NEAR-THRESHOLD PHOTOIONIZATION OF SILICON CLUSTERS IN THE 248-146 NM REGION - IONIZATION-POTENTIALS FOR SI(N)

Photoionization thresholds for silicon clusters in the size range Si2 to Si200 have been examined by laser photoionization with detection, by reflectron time-of-flight mass spectrometry. Stimulated Raman scattering light of narrow bandwidth 193 nm radiation was used as the ionization light source down to 146 nm. A characteristic size dependence of ionization potentials. (IPs) was found for clusters smaller than 22 atoms, featuring major maxima at n = 10 and 20. We also found a large gap in IPs between n = 20 and 22. This gap was ascribed to the structural transition of neutral clusters in analogy with that found recently for small silicon cluster ions. The IPs for larger clusters with n = 100-200 silicon atoms were found to be 5.0-5.17 eV, which are lower than the ionization energy but still higher than the work function of bulk Si(111) surface. This difference was discussed with relation to the nature of surface states for both phases.