Two Potential Energy Surfaces for the He-CO System

In 1994, two different two-dimensional models for the He-CO potential energy surface were determined by fitting quantum-mechanically simulated infrared spectra generated from trial potential energy functions, to discrete infrared transition energies of He-CO bimers mesured by A.W.R. McKellar [J.Chem.Phys. 101, 39 (1994)]. For both surfaces, the simulated spectra generated from the recommended models agree (on average) with the experimental data for 3He-CO and 4He-CO to within 1.6 - 1.8 times the estimated experimental uncertainties of +/-0.001 cm-1. The small remaining disagreement with experiment was attributed to the fact that the effect of the vibrational and centrifugal stretching of the CO monmer was approximated by a constant band-origin shift. Moreover, for both potential energy surfaces the (relative) intensities in the simulated spectra agree with experiment to within the uncertainties of the latter.

The first of these potential energy surfaces is the semi-empirical "V333" surface of Chuaqui et al. [J.Chem.Phys. 101, 39 (1994)], which is an empirical anisotropic analytic function constrained to have the correct theoretically-predicted (anisotropic) long-range tail. The second is the theoretically based "XC(fit)" surface of Bissonnette et al. [Faraday Disc.Chem.Soc.(London) 97, 81 (1994)], which was `morphed' to give good agreement with the infrared Van der Waals bimer spectra. While the V333 potential yields slightly better agreement with the high resolution IR data, which depend mainly on the potential well and low energy wall, Dham and Meath [Mol.Phys. 88, 339 (1996)] showed that the XC(fit) surface gave distinctly better predictions of a number of dilute gas properties (binary diffusion, interaction viscosity and mixture viscosity coefficients), which depend mainly onf the position and anisotropy of the short-range repulsive wall. Moreover, both give quite similar predictions for low energy elastic and inelastic differential scattering cross sections, and yielded similar levels of (dis)agreement with experimental very low temperature (T < 20 K) line broadening and shifting coefficients for pure rotational transitions of CO in He (see Thachuk et al. [J.Chem.Phys. 105, 4005 (1996)]).

Fortran subroutines for generating these V333 and XCfit potential energy surfaces for the He-CO system may be downloaded by filling in the requested information on the forms associated with the links below.

Subroutine V333HeCO will be e-mailed to anyone who fills in the form: Download the V333 He-CO Potential Energy Function Subroutine .

Subroutine XCfitHeCO will be e-mailed to anyone who fills in the form: Download the XCfit He-CO Potential Energy Function Subroutine .

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