Isotopic ordering in adsorbed hydrogen monolayers

Quantum ordering, mixing, and mobility of isotopic hydrogen mixtures adsorb ed on graphite (0001) is studied in the monolayer range by diffraction, sma ll angle (SANS), and quasielastic neutron scattering (QENS). Solid mixtures of H-2-D-2 commensurate with the graphite substrate remain random isotopic mixtures from 18 K down to 3 K. Incommensurate solid mixtures of H-2-D-2 a t monolayer completion with and without a partial second layer adsorbed on top show a tendency towards phase separation and clustering as the temperat ure 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 SAN S measurements discussed in the text. Clustering slows down or stops at app roximate to 8 K without reaching the full isotopic phase separation predict ed on quantum-mechanical grounds. The QENS measurements measured the mobili ty of pure HD and HD-D-2 mixtures for two coverages. The mobility in the mi xture is enhanced over the one of pure HD at the same coverage and temperat ure, an effect which can be related to a bond softening of the heavier D-2 molecule due to quantum motion. [S0163-1829(99)01936-0].