Interchannel resonances at a threshold

Fermi's Golden Rule, the perturbation theoretic formula for calculating the halfwidth of a resonance, is not applicable to the case of a transition from a bound state into an open channel, when the energy of the bound state is exactly at the threshold of the continuum. We study solvable models of this phenomenon. The exact results coincide in leading order with the formulas found by modifying Fermi's Golden Rule. (C) 1996 American Institute of Physics.