Resonance effects in low-energy electron scattering by HBr

Earlier ab initio calculations for electron scattering by HBr are analysed to obtain the behaviour of the S-matrix poles in the complex momentum plane as a function of the internuclear distance, This analysis shows the existence of a pole corresponding to a bound state at large internuclear distances as well as a pole corresponding to a shape resonance at small internuclear distances. It is found that the bound-state pole does not evolve into the shape resonance pole as the internuclear distance decreases but instead splits into two mirror image poles which move onto unphysical Riemann sheets of the complex momentum plane confirming model-potential calculations by Herzenberg and Saha. The physical implications of the mirror image poles and the shape resonance pole are also discussed.