Coherent Interactions within the Atomic Continuum

Abstract

In the traditional view of photoionization of a one-valence-electron atom, the absorption of the photon raises the electron from a bound state into the continuum instantly, so to speak, and irreversibly. There is no characteristic time (analogous to the spontaneous lifetime of a discrete excited state) which we can associate with “how long” the electron “stays” in the continuum energy state to which it was raised. Another side of the same picture is that the dependence of the bound-free matrix element that determines the cross-section on the photon energy is smooth, exhibiting a slow variation originating from the oscillatory behavior of the wavefunctions. It shows no resonance-like structure. The situation changes significantly when the photoabsorption raises two electrons into the continuum. Then we encounter doubly excited discrete states embedded in (degenerate with) the single-electroncontinuum)^{[1, 2]}. The process (at least as long as the field is not too strong) is still irreversible, but there is now a characteristic time which can be viewed as the lifetime of the discrete state that has been formed in the continuum. The dependence of the photoionization cross section on the photon energy is no longer smooth but exhibits maxima and minima reflecting the interference between the amplitudes of the transition to the continuum and discrete parts of the wavefunction. By exciting the appropriate superposition of discrete and continuum wavefunction, we achieve a temporary localization and stabilization of an electron whose energy is above the ionization threshold. These so-called autoionizing states (or resonances) can have lifetimes ranging from less than a pico-second to microseconds, or in rare cases even be metastable against autoionizationl^[3], depending on the atom and the configuration.