Formation and emission of gold and silver carbide cluster ions in a single C60- surface impact at keV energies: Experiment and calculations.

Impact of fullerene ions (C₆₀^-) on a metallic surface at keV kinetic energies and under single collision conditions is used as an efficient way for generating gas phase carbide cluster ions of gold and silver, which were rarely explored before. Positively and negatively charged cluster ions, Au_nC_m⁺ (n = 1-5, 1 ≤ m ≤ 12), Ag_nC_m⁺ (n = 1-7, 1 ≤ m ≤ 7), Au_nC_m^- (n = 1-5, 1 ≤ m ≤ 10), and Ag_nC_m^- (n = 1-3, 1 ≤ m ≤ 6), were observed. The Au₃C₂⁺ and Ag₃C₂⁺ clusters are the most abundant cations in the corresponding mass spectra. Pronounced odd/even intensity alternations were observed for nearly all Au_nC_m^+/- and Ag_nC_m^+/- series. The time dependence of signal intensity for selected positive ions was measured over a broad range of C₆₀^- impact energies and fluxes. A few orders of magnitude immediate signal jump instantaneous with the C₆₀^- ion beam opening was observed, followed by a nearly constant plateau. It is concluded that the overall process of the fullerene collision and formation/ejection of the carbidic species can be described as a single impact event where the shattering of the incoming C₆₀^- ion into small Cm fragments occurs nearly instantaneously with the (multiple) pickup of metal atoms and resulting emission of the carbide clusters. Density functional theory calculations showed that the most stable configuration of the Au_nC_m⁺ (n = 1, 2) clusters is a linear carbon chain with one or two terminal gold atoms correspondingly (except for a bent configuration of Au2C+). The calculated AuCm adiabatic ionization energies showed parity alternations in agreement with the measured intensity alternations of the corresponding ions. The Au₃C₂⁺ ion possesses a basic Au2C2 acetylide structure with a π-coordinated third gold atom, forming a π-complex structure of the type [Au(π-Au2C2)]+. The calculation shows meaningful contributions of direct gold-gold bonding to the overall stability of the Au₃C₂⁺ complex. [ABSTRACT FROM AUTHOR]