Core-hole/hole coincidences – Excitation dynamics and next generation ESCA
A certain class of coincidence measurements Coincidence methods allows to observe multi-hole states in ionic particles such as atoms, molecules and clusters. Such measurements involve at least the detection of a recoil-ion together with one electron  or the detection of two electrons, e.g., using a magnetic bottle time-of-flight spectrometer . One may identify double core-hole states by requiring the detection of two inner-shell Auger electrons as well as a consistent photo electron . Within a single atom, multi-electron excitation processes may proceed via the shake-off mechanism, influenced significantly by electron-correlation effects in the initial state . Contrary, statistically independent multi-photon ionization processes may be triggered by the high virtual photon field of projectile ions  or by the high photon densities in the short pulse of a Free-Electron Laser (FEL) Free-Electron Lasers. This ionization mechanism would also result in ionic target atoms with larger and even very large distances within a molecule or a cluster. A third mechanism, which however favors multi-hole states at shorter atomic distances, relies on dynamic correlation (electron-electron collision) in the intermediate state. One of the current hot topics in electron dynamics is the distinction and quantitative investigation of these three multi-ionization mechanisms. The production and analysis of multi-hole states, however, points also to a new application that is described in the following.
The production and identification of a double-core-hole (DCH) state, after multi-photon ionization (at an FEL) or double ionization via dynamic correlation (at BESSY II) enables an improved electron spectroscopy for chemical analysis (ESCA). The spectroscopy of Auger electrons from such a DCH state is sensitive to the interaction of the emitting atom with the positive charge of the (separated) second core hole. This interaction leads to an Auger energy reduction (in atomic or Hartree units) of 1/|Ri,j|, where Ri,j is the (localized) hole/hole distance in atomic units (equal to the Bohr radius). Thus, high resolution spectroscopy on such states in molecules or clusters should show several satellite lines with line shifts related to atomic distances and intensities related to the multiplicity of a certain distance within a molecule. Thus, we expect to be sensitive on the chemical structure and able to distinguish isomers. It is clear that this DCH spectroscopy at BESSY yields distance information similar as the EXAFS technique. The coincidence requirement, however, may fix the target nuclear charge of both active atoms with high precision, not only the emitter atom as in EXAFS. Furthermore, application of this technique at an FEL would relate atoms with large distances, far beyond the sensitivity range of EXAFS.
Selected references on experiments using coincidences between electrons and other electrons or target recoil-ions:
- "Evidence for Electron Correlation in the Two-Electron Continuum During Double Ionization in 300-keV p + He Collisions" B. Skogvall and G. Schiwietz, Phys.Rev.Lett. 65, 3265 (1990)
- "Double Core Hole Creation and Subsequent Auger Decay in NH3 and CH4 Molecules" J. H. D. Eland, M. Tashiro, P. Linusson, M. Ehara, K. Ueda, and R. Feifel, Phys. Rev. Lett. 105, 213005 (2010)
- "Double Ionization of Helium by 40 MeV Protons" G. Schiwietz, G. Xiao, P.L. Grande, B. Skogvall, R. Köhrbrück, B. Sulik, K. Sommer, A. Schmoldt, U. Stettner, and A. Salin, Europhys.Lett. 27, 341-346 (1994)