Photoelectron photoion coincidence measurements of selenium cluster beam. I. Evidence for the Coulomb explosion

The photoelectron photoion coincidence (PEPICO) spectra have been measured for Set and larger cluster, "Se-5", at photon energies of 12.63 keV and 12.68 keV (i.e., on the both sides of the Se K-edge) by utilizing synchrotron radiation at the BL-12C station in Photon Factory, KEK. In the PEPICO spectra of Set, broad peaks corresponding to z/n = 1 to 7 are observed and no clear peaks are seen at the half-integer of tin. Most of the peaks are split into asymmetric doublets. These facts indicate that multiply charged ions are formed by the vacancy cascade as a consequence of X-ray absorption and exploded due to the Coulomb repulsion (Coulomb explosion). The experimentally observed peak width in the PEPICO spectrum can be interpreted quantitatively by assuming that the maximum initial charge before the Coulomb explosion is equal to 8. Although there is large apparent difference between the branching ratios of Set and "Se-5", they are well reproduced on the both sides of the K-edge, by considering that the charges are randomly distributed within the clusters before the Coulomb explosion.