Coincidence, multiphoton and non-linear methods
Coincidence measurements allow to relate different properties of individual excitation, relaxation and decay processes after the interaction of photons with matter. Such measurements yield multi-dimensional cross sections involving the simultaneous detection of, e.g., a scattered or an emitted photon, one or more ejected electrons and/or desorbed or excited/ionized atomic/molecular target species. Using this method, the multi-particle dynamics may be investigated in great detail and processes may be uncovered that are otherwise hidden below large background signals. Initial-state, final-state or dynamic electron correlation [1,2] as well as shake-off  may, e.g., directly show up in the coincidence spectra and the coupling between electronic and atomic degrees of freedom may be investigated as well.
As an example, the measurement of coincidences in photon-atom/molecule interactions may involve the following (see the animation below): A photon beam interacts with a single target atom/molecule that effuses from a gas inlet just above the scattering center. The target is ionized and an ejected electron (green track) may be detected with an electron time-of-flight (TOF) analyzer. After a small delay, a positive potential is applied to the gas inlet system. The transmitted part of the photon beam pulse may be detected as a time-reference signal for the TOF measurement(s). The pulsed positive potential at the gas inlet repels the positively charged recoil ion (light blue track) into a recoil time-of-flight analyzer, where the time information is used to determine the charge state and mass of the recoil ion. Now we have a coincidence between emitted ELECTRON and detected RECOIL-ION. In case of a so-called true coincidence, they belong to the same excitation event. This means, we know the target mass and charge state, as well as the momentum vector of the ejected electron. If all particles involved in this excitation event are detected, we have performed a complete experiment and conservation of energy as well as linear and angular momentum may directly be checked. From these checks it is directly evident whether a particle or photon has escaped the detection.
Animated figure: Time-evolution of a coincidence event, involving the transmitted photon pulse, an emitted electron and an accelerated target recoil-ion.
Within the institute G-I2, we are heading towards efficient experimental techniques on Core-hole/hole coincidences – Excitation dynamics and next generation ESCA double-hole coincidences, that will advance our current knowledge on the excitation dynamics in multi-electron systems, leading in addition to new applications in the determination of chemical structures. 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 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)
- "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)