Electron excitation and bombardment effect induced by polyatomic ions

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We studied the electronic energy-loss of swift polyatomic ions in solids and the secondary electron emission under their bombardment. First, we developed the self-consistent average charge theory for cluster ions. Our theory predicts the cluster charge average per ion is less than that of the isolated ion at an equivalent speed. The dependences on the number of constituent atoms, on the foil thickness, and on the spatial structure were found to be well explained. By taking into the coulomb explosion and the cluster average charge, we calculated the electronic energy loss of carbon cluster ions with the MeV/atom energies. In spite of the reduction of the average charge per ion, the energy loss per ion of the cluster is greater than that of the single ion at an equivalent speed(this is called the 'positive' cluster effect). In our treatment, the 'negative' cluster effect is also predicted for lower ion speeds. We found that the threshold speed in this cluster effect is located around two times Bohr velocity for the carbon clusters, and it does not so strongly depend on the foil thickness. Next, we investigated the secondary electron(SE) emission from carbon foil by the MeV/atom carbon clusters. The relative SE energy spectra per ion calculated on the basis of the three-step model showed the sublinear effect. This result is consistent with the recent experimental data by Kudo et al. The relative SE yield is also related with the cluster stopping power and with the cluster average charge. Finally we studied the multiple ionization process by the ionization probability in the 'energy transfer model'. The calculated cross sections agree with the data for rare gas atoms even for the incidence of swift highly charged ions, without any fitting parameters.