Collision Transfer of Vibrational Energy from Nitrogen and Methane to the Carbon Dioxide Molecule

Abstract

The sound absorption of CO2 has been measured at 95°C from 10 to 5 × 106 Hz/atm with 99%–92.5% N2 and at 80°C from 2 × 104 to 4 × 107 Hz/atm with 10%–90% CH4 admixed. N2 has been chosen because of its good resonance to the asymmetric valence-bond vibration of CO2 (ν3), while CH4 resonates approximately with the CO2 levels at about 1900°K. The results were analyzed by a modified “binary two-level” theory of the quantum exchange mechanism between the two molecules. From the CO2–N2 measurements, the reciprocal de-excitation probabilities of ν3 were obtained to be 1 / P = 58 000, 1 / P = 40 000···130 000 in collisions with N2 and CO2, respectively. The de-excitation of the first bending vibration level in collisions with nitrogen needs 1 / P = 58 000 collisions. The same analysis applied to the CO2–CH4 mixtures yielded the following transition probabilities: de-excitation of CO2 (0110) by CH4, 1 / P = 1000; resonance production of two states (0110) from one at 1900°K in collision with ground-state CO2 molecules, 1 / P = 150; quantum exchange from excited CH4 to the CO2 levels at 1900°K, 1 / P = 2600; de-excitation of CH4 in collisions with CO2, 1 / P>106. The fact that the relaxing molar heat capacity of the CO2–CH4 system exceeded its total vibrational heat capacity is explained as an influence of rotational–vibrational transitions, in which preferentially the vibration is de-excited and the rotation excited. Such processes also would explain the high efficiency of the (fast-rotating) methane for the de-excitation of the CO2 bending vibration.