Asymmetry of the intercollisional interference dips in the collision-induced absorption spectrum of molecular hydrogen

Quadrupole-allowed and collision-induced fundamental band absorption in ${\mathrm{H}}_2$ has been studied at densities between 1.3 and 2.7 amagats and at high resolution (0.009 ${\mathrm{cm}}^{-1\mathrm{}}$). The absorption dips observed for several $Q$- and $S$-branch transitions are asymmetric about the center frequency. The asymmetry is attributed to collisional phase shifts, and a simple expression for the dip line shape is developed. When this dip line-shape function is combined with a Galatry profile for the narrow quadrupole absorption features, the data can be satisfactorily fitted. The resulting width and phase-shift parameter values for each dip feature are consistent with one another and with the assumptions used in the derivation of the line-shape function.