Equilibrium dimer concentrations in gases and gas mixtures

Abstract

It is shown that dimer formation in gases is controlled by direct two-body contact collisions within a certain restricted range of energies, due to dimer-bond vibration to translation and to rotation energy conversions. This is in contrast to prior dimer theories which hold that three-body collisions are necessary. Association and dissociation transitions are found to proceed through the highest vibrational quantum level just below the dissociation limit of the intermolecular bond. Assuming Lennard-Jones or Morse interaction potentials, and anharmonic-oscillator vibrational energy levels for molecules captured in the intermolecular potential well, a simple formula is derived to calculate equilibrium dimer concentrations as a function of temperature. As inputs, two intermolecular potential parameters (well depth D α and stretch vibration ν α ) are required, besides molecular masses and mixing fractions. A correction is provided for dimers of molecules with low-energy wagging vibrations like UF 6, which absorb excess kinetic energy in a collision. Calculated dimer concentrations and trends agree much better with experiment than earlier formulations.