Distribution of internal energy in CO and CO2 vibrationally excited by a hot platinum surface

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Abstract
Changes in the internal state of CO and CO2 molecules scattered by a hot platinum surface have been characterized by Fourier transform infrared emission spectroscopy. The only molecules which can be detected are those that are vibrationally excited for CO and excited to the asymmetric stretch mode for CO2. The distribution of rotational energy in these molecules is Boltzmann for CO2 but not for CO. In both cases, the rotational energy is substantially less than the equilibration values based on the surface temperature. The accommodation coefficient for the asymmetric stretch vibrational mode of CO2 drops from 0.22 at 700 K to 0.16 at 1500 K. Very few of the molecules which are vibrationally excited in the asymmetric stretch mode are also excited in either the bending or symmetric stretch mode. For CO, the vibrational accommodation coefficient is about 0.7. However, the fraction of the molecules excited to the v=2 vibrational level compared to the v=1 level indicates complete vibrational accommodation at the surface temperature. This implies a partitioning of CO molecules between those that completely accommodate vibrationally and those that do not accommodate at all.