Higher Ionization Energies from Sequential Vacuum-Ultraviolet Multiphoton Ionization of Size-Selected Silicon Cluster Cations

The second to fifth ionization energies and the appearance sizes of the higher charge states of silicon clusters with 6–92 atoms are determined by sequential vacuum-ultraviolet multiphoton ionization of size-selected cluster ions in a cryogenic ion trap. The higher ionization energies are well described by the charging energy in a spherical droplet model. In contrast to the electron affinity and the first ionization energy of silicon clusters, the higher ionization energies seem to change less abruptly at the prolate-to-spherical shape transition.