Complex scattering lengths in multi-channel atom–molecule collisions

It is shown that in the presence of inelastic scattering, zero energy elastic and inelastic scattering can be characterized by a complex scattering length, the imaginary part of which is related directly to the total inelastic scattering cross section. Collisions between H atoms and vibrationally excited H2 molecules are investigated. Zero energy cross sections for all vibrational states of H2 and the corresponding complex scattering lengths are reported using accurate quantum mechanical calculations. We obtain large values of the elastic cross sections which we attribute to s-wave bound and quasi-bound states of H⋯H2(v). The energies and lifetimes of the quasi-bound states are extracted from the complex scattering lengths.