Observation of U -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 H₂ 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 H₂. A measurement of the absorption intensity was made for the case of high-purity parahydrogen.