Size-Selective Extended X-ray Absorption Fine Structure Spectroscopy of Free Selenium Clusters

In a recent paper [M. Yao et al., J. Synchrotron Radiat. 8, 542 (2001)], we proposed a new method for the size-selective EXAFS (extended x-ray absorption fine structure) of neutral-free clusters, in which not only the x-ray absorption process but also the deexcitation processes are utilized as the structural information. In order to verify this method experimentally, we have developed the synchronous measurements of EXAFS and photoelectron photoion coincidence and carried them out for a Se cluster beam by utilizing the third-generation intense x-ray source. The EXAFS spectra for Se small clusters have been obtained and compared critically with theoretical predictions.

Figure 1

The total ion yield (TIY) versus x-ray photon energy for the Se cluster beam. Unusual data points due to the Bragg reflection from the monochromator crystal were omitted. The inset shows the EXAFS spectrum ${\chi }_T(k)$ deduced from the TIY as a function of the photoelectron wave number.

Figure 2

The PEPICO spectrum versus time of flight for the Se cluster beam at the preedge region (a), at the resonance absorption region (b), at the beginning (c), and end (d) of the EXAFS region.

Figure 3

The partial EXAFS spectrum ${\chi }_z(k)$ for $z=1$ to 7 deduced from the partial ion yields.

Figure 4

Size-selective EXAFS spectra $\chi (k)$ for ${\mathrm{S}\mathrm{e}}_2$ [circles in (a)] and ${\mathrm{S}\mathrm{e}}_{n\ge 5}$ [squares in (b)]. $\chi (k)$ for v-Se and m-Se are shown by the lines in (a) and (b), respectively. The dashed lines are the filtered Fourier transform of the first peak.