Collisional broadening and spectral shapes of absorption lines of free and nanopore-confined O₂ gas

This paper presents fully ab initio calculations of the broadenings and spectral shapes of O₂ infrared absorption lines in a free gas and when confined in nanoporous media. These calculations are performed, without use of any adjusted parameter, using a recently proposed approach that is based on requantized classical molecular-dynamics simulations. This involves studying the time evolutions of the translational and rotational motions of large numbers of molecules taking molecule-molecule and molecule-surface collisions into account through realistic interaction potentials. These simulations enable predictions of dipole autocorrelation functions whose Fourier-Laplace transforms yield the associated spectra. Comparisons are then made with broadening coefficients and line shapes provided by new and previous experiments. The good agreement between calculated and measured results confirms the veracity of the proposed model for a free gas and shows that the effects of confinement, which induce significant modifications to the line shapes, are correctly predicted. The need for improved characterization of the shape and size of pores in random nanoporous media is highlighted.