Stability of rare gas cluster ionsPhotoionization 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.

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Photoionisation experiments were performed with heterogeneous Ar-Xe-clusters produced by supersonic expansion of argon gas with small quantities of xenon added to it. A threshold-electron photoionisation (TEPICO) technique was used to obtain time of flight cluster mass spectra. These mass spectra show particularly strong intensities for Ar12Xe + and ArlsXe + which are attributed to the extraordinary stabilities of these cluster ions. Maxima in the ionic size distribution around ArTXe + are related to a particular abundance of neutral Ar12Xe which is fragmented after ionization. These stabilities are explained in terms of geometries consisting of a central Xe atom or ion surrounded by shells of Ar atoms. Filled shells exhibit particular strong bonding because these exhibit the largest number of atom-atom bonds. This conclusion is supported by simple theoretical calculations. The ionization process is discussed in terms of two direct and one indirect ionization channels the latter one proceeding via an intermediate electronic excitation of the Ar component in the neutral cluster.

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