Observation ofU-branch transitions in the infrared fundamental band of solid hydrogen

$U$-branch transitions corresponding to 5←1 and 4←0 in the rotational quantum number $J$ of the ${\mathrm{H}}_2$ molecule have been clearly observed in the infrared fundamental band with a 3.6-cm absorbing path of the solid at ∼ 10 K. These transitions arise because of the hexadecapolar contribution to the intermolecular interaction, and this work represents the first observation of such absorption in solid ${\mathrm{H}}_2$. A measurement of the absorption intensity was made for the case of high-purity parahydrogen.