Melting of ap−H2monolayer on a lithium substrate

Abstract

Adsorption of $p\text{\ensuremath{-}}{\mathrm{H}}_{2}$ films on alkali metals substrates at low temperature is studied theoretically by means of path integral Monte Carlo simulations. Realistic potentials are utilized to model the interaction between two $p\text{\ensuremath{-}}{\mathrm{H}}_{2}$ molecules, as well as between a $p\text{\ensuremath{-}}{\mathrm{H}}_{2}$ molecule and the substrate, assumed smooth. Results show that adsorption of $p\text{\ensuremath{-}}{\mathrm{H}}_{2}$ on a lithium substrate, the most attractive among the alkali, occurs through completion of successive solid adlayers. Each layer has a two-dimensional density ${\ensuremath{\theta}}_{e}\ensuremath{\approx}0.070\phantom{\rule{0.3em}{0ex}}{\mathrm{\AA{}}}^{\ensuremath{-}2}$. A solid $p\text{\ensuremath{-}}{\mathrm{H}}_{2}$ monolayer displays a higher degree of confinement, in the direction perpendicular to the substrate, than a monolayer helium film, and has a melting temperature of about $6.5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The other alkali substrates are not attractive enough to be wetted by ${\mathrm{H}}_{2}$ at low temperature. No evidence of a possible superfluid phase of $p\text{\ensuremath{-}}{\mathrm{H}}_{2}$ is seen in these systems.