Photoionization thresholds of rare gas clusters

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Abstract
Photoionization efficiency curves of rare gas clusters generated by supersonic expansion of pure argon, krypton, and xenon gas have been measured as a function of photon energy using light from the Berlin Electron Synchrotron Facility (BESSY). Assuming that the relative shapes of the efficiency curves are independent of cluster size, ionization potentials for each cluster can be extracted from the data by using the known absolute ionization thresholds of the dimer or trimer ions. The data agree with the model that ionic clusters are composed of an ionic substructure surrounded by shells of atoms. In particular, it has been confirmed that Ar+3 is the central substructure in Ar+n for n<15±1. In addition we have estimated the ratio of the covalent binding energy to the charge-induced dipole binding energy for the ions. The larger dipole-induced binding energy in the case of xenon may explain the differences between the mass spectra of Xe and Ar clusters.