Absolute Definition of Phase Shift in the Elastic Scattering of a Particle from Compound Systems

The projection of the target wave function on the total wave function of a scattered particle interacting with the target system is used to define an absolute phase shift including any multiples of π. With this definition of the absolute phase shift, one can prove rigorously in the limit of zero energy for s‐wave electrons scattered from atomic hydrogen that the triplet phase shift must approach a nonzero multiple of π. One can further show that at least one π of this phase shift is not connected with the existence of a bound state of the H− ion.