Collision Spectroscopy

Electron-ion collision spectroscopy utilizes energy-sharp features such as steps, thresholds and, in particular, resonant structures in the cross section of electron-ion collision processes in order to obtain precision spectroscopic information. This comprises not only excitation or transition energies but also deep insights into Matrix elements and, hence, the collision dynamics from high-resolution absolute cross-section data.

Dielectronic Recombination – the Primary Spectroscopic Tool for Collision Spectroscopy

The cross section of the process of dielectronic recombination (DR) features pronounced energy-sharp resonant structures. Theses resonances can be measured with high experimental resolution at storage-ring electron coolers or merged-beams free-electron targets, in particular at low electron-ion collision energies.

DR is a two-step process:

X^q+ +e− → X^{(q−1)+∗∗} → X^(q−1)+∗ +photons.

In the initial step termed dielectronic capture (DC; time-inverse to autoionization) a free electron is resonantly captured. DR collision spectroscopy is sensitive to this first step of DR: the electron-ion collision energy is scanned in fine steps. For center-of-mass (CM) collision energies that match the resonance energies a higher count rate of recombined ions, X^(q−1)+ , is recorded. DR collision spectroscopy can be regarded as Auger spectroscopy in inverse kinematics.

This experimental approach possesses high sensitivity and high precision allowing for the investigation of processes with low resonance strengths and/or with low intensity beams such as rare isotopes. Experiments with only a few hundred ions produced and injected every 10 minutes into the storage ring were successfully conducted. The list of physics questions studied with electron-ion collision spectroscopy at storage rings is long (> 100 Publications) and compromises among others:

• Precision studies of fundamental interactions, in particular on QED in strong fields or on time-reversal symmetry.
• Atomic structure and dynamics investigations in the relativistic domain.
• Correlated multiple-electron excitation processes.
• Influence of external fields on photorecombination.
• Measurement of rate coefficients for application in plasma physics or in astrophysics.
• Investigations of metastable ions including excitations from the metastable state, life time studies, and the hyperfine induced quenching of metastable states.
• Application of DR spectroscopy for the investigation of nuclear properties such as nuclear charge radii, nuclear magnetic moments and the mutual interaction of atomic electrons and the nucleus. More recently, the scope of this nuclear physics related experiments is extended to studies of radioisotopes and to studies of nuclear isomers.

Thresholds, steps and resonances in ionization

Not only the electron capture channel of electron-ion collision features well-defined and energy-sharp structures in the cross section : Following the above described DC, one, two or more electrons can be emitted from the intermediate resonant state, leading also to resonances in the excitation or in the (multiple-) ionization cross section.

In addition, after a electron-impact excitation excess energy can also be transferred into the emission of Auger electrons in so-called excitation-autoionization sequences that lead to a net-ionization. Step-like features with sharp edges become apparent as well in the excitation process itself but also in the (much easier to measure) ionization pathway.