Photon-electron coincidence experiments at synchrotron radiation facilities with arbitrary bunch modes

Efficient neutralization of core ionized species in an aqueous environment

Core ionization dynamics of argon-water heteroclusters Ar_M[H₂O]_N are investigated using a site and process selective experimental scheme combining 3 keV electron irradiation with Auger electron-ion-ion multi-coincidence detection. The formation of Ar 2p^-1 vacancies followed by non-radiative decay to intermediate one-site doubly ionized states Ar²⁺(3p^-2)-Ar_M-1[H₂O]_N and subsequent redistribution of charge to the cluster environment are monitored. At low argon concentrations the emission of an [H₂O]_n'H⁺/[H₂O]_n''H⁺ ion pair is the dominant outcome, implying on high efficiency of charge transfer to the water network. Increasing the condensation fraction of argon in the mixed clusters and/or to pure argon clusters is reflected as a growing yield of Ar_m'⁺/Ar_m''⁺ ion pairs, providing a fingerprint of the precursor heterocluster beam composition. The coincident Auger electron spectra, resolved with better than 1 eV resolution, show only subtle differences and thereby reflect the local nature of the initial Auger decay step. The results lead to better understanding of inner shell ionization processes in heterogeneous clusters and in aqueous environments in general.