  At high energies, single-photon photodetachment of alkali negative ions
populates final states where both the ejected electron and the residual valence
electron possess high angular momenta. The photodetached electron interacts
strongly with the anisotropic core, and thus the partial cross sections for
these channels display non-Wigner threshold behavior reflecting these large,
and occasionally repulsive, interactions. Our fully quantum mechanical
theoretical study enables a deeper interpretation of these partial cross
sections. Comparisons of the behavior in different channels and between
different atomic species - sodium, potassium, and cesium - show the critical
role of near-degeneracies in the energy spectrum and demonstrate that much of
the behavior of the partial photodetachment cross sections stems from the
permanent, rather than induced, electric dipole moments of these
nearly-degenerate channels. This provides a concrete example of a system where
negative dispersion forces play a decisive role.
