Predissociation of weak-anisotropy Van der Waals molecules. Theory, approximations and practical predictions

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Abstract
Widths and energies for predissociating levels of H2–inert-gas Van der Waals molecules are calculated by solving the close-coupling equations for accurate potential-energy surfaces. These exact results are used to test several approximate schemes previously employed for calculating level energies and widths. None of these is found to be entirely satisfactory, and a new method is proposed which gives more accurate results. In this new method (SEPTOC) the secular equations are solved in matrix form for the closed-channel manifold, and the open channels are treated by perturbation theory. The limited form of SEPTOC tested here gives very promising results. The level widths are found to be very sensitive to the potential used, particularly in the repulsive region. For H2–Ar, two different potentials which both reproduce the level energies are found to give level widths which differ by ca. 30%. This is the first conclusive demonstration that measurements of predissociation widths can give potential information different from that yielded by the level energies.

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