Molecular hydrogen in porous Vycor glass

Raman measurements were performed of the lowest rotational J=0→2 and the intramolecular vibrational transition of p-${\mathrm{H}}_2$ confined in a Vycor rod. From the dependence on the amount of ${\mathrm{H}}_2$ adsorbed in the system, we are able to assign specific features in both spectra to molecules which are in contact with the surface. Studies of the temperature dependence show that below 12 K, the system is in a metastable state, which may result in a suddenly freezing in of the molecules, evidenced by a quenching of both transitions. For sufficiently slowly cooled systems, the molecules are able to orient into a phase which is neither hcp nor fcc. Evidence for the occurrence of islands of ${\mathrm{H}}_2$ is provided. For the smaller filling fractions, our results show that a part of the adsorbed molecules may remain in the liquid phase down to lower temperatures than so far observed.