Electron-Atom Resonances

U. Fano

F. Giusiano, G. Mambriani

The analysis of a theoretical equation for the double differential cross-section of electron-atom collisions, in which "bremsstrahlung" (BS) photons are produced, and in particular of its dependence on both photon energy and electron scattering angle, shows that there is an optical sector of electron-atom resonances, that is, the appearance of resonance dips in BS cross-section. When considering, for instance, the resonance that the rare gases exhibit at nearly 0.5 eV below excitation threshold, this optical sector, or in other words the BS photon energy distribution, presents one or two resonancelike structures whose positions depend on both electron energy and scattering angle. By suitably choosing these parameters, one of these optical-resonance images can be made to occur at easily detectable photon energies. A successful experiment could be performed by utilizing an electron-photon coincidence method such as those used for studying coherence and correlation in electron-atom collisions. Particular conditions for such an experiment are considered, and its possible relevance for better insight into electron-atom resonances is outlined.

A. Eckey, A. Jacob, Alexander B Voitkiv +1 more

Electron scattering and dielectronic recombination with an ion in the presence of a neighboring atom is studied. The incident electron is assumed to be captured by the ion, leading to resonant excitation of the atom which afterwards may stabilize either by electron reemission or radiative decay. We show that the participation of the atom can strongly affect, both quantitatively and qualitatively, the corresponding processes of electron scattering and recombination. Various ion-atom systems and electronic transitions are considered. In particular we show that electron scattering under backward angles may be strongly enhanced, provided the incident energy lies very close to the resonance. Besides, we derive the scaling behavior of two-center dielectronic recombination with the principal quantum numbers of the participating atomic states. The resonant enhancement in this process is shown to be so strong that it can compete with ordinary single-center recombination even after integrating over a rather broad distribution of incident electron energies.

Marvin H. Mittleman

A recent experiment in which an electron scattering off an atom at a resonance energy is excited to a higher resonance energy by absorption of a laser photon is analyzed. There is more than one mechanism for the reaction, but the cross section, as a function of the laser frequency, is shown to have only one peak for each pair of electron-atom resonance states which can be connected by absorption of a photon. The width of the peak is the width of the upper resonance state.