The chemistry and physics of molecular surfaces

Ionization potentials of MgN (N = 7-56) clusters formed by spontaneous collapse of magnesium foam in helium nanodroplets

The ionization potentials of magnesium clusters (Mg_N, N = 7-56) are determined by doping ultracold helium nanodroplets (He_M, M ≈ 52 000) with Mg atoms. Inspecting the particle size distributions resulting from non-resonant, short-wavelength, single-photon ionization gives evidence that beyond a certain ensemble size, the developing foam structure undergoes a spontaneous collapse on the way to the laser interaction region. As a result, hot Mg clusters form in the relaxation process. The spontaneous collapse manifests in a substantial change in the size distributions, when recording mass spectra at wavelengths shorter than 272 nm. Tracing individual Mg_N signals as a function of laser photon energy allows extraction of size-specific ionization potentials, which for small clusters show a good agreement with results obtained from density functional theory simulations. The further development is compared to calculations based on the liquid drop model. However, even when quantum effects are included, the simple scaling law is not able to reproduce the development of the ionization potentials. The results suggest that small neutral magnesium clusters behave as non-metallic. The comparison to electron affinities and band gaps obtained from photoemission experiments on Mg_N^- provides information on the charge state dependence of the non-metal-to-metal transition and properties like the Mulliken electron negativity.

REACTIONS OFTRANSITIONMETALCLUSTERS WITHSMALLMOLECULES

Atoms and small molecules react with transition metal clusters in ways that are analogous to the physisorption and chemisorption reactions observed on the corresponding extended metal surface. However, often underlying these similarities are size-dependent variations in the reaction mechanisms and rates, the interpretation of which requires a detailed understanding of the structures of both the bare metal cluster substrates and the cluster-molecule complexes. Although polyatomic transition metal clusters cannot be characterized by the traditional methods of molecular spectroscopy, the combination of other physical and chemical probes can provide qualitative and semiquantitative structural information. These techniques, when combined with equilibrium geometries calculated using ab initio or semiempirical methods, provide a detailed picture of the structural origin of metal cluster reactivity and its variation with size.