Photoelectron Photoion Coincidence Measurements of Selenium Cluster Beam. II. Photon Energy Dependence

The photon energy dependence of the photoelectron photoion coincidence (PEPICO) spectra near the Se K-edge have been measured for Se 2 and larger species such as Se 5 at the BL-12C in Photon Factory. The PEPICO spectra reveal that the multiply charged ions, Se z + , are produced as a result of the de-excitation process following the inner-shell excitation. Two prominent structures corresponding to the π * and σ * resonances are clearly observed in the energy dependence of the branching ratio from Se 2 , while only one structure corresponding to the σ * resonance can be seen for the larger species. At the σ * resonance, the production of Se + and Se 2+ is enhanced, in comparison with the direct ionization where the core-hole states are completely relaxed before the dissociation. The energies of the π * and σ * resonance relative to the 1 s ionization potential are estimated at -10.0±0.5 eV and -5±1 eV, respectively. In addition, we propose a new method for the size-selective structural analysis of clusters...