Spectroscopic aspects of subthreshold Siegert states

The Siegert states are approached in the framework of Bloch–Lane–Robson theory of quantum collisions. Both the bound and the quasi-stationary Siegert states are subject of the equation relating the channel R-matrix element to the logarithmic derivative. The Siegert state dependence on the decay channel parameters results in channel renormalization of reduced widths, especially near threshold. The neutron subthreshold and the electron Rydberg states are examples of subthreshold Siegert states. The Siegert approach results in Heisenberg’s S-matrix formula for the bound state and the subthreshold resonance. The Siegert state residue is a spectroscopic asymptotic normalization constant. The decay width of the electron Rydberg channel resonance is, up to a factor, the electron strength function.