Near infrared absorption of fluid CO and CO2

First and second stretching overtones of pure CO and some combination and overtone modes of CO2 between 3200 cm−1 and 7300 cm−1 have been measured in an extended density range to maximum pressures and temperatures of 3100 bar and 500 K, respectively. Near infrared bandshapes of both CO overtones, almost up to the critical density, are adequately represented by summing up over the individual rotation-vibration lines of the gaseous spectrum with half-widths proportional to density. CO band positions, with increasing density, linearily shift to lower wavenumbers whereas in CO2, after an initial decrease, maximum wavenumbers become constant or even increase toward the highest densities which suggests that repulsive interactions come into play. Vibrational intensities are independent of temperature and show only minor variations with density which enables concentration measurements on CO and CO2 in fluid phase equilibria and reactions via high-pressure near infrared spectroscopy.