Vibrational Relaxation Times of a Number of Polyatomic Gases Derived from Measurements of Acoustic Absorption

Acoustic attenuation in gas mixtures with nitrogen: experimental data and calculations

Attenuation in a gas results from a combination of classical attenuation, attenuation from diffusion, and attenuation due to molecular relaxation. In previous papers [J. Acoust. Soc. Am. 109, 1955 (2001); 110, 2974 (2001)] a model is described that predicts the attenuation from vibrational relaxation in gas mixtures. In order to validate this model, the attenuation was measured using a pulse technique with four transducer pairs, each with a different resonant frequency. The attenuation calculated using the model was compared to the measured values for a variety of gases including: air, oxygen, methane, hydrogen, and mixtures of oxygen/nitrogen, methane/nitrogen, carbon dioxide/nitrogen, and hydrogen/nitrogen. After the measured data is corrected for diffraction, the model matches the trends in the measured attenuation spectrum for this extensive set of gas mixtures.

Acoustic attenuation in three-component gas mixtures--theory

Vibrational relaxation accounts for absorption and dispersion of acoustic waves in gases that can be significantly greater than the classical absorption mechanisms related to shear viscosity and heat conduction. This vibrational relaxation results from retarded energy exchange between translational and intramolecular vibrational degrees of freedom. Theoretical calculation of the vibrational relaxation time of gases based on the theory of Landau and Teller [Phys. Z. Sovjetunion 10, 34 (1936); 1, 88 (1932): 2, 46 (1932)] and Schwartz et al. [J. Chem. Phys. 20, 1591 (1952)] has been applied at room temperature to ternary mixtures of polyatomic gases containing nitrogen, water vapor, and methane. Due to vibrational-translational and vibrational-vibrational coupling between all three components in ternary mixtures, multiple relaxation processes produce effective relaxation frequencies affecting the attenuation of sound. The dependence of effective relaxation frequencies and the attenuation on mole fractions of the constituents was investigated. The acoustic attenuation in a mixture that is primarily nitrogen is strongly dependent on the concentrations of methane and water vapor that are present. However, the attenuation in a mixture that is primarily methane is only weakly dependent on the concentrations of nitrogen and water vapor. The theory developed in this paper is applicable to other multicomponent mixtures.

Energy transfer in polyatomic molecules I. Catalysis of energy transfer with isotopic molecules

Dispersion of sound has been com pared for ethylene and tetradeutero-ethylene. Relaxation times of 0.26 and 0.14 u s for these molecules follow the general trend with molecular frequencies predicted by theory. A quantitative account of th e observed ratio 1:2 of relaxation times can best be given by introducing the notion of ‘complex collisions’ and by abandoning the concept that the rate of activation of the lowest vibrational mode fully controls the rate of activation of higher modes. Small amounts of C 2 D 4 or C 2 D 2 H 2 act as energy transfer catalysts for ethylene. Both H 2 O and D 2 O likewise act as catalysts, approximately to the same extent. Theoretical interpretations of these findings are discussed.