Note on Thermal Relaxation of CO2 in Presence of H2O and D2O Molecules

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The large effect of water vapor on the thermal relaxation of carbon dioxide has not yet been satisfactorily explained. In order to have more information on the characteristics of the phenomena involved, we have tried to compare the effects produced by H2O and D2O molecules on the relaxation of carbon dioxide by determining the changes in the dispersion curve. Commercial CO2 was used. The shape of the dispersion curve shows a departure from the single relaxation time theoretical curve, which increases with the H2O and D2O content. If this result is confirmed by others, it would indicate that these impurities strongly affect the mechanism of energy transfer among CO2 molecules. The objection, however, can be made that we have assumed that during the experiments the water absorbed on the walls of the interferometric system did not vary. In any case the comparison between the effects of H2O and D2O can be made. We find the values 1.86 and 2.50 for the respective ratios of the transition probabilities in the collision H2O-CO2 and D2O-CO2, the two values being determined from two different sets of mixtures. An approximate theoretical calculation of the ratio of the transition probabilities considering only the mass change of the added impurity, gives a value 1.25. The same ratio would instead be less than unity if the important process were the energy exchange between rotation of H2O or D2O molecules and vibration of CO2 molecules. The latter mechanism does not seem therefore to be responsible for the large decrease of relaxation time when water vapor is added to carbon dioxide.