Density dependence of the intramolecular distance in solidH2: A. Spectroscopic determination

We present an analysis of the measurements of the roton bands in p-${\mathrm{H}}_2$ at T=6 K. The quadrupole-quadrupole interaction gives the dominant anisotropic contribution up to at least 40 GPa and leads to a splitting of the J=0→2 rotational transition into three equally spaced branches. In the framework of the Van Kranendonk theory, the effect of the anisotropic interaction can be accounted for and the remaining variation of the roton frequency is then related to the change of the bond length of the ${\mathrm{H}}_2$ molecule, ${\mathit{R}}_{\mathrm{intra}}$. The present determination of ${\mathit{R}}_{\mathrm{intrais}}$ semiquantitatively valid but points to an interesting variation, with a minimum around 30 GPa. By analogy with measurements done on ${\mathrm{I}}_2$ and ${\mathrm{Br}}_2$, and with ab initio calculations, this demonstrates the importance of charge-transfer interaction in dense ${\mathrm{H}}_2$, even at pressures which are far from the regime of molecular dissociation.