Librational relaxation and IR line broadening of matrix isolated CO

The IR line profile corresponding to the fundamental vibrational transition of CO isolated in an Ar matrix has been studied as a function of the temperature. The spectrum consists of two superimposed bands, a sharp one and a broad one whose linewidths are respectively 0.45 and 2.7 cm^?1 at 10 K. The strong temperature dependence of the CO lineshape is mainly due to the large change in the intensity ratio of the two components. Such a behaviour is accounted for by relaxation of librational populations. The calculated contributions of the processes connecting the ground and first excited librational states to the IR linewidth are of the same order of magnitude as the values experimentally measured. However the experimental lineshape of the sharp component being rather gaussian than lorentzian, it is concluded that librational relaxation contributes significantly to the broadening of the infrared CO band but not exclusively.