Perturbation theory and atornic resonances since Schrodinger's time

Quantum theory makes a sharp distinction between bound states and scattering states, the former associated with point spectrum and the latter with continuous spectrum. Resonances associated with quasi-stationary states bridge this distinction, and have posed mathematical challenges since the beginning of the Schrodinger theory. Here the development of the mathematical underpinnings of resonance theory in atomic physics is reviewed, with particular reference to the role of the (DC) Stark effect, and time-independent perturbations of bound states in the two-body problem in atomic and molecular physics.