Stability patterns and ionization potentials of CrnOm clusters (n=3-50, m=0, 1, 2)

Chromium and chromium oxide clusters (CrnOm, n = 3-50, m = 0, 1, 2) were pr oduced by a laser vaporization cluster source, and investigated by laser ph otoionization spectroscopy and photofragmentation spectroscopy in combinati on with mass spectrometry. Ionization potentials for CrnOm clusters (m = 1, 2) deduced from photoionization spectroscopy measurements show a smoothly decreasing trend with size similar to the behavior of other transition meta ls. Superimposed on this is a diffuse step from 8 to 11 chromium atoms, pos sibly indicating a structural transition. Mass-abundance spectra recorded w ith low-fluence, high-photon-energy ionization (6.4 eV) reveal stability st eps for CrnOm clusters (n = 3-50, m = 1, 2), corresponding to icosahedral g eometrical shell closings insensitive to the oxygen content. These magic nu mbers are also found for CrnOm+ clusters (n = 3-50, m = 0, 1, 2) investigat ed with photofragmentation.