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arXiv:1805.03458 · Physical Review A

Resonant electron scattering and dielectronic recombination in two-center atomic systems

A. Eckey, A. Jacob, Alexander B Voitkiv, Carsten Müller

Published Jul 23, 2018• 12 citations
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Abstract

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.

DOI
10.1103/physreva.98.012710

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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.
We consider radiative recombination and photoionization in an atomic system, which consists of two subsystems AAA and BBB. These subsystems are well separated in space and it is supposed that AAA has a lower ionization potential. In such a case photoionization of AAA and recombination of an incident electron with A+{A}^{+}A+ can be strongly influenced, via two-center electron-electron correlations, by resonant electron dipole transitions induced in BBB. A theoretical description of these two-center resonant dielectronic processes is presented.
A free electron can form a bound state with an atomic center AAA upon photoemission (radiative recombination). In the presence of a neighboring atom BBB, such a bound state can, under certain conditions, be also formed via resonant transfer of energy to BBB, with its subsequent relaxation through radiative decay (two-center dielectronic recombination). This two-center process is very efficient in the ``static'' case where AAA and BBB form a weakly bound system, dominating over single-center radiative recombination up to internuclear distances as large as several nanometers. Here we study its dynamic variant in which recombination occurs when a beam of species AAA collides with a gas of atoms BBB and show that, even though the average distance between AAA and BBB in collisions is orders of magnitude larger than the typical size of a bound system, the two-center recombination can still outperform the single-center radiative recombination.
Although dielectronic and radiative recombination are usually treated as distinct noninterfering processes, Alber, Cooper, and Rau [Phys. Rev. A 30, 2845 (1984)] have recently presented a scattering- or S-matrix analysis which provides a unified description of these processes. This description employs a diagonalization of the atom plus radiation field Hamiltonian using a limited basis set consisting of one discrete autoionizing state, a single-electron continuum, and a single-photon continuum. In the present work we extend Mo?ller scattering operator and resolvent operator techniques, which have previously been used to discuss the decay of prepared systems, in order to provide an S-matrix analysis of the electron-ion photorecombination process near an isolated autoionizing resonance. We explicitly allow for degenerate magnetic sublevels of the atomic system and for multiple angular momentum contributions in the partial-wave expansion of the electron-continuum eigenstate. After the introduction of the pole approximation, in which only the \ensuremath{\delta}-function term is retained in the evaluation of the various self-energies that occur in the diagonalization of the Hamiltonian for the combined many-electron radiation-field system, we obtain the total electron-ion photorecombination cross section as the sum of the radiative and dielectronic recombination contributions together with the conventionally ignored interference term.The radiative and dielectronic recombination cross sections reduce to the familiar forms when the continuum-continuum coupling effects are neglected. Alternatively, the combined cross section for the entire electron-ion photorecombination process may be represented by a modified Fano line profile, which is shifted and broadened as a result of the coupling between the autoionization and radiation continua. Recombination processes that involve more than a single state of the initial ion, of the autoionizing resonance, or of the final system can be treated by appropriately augmenting the unperturbed basis set. It is anticipated that the effects of the interference between radiative and dielectronic recombination and of the continuum-continuum coupling will be most important for individual transitions involving low-lying autoionizing levels and will probably be negligible for the total dielectronic recombination rates due to the highly excited levels.
Photoionization of an atom AAA by a strong laser field in the presence of a spatially well-separated neighboring atom BBB is considered. The laser field frequency is assumed to lie below the ionization potential of atom AAA and be resonant with a dipole-allowed transition in atom BBB. In this situation, the ionization may occur either directly by multiphoton absorption from the laser field at the first atomic center or via an indirect pathway involving two-center electron-electron correlations, where the neighbor atom BBB is first photoexcited and, afterwards, transfers its energy upon deexcitation radiationlessly to atom AAA. Considering monochromatic as well as bichromatic laser fields, we study various coupling regimes of the photoionization process and identify experimentally accessible parameter domains where the two-center channel is dominant.
This volume offers a timely look at Rydberg states of atoms in external fields and dielectronic recombination. Each topic provides authoritative coverage, presents a fresh account of a flourishing field of current atomic physics and introduces new opportunities for discovery and development. Topics considered include electron-atom scattering in external fields; observations of regular and irregular motion as exemplified by the quadratic zeeman effect and other systems; Rydberg atoms in external fields and the Coulomb geometry; crossed-field effects in the absorption spectrum of lithium in a magnetic field; precise studies of static electric field ionization; widths and shapes of stark resonances in sodium above the saddle point; studies of electric field effects and barium autoionizing resonances; autoionization and dielectronic recombination in plasma electric microfields; dielectronic recombination measurements on multicharged ions; merged beam studies of dielectronic recombination; Rydberg atoms and dielectronic recombination in astrophysics; and observations on dielectronic recombination.
We have studied two-electron one-photon transitions in dielectronic recombination with H-like ions (Z=5−−54Z=5--54Z=5−−54) within ab initio QED theory. It was found that interference between dielectonic recombination with two-electron one-photon transitions and radiative recombination is prominent for light and medium ions being large enough for experimental observation. The interference manifests itself firstly in a sharp Fano-like resonance structure and secondly in a noticeable distortion of the photon angular distribution of the radiative recombination.