Core-level interatomic Coulombic decay in van der Waals clusters

We report on the experimental observation of the direct decay of a core vacancy in van der Waals clusters by emission of a fast electron from a neighboring atom. The process can be regarded as an interatomic Coulombic decay of core holes (core-level ICD). We identify it unambiguously by electron-electron and electron-electron-photon coincidence spectroscopy of the decay of 2p vacancies in Ar clusters. While several earlier works reported the absence of this channel, we find core-level ICD to be of considerable significance and quantify the branching ratio of this nonlocal electron emission to conventional local Auger decay as (0.8±0.2)%. Our results are supported by calculations on smaller clusters and show a reasonable agreement. This report on a successfully performed electron-electron-photon coincidence experiment provides a perspective for explorations of matter exposed to ionizing radiation. The observed core-level ICD is proposed to be of general importance for studies on charge redistribution after core-level photoionization where van der Waals clusters are often used as prototype systems.