PHOTOIONIZATION SPECTROSCOPY OF SMALL MERCURY CLUSTERS IN THE ENERGY-RANGE FROM VACUUM-ULTRAVIOLET TO SOFT-X-RAY

We have measured the single and double photoionization efficiency of Hgn mass selected clusters as a function of photon energy between 30 and 110 eV for masses ranging from n=1 to 36 for the singly ionized and from n=5 to 70 for the doubly ionized clusters, using the light beam issued from a wiggler at the BESSY synchrotron. The results show in all cases a spectrum similar to the 5d→εf shape resonance previously observed in the isolated atom. In contrast, the expected 5p→6p transition allowed by hybridization of the 6s and 6p orbitals is not observed. We also compared the mass spectra of singly and doubly charged mercury clusters ionized by electron impact (30-90 eV) to those of clusters ionized by synchrotron radiation (30-70 eV). We found that the ratio Hgn2+/Hgn + was higher for photoionization at low energy and for electron impact ionization at high energy. Finally, we estimated the critical stability size of the triply charged clusters. © 1995 American Institute of Physics.