Observation of Rotational Excitations ofH2Adsorbed on Ag Surfaces

High-resolution electron-energy-loss studies of ${\mathrm{H}}_2$ on Ag at ∼ 10 K reveal both rotational and rotational-vibrational excitations of molecularly adsorbed and condensed ${\mathrm{H}}_2$. The rotational losses for submonolayers of adsorbed ${\mathrm{H}}_2$ indicate unhindered rotational motion as well as a H-H internuclear separation which is within ∼2% of that of condensed ${\mathrm{H}}_2$. Adsorbed ${\mathrm{H}}_2$ is also converted from predominantly orthohydrogen to parahydrogen.