The dip observed in the Q branch of the pressure-induced vibrational spectra at high densities is shown to be an interference effect due to the correlations existing between the dipole moments induced in successive binary collisions. A similar dip is predicted to exist in the translational spectra of inertgas mixtures at zero frequency. This intercollisional interference effect has the same origin as the dip in the spectral density of the intermolecular force, discussed by Purcell in connection with nuclear electric dipole relaxation. The effect does not occur for the anisotropic part of the induced dipole moments, and this explains the observed absence of any splitting of the S lines and of the QQ component of the Q branch of the induced infrared spectra.