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

Abstract.

Chromium and chromium oxide clusters (Cr _n O _m , n=3–50, m=0,1,2) were produced by a laser vaporization cluster source, and investigated by laser photoionization spectroscopy and photofragmentation spectroscopy in combination with mass spectrometry. Ionization potentials for Cr _n O _m clusters (m=1,2) deduced from photoionization spectroscopy measurements show a smoothly decreasing trend with size similar to the behavior of other transition metals. Superimposed on this is a diffuse step from 8 to 11 chromium atoms, possibly indicating a structural transition. Mass-abundance spectra recorded with low-fluence, high-photon-energy ionization (6.4 eV) reveal stability steps for Cr _n O _m clusters (n=3–50, m=1,2), corresponding to icosahedral geometrical shell closings insensitive to the oxygen content. These magic numbers are also found for Cr _n O _m ⁺ clusters (n=3–50, m=0,1,2) investigated with photofragmentation.