  We report on the first experimental observation of a new threshold behavior
observed in the 5g partial channel in photodetachment of K-. It arises from the
repulsive polarization interaction between the detached electron and the
residual K(5g) atom, which has a large negative dipole polarizability. In order
to account for the observation in the K(5g) channel, we have developed a
semiclassical model that predicts an exponential energy dependence for the
cross section. The measurements were made with collinear laser-ion beams and a
resonance ionization detection scheme.
