TRANSLATIONAL BAND OF GASEOUS-HYDROGEN AT LOW-TEMPERATURE

A comparison is made between the observed submillimeter absorption spe ctrum of normal hydrogen gas at three different temperatures near 25 K and a fully quantum mechanical ab initio calculated spectrum. Measure ments cover the wavenumber range 20-320 cm(-1), where, under the exper imental conditions of low temperature and low pressure, the pure trans lational band is observed virtually isolated from the S(0) rotational line. The theoretical analysis shows that the H-2 spectrum is primaril y due to transient collision-induced dipole moments in pairs of collid ing molecules, i.e., due to free-free transitions. Bound-free transiti ons of the H-2 dimer are responsible for an additional similar to 10% of the absorption intensity. The agreement between the theoretical spe ctra and the measurements is excellent, thereby verifying that the int ermolecular potential, the wave functions, and the model of the induce d dipole moment have all been accurately represented. As a consequence , it is possible to predict with confidence the absorption spectrum of H-2 in physical conditions not readily accessible to laboratory measu rements.