The nature of the absorption bandshape density evolution for the first overtone of CO compressed by N2

Infrared spectra of the 2←0 absorption band of CO, compressed by N₂ at various densities up to 220 amagat at about 296 and 187 K have been analyzed. To interpret these spectra a line-by-line model has been applied, based on empirical corrections of the Lorentzian line-shape, which account roughly for the effects of line-mixing and the finite duration of collisions. At room temperature, this model provides a satisfactory agreement with experimental bandshapes, especially for the lower N₂ densities. At low temperature, the calculated absorption differs significantly from that observed around the band center, with a discrepancy increasing with the N₂ density. This difficulty is overcome by accounting for the binary—van der Waals complexes— CO…N₂ formation. The extent of these CO…N₂ complexes has been estimated from two models of bandshape fitting the experimental profiles and by a simple thermodynamic calculation.