Equilibrium dimer concentrations in gases and gas mixtures

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 di mer-bond vibration to translation and to rotation energy conversions. This is in contrast to prior dimer theories which hold that three-body collision s are necessary. Association and dissociation transitions are found to proc eed through the highest vibrational quantum level just below the dissociati on limit of the intermolecular bond. Assuming Lennard-Jones or Morse intera ction potentials, and anharmonic-oscillator vibrational energy levels for m olecules captured in the intermolecular potential well, a simple formula is derived to calculate equilibrium dimer concentrations as a function of tem perature. As inputs, two intermolecular potential parameters (well depth D- alpha and stretch vibration v(alpha)) are required, besides molecular masse s and mixing fractions. A correction is provided for dimers of molecules wi th low-energy wagging vibrations like UF6, which absorb excess kinetic ener gy in a collision. Calculated dimer concentrations and trends agree much be tter with experiment than earlier formulations. (C) 2001 Elsevier Science B .V. All rights reserved.