A two-dimensional quantum crystal: H2 on NaCl(100)

Abstract

Monolayers of H2 on NaCl(100) at 5.2 K have been prepared from both normal and para-enriched hydrogen and examined by polarized infrared spectroscopy. Vibrational features associated with J=0 and J=1 molecules are distinguished. The spectroscopic data are consistent with hydrogen adsorbed to two distinct sites: surface Na+ ions and Cl− ions. For the para-enriched sample a two-dimensional crystal structure is proposed in which J=0 molecules reside on Na+ ions and J=1 molecules cover Cl− sites. The polarization measurements show that all transition dipoles are perpendicular to the NaCl(100) surface. The mechanism for the infrared absorption is through electric field induced transition dipoles.