The absorption spectrum of the ν = 1 ← 0 band of HD has been investigated at a temperature of 77 K and for densities in the range of 15 to 140 amagat. The band consists of two components: a broad collision-induced quasi continuum arising from dipoles induced during molecular collisions; and a dipole-allowed part arising from the small permanent electric dipole moment of the free HD molecule. The interference effects which occur between these two components were studied for the dipole-allowed R1(0) and R1(1) transitions. These transitions exhibited increasingly large asymmetries and changes in intensity at high density, but their behaviours were quite different from each other. The shape of each transition could be well represented by a series of Fano line profiles, and the evolution of shape and intensity with density could be accounted for by the formulation of Herman, Tipping, and Poll. However, the precise interpretation of the phase shift parameters arising in the theory is not clear.
