Translational Band of Gaseous Hydrogen at Low Temperature

Abstract

A comparison is made between the observed submillimeter absorption spectrum of normal hydrogen gas at three different temperatures near 25 K and a fully quantum mechanical ab initio calculated spectrum. Measurements cover the wavenumber range 20-320 cm-1, where, under the experimental conditions of low temperature and low pressure, the pure translational band is observed virtually isolated from the S(0) rotational line. The theoretical analysis shows that the H2 spectrum is primarily due to transient collision-induced dipole moments in pairs of colliding molecules, i.e., due to free-free transitions. Bound-free transitions of the H2 dimer are responsible for an additional ~10% of the absorption intensity. The agreement between the theoretical spectra and the measurements is excellent, thereby verifying that the intermolecular potential, the wave functions, and the model of the induced dipole moment have all been accurately represented. As a consequence, it is possible to predict with confidence the absorption spectrum of H2 in physical conditions not readily accessible to laboratory measurements.