Vibrational spectroscopy ofH2in porous Vycor glass: First evidence for the bilayer structure

Infrared-absorption measurements have been performed on the fundamental band of physisorbed ${\mathrm{H}}_2$. Distinct vibrational, rotational, and adsorbate phonon features induced by surface and intermolecular interactions are identified. When compared with adsorption isotherms the surface-enhanced vibrational line saturates at 40% of the surface layer coverage which results from the short range of the surface-induced dipole in Steele’s bilayer model and is inconsistent with a high-density monolayer phase. Our results also strongly point to the formation of islands at low coverages.