Anomalous density dependence of the v₁ band of carbon dioxide in pressure-induced absorption

The anomaly in the density variation of the integrated absorption of the pressure-induced v 1 band of carbon dioxide is shown experimentally to be represented by a linear term in the density expansion of the intensity, which is important only in the lower density region (0–20 amagat). A calculation indicates that the linear term arises from the infrared-active absorption of the v 1 band of C 16 O 18 O.