic hydrogen mixtures adsorbed on graphite (0001) is studied in the monolayer range by diffraction, small angle (SANS), and quasielastic neutron scattering (QENS). Solid mixtures of H₂-D₂ commensurate with the graphite substrate remain random isotopic mixtures from 18 K down to 3 K. Incommensurate solid mixtures of H₂-D₂ at monolayer completion with and without a partial second layer adsorbed on top show a tendency towards phase separation and clustering as the temperature is reduced below 20 K. This tendency is monitored both by the change of the diffraction peak intensity with temperature and by features of the SANS measurements discussed in the text. Clustering slows down or stops at ≈8 K without reaching the full isotopic phase separation predicted on quantum-mechanical grounds. The QENS measurements measured the mobility of pure HD and HD-D₂ mixtures for two coverages. The mobility in the mixture is enhanced over the one of pure HD at the same coverage and temperature, an effect which can be related to a bond softening of the heavier D₂ molecule due to quantum motion.