X-ray Absorption Fine Structure of Free Selenium Clusters

Although clusters have attracted great interest as intermediate states between atomic and condensed states of matter [1], general methods for the size-selective structural analysis of neutral clusters have not been established so far. Recently we measured the Pho toe lec t ron Pho to ion Coincidence (PEPICO) spectra for selenium dimer Se2 and larger species such as Se5 [2,3]. The PEPICO spectra reveal that the multiply charged ions are produced as a result of de-excitation processes following the K -shell excitation and fragmented to atomic ions with z charges, Sez+, owing to the Coulomb explosion (see Fig. 1). The branching ratios from the parent cluster to the daughter ions exhibit clear dependence on the size N of the parent c lusters. In part icular, the branching ratios in the non-resonant absorption region are well reproduced considering that the charges were randomly distributed within the clusters before the Coulomb explosion. Based upon these findings, we proposed a new method for the size-selective EXAFS of neutral free clusters by utilizing the PEPICO measurements [3]. The basic relations we have suggested are the simultaneous equations of the X-ray absorption coefficients per atom, α (hν), parameterized by z, as follows: