Molecular hydrogen trapped within {111}-oriented platelets in silicon is studied by means of Raman scattering and first principles theory. The rotational transition $S_0\left(0\right)$ ($J=0\to J=2$) of para-${\mathrm{H}}_2$ (nuclear spin $I=0$) at 353 ${\mathrm{cm}}^{-1}$ is used as a probe. We find that for temperatures below 100 K the $S_0\left(0\right)$ Raman line starts to broaden asymmetrically, which is interpreted as the onset of a phase transition from a state with a short-range order (“gaseous” or “liquid” phase) to a two-dimensional molecular crystal lying in the {111} plane of silicon. The shape of the $S_0\left(0\right)$ line at helium temperatures strongly depends on the relative content of ortho- (nuclear spin $I=1$) and para-${\mathrm{H}}_2$ revealing the details of the intermolecular interaction. A comprehensive theoretical analysis based on ab initio calculations, molecular dynamics simulations, and rotational spectra modeling reveals that the phase transition to the crystalline state of the two-dimensional hydrogen does occur at temperatures substantially higher compared to those of bulk ${\mathrm{H}}_2$.

• Received 24 January 2018
• Revised 25 February 2018

DOI:https://doi.org/10.1103/PhysRevB.97.125307