Vibrational–vibrational coupling in air at low humidities

Abstract

Calculations of sound absorption in air are traditionally based on the assumption that molecular relaxations in nitrogen and oxygen are independent. In binary mixtures of these two gases, however, they are not independent; rather, molecular relaxation is known to be controlled by a very strong vibrational–vibrational (V–V) coupling, which influences both the relaxation frequencies and the relaxation strengths. This article shows that small concentrations of the air constituents carbon dioxide and water vapor, which themselves possess a strong V–V coupling to N2 and O2, serve to decouple the N2 and O2 relaxations. To characterize the N2–O2 coupling a ‘‘coupling strength’’ is derived, which depends upon the constituent concentrations and the related reaction rate constants. It is found that the molecular relaxations associated with N2 and O2 in air experience a gradual transition from strong to weak coupling as the humidity increases beyond approximately 0.001 mole %.