Multi-coincidence spectroscopy on interatomic Coulombic decay

The inner-shell ionized states in matter are subject to Auger decay. Auger spectra are considered as fingerprint images of the atom where the inner-shell hole is created. About a decade ago, Cederbaum and coworkers [1] proposed a new mechanism of electronic decay where the environment plays a role. For an isolated atom or molecule with an innervalence vacancy, Auger decay is often energetically forbidden but interatomic or intermolecular Coulombic decay (ICD) may occur when another species is in close proximity. Later, the experimental evidences of ICD were provided by electron spectroscopy [2] and by multicoincidence spectroscopy [3]. ICD can take place also after Auger decay as second-step decay [4]. We have been investigating ICD after Auger decay in rare-gas dimers, such as Ar2 [5], ArKr [6], and Ne2 [7], as well as trimers [8] and larger clusters, using momentum-resolved electron-ion multi-coincidence spectroscopy. By this method, we can measure three-dimensional momentum of the photoelectron and/or ICD electron together with those of two or three ions in coincidence and extract angular and energy correlations between the photoelectron/ICD electron and fragment ions. This allows us to identify the ICD channels without any ambiguity and, furthermore, to classify the ICD channels into two classes, one via virtual photon exchange and the other via electron exchange. The present work has been carried out in fruitful collaboration with a number of colleagues whose names can be found in the authors’ list of Refs. [5-8], with approval of JASRI, and supported by JSPS, MEXT, BMBF, and DFG.