a Theoretical Analysis of Discrete State-Continuum Atomic Interactions.

Aspects of the decay of discrete states into continua for several atomic systems and models are considered theoretically. In the first chapters, resonant multiphoton ionization of atoms is considered. The free-electron continuum is treated perturbatively and with matrix elements determined using the quantum defect method, but resonant radiative coupling between bound states is treated non-perturbatively. Turn -on effects are studied for the ionization of sodium, and it is found that continuum structure can lead to non-monotonic ionization probability in short-time regimes when a wide range of continuum states can be populated. Turn-on effects are also studied for four-photon (three-photon resonant) ionization of cesium for 15 psec laser pulses, and it is shown that laser turn-on gives a line shape with width nearly independent of laser intensity. In subsequent chapters it is shown how the Moller operators of scattering theory can be used to diagonalize discrete state-continuum systems and to describe the decay of prepared states, and the technique is applied to several model systems. The problem of a single state embedded in a free-electron continuum (the "Fano problem") is analyzed in detail and the importance of the standard "pole approximation" is discussed. Threshold effects and the possibility of incomplete ionization are investigated. Vacuum field fluctuations (spontaneous emission) coupling the discrete state and continuum to some bound atomic state are considered and the importance of the coupling between the electron and photon continua is investigated. A "modified Fano profile" is presented. Finally the problem of laser-induced autoionization is considered. It is shown how interference effects arise in the decay and population can be trapped in the atom. It is found that spontaneous radiative decay partially destroys and provides a means of studying the coherence responsible for the trapping.