Theory of collision-induced absorption for spherical top molecules

Exact low-order classical moments in collision-induced bands by linear rotors: CO2-CO2

Exact and general analytic expressions are reported for the integrated intensity and the width of collision-induced absorption (CIA) and collision-induced scattering (CIS) bands by gases of centrosymmetric linear molecules. These expressions provide significant insight and allow assignment of partial second moments to the degrees of freedom of the colliding molecules. The expressions are applied to ambient CO(2), whose collisional spectra are reputed to be useful probes for terrestrial and planetary atmospheres. Compelling evidence of the substantial role of hitherto missing polarization and polarizability mechanisms is provided and is in remarkable agreement with experimental observation. Our findings allow the long-overdue simple interpretation of CIA and CIS by CO(2)-CO(2) without the need to resort to short-range interactions to offset the discrepancies between theory and experiment.

Molecular dynamics study of the stability of methane structure H clathrate hydrates

Molecular dynamics simulations are used to study the stability of structure H (sH) methane clathrate hydrates in a 3 x 3 x 3 sH unit cell replica. Simulations are performed at experimental conditions of 300 K and 2 GPa for three methane intermolecular potentials. The five small cages of the sH unit cell are assigned methane guest occupancies of one and large cage guest occupancies of one to five are considered. Radial distribution functions, unit cell volumes, and configurational energies are studied as a function of large cage CH(4) occupancy. Free energy calculations are carried out to determine the stability of clathrates for large cage occupancies. Large cage occupancy of five is the most stable configuration for a Lennard-Jones united-atom potential and the Tse-Klein-McDonald potential parametrized for condensed methane phases and two for the most stable configuation for the Murad and Gubbins potential.

The Continuum of Water Vapor Mixed with Methane: Absolute Absorption at 239 GHz and Linewidth Calculations

In the aim of achieving a better understanding of the molecular mechanisms involved in the absorption continuum of the water-vapor rotational spectrum, various H(2)O-X mixtures are investigated in an atmospheric window in the mmw range. In the present study, the X mixing gas is methane. Absolute absorption rates have been measured at 239 GHz. The dependence on pressure as well as temperature has been obtained. The experimental data are compared with models using conventional line profiles. As these models require the knowledge of the collisional half-widths of H(2)O broadened by CH(4), theoretical calculations using the complex Robert-Bonamy formalism have been carried out. A "continuum effect" is observed when comparing the experimental absorption with the models, as well as for the magnitude of the absorption discrepancy and for the strong temperature dependence of this absorption. The effect is compared for various X mixing gases to the collisional broadening efficiency in the impact approximation. Copyright 2000 Academic Press.