Close similarity of the electronic structure and electron correlation in gas-phase and solid C60

Peculiar covalent bonding of C60/6H-SiC(0 0 0 1)-(3 × 3) probed by photoelectron spectroscopy

High resolution photoemission with synchrotron radiation was used to study the interface formation of a thin layer of C₆₀ on 6H-SiC(0 0 0 1)-(3 × 3), characterized by protruding Si-tetramers. The results show that C₆₀ is chemisorbed by orbital hybridization between the highest-occupied molecular orbital (HOMO) and the p _z orbital of Si adatom at the apex of the tetramers. The covalent nature of the bonding was inferred from core level as well as valence band spectra. The Si 2p spectra reveal that a large fraction (at least 45%) of the Si adatoms remain unbound despite the reactive character of the associated dangling bonds. This is consistent with a model in which each C₆₀ is attached to the substrate through a single covalent C₆₀-Si bond. A binding energy shift of the core levels associated with sub-surface Si or C atoms indicates a decrease of the SiC band bending caused by a charge transfer from the C₆₀ molecules to the substrate via the formation of donor-like interface states.

Spectromicroscopy of C60 and azafullerene C59N: Identifying surface adsorbed water

C60 fullerene crystals may serve as important catalysts for interstellar organic chemistry. To explore this possibility, the electronic structures of free-standing powders of C60 and (C59N)2 azafullerenes are characterized using X-ray microscopy with near-edge X-ray adsorption fine structure (NEXAFS) spectroscopy, closely coupled with density functional theory (DFT) calculations. This is supported with X-ray photoelectron spectroscopy (XPS) measurements and associated core-level shift DFT calculations. We compare the oxygen 1s spectra from oxygen impurities in C60 and C59N, and calculate a range of possible oxidized and hydroxylated structures and associated formation barriers. These results allow us to propose a model for the oxygen present in these samples, notably the importance of water surface adsorption and possible ice formation. Water adsorption on C60 crystal surfaces may prove important for astrobiological studies of interstellar amino acid formation.

X-ray induced fragmentation of size-selected salt cluster-ions stored in an ion trap

A method combining X-ray synchrotron radiation and mass spectrometry is presented. Ammonium bisulphate cluster ions in an ion trap exhibit well-defined core-level absorption edges in the fragment-ion abundance spectra.

Size dependence of L2,3 branching ratio and 2p core-hole screening in x-ray absorption of metal clusters

Resonant 2p x-ray absorption spectra of size-selected transition metal ions and clusters consisting of 1<or=n<or=200 atoms are reported. Remnants of atomic multiplet splitting in L(2,3)-edge x-ray absorption can be resolved up to the trimer, above which the overall line shape is already bulklike. A strong cluster size dependence of the L(2,3) branching ratio was found for titanium, vanadium, and cobalt clusters. While 3d electron delocalization increases with cluster size, the apparent 2p spin-orbit splitting remains constant within the error bars. The size dependence of the L(2,3) branching ratio can be attributed to cluster size specific screening of the 2p-3d Coulomb interaction by 3d/4s valence electrons.

Polarized near-edge x-ray-absorption fine structure spectroscopy of C60-functionalized 11-amino-1-undecane thiol self-assembled monolayer: Molecular orientation and Evidence for C60 aggregation

Near-edge x-ray-absorption fine structure (NEXAFS) spectroscopy was adopted to probe the unoccupied electronic states of C60 anchored onto an organized assembly of 11-amino-1-undecane thiol on Au(111). The polarization dependence of the intensity of pi* resonance associated with C60 pi network revealed the self-assembled monolayer (SAM) system to be oriented with an average molecular tilt angle of 57 degrees with respect to the surface normal. Invoking the absence of solid-state band dispersion effects and in comparison to solid C60 and /or 1-ML C60/Au(111), the electronic structure of the resulting assembly was found dominated by spectral position shift and linewidth and intensity changes of the lowest unoccupied molecular orbital (LUMO), LUMO+1, and LUMO+2 orbitals. The latter implied hybridization between N Pz of -NH2 group of thiolate SAM and pi levels of C60, resulting in a nucleophilic addition with a change in the symmetry of C60 from Ih to C1 in the SAM. Occurrence of a new feature at 285.3 eV in the NEXAFS spectrum, assigned previously to pi* graphitic LUMO, signified the formation of aggregated clusters, (C60)n of C60 monomer. Low tunneling current scanning tunneling microscopy confirmed them to be spherical and stable aggregates with n approximately 5.

Imaging delocalized electron clouds: photoionization of C(60) in Fourier reciprocal space

The dynamics of the photoionization of the two outermost orbitals of C(60) has been studied in the oscillatory regime from threshold to the carbon K edge. We show that geometrical properties of the fullerene electronic hull, such as its diameter and thickness, are contained in the partial photoionization cross sections by examining ratios of partial cross sections as a function of the photon wave number in the Fourier conjugated space. Evaluated in this unconventional manner photoemission data reveal directly the desired spatial information.