Ultraviolet photoelectron spectroscopy of Nb − 4 to Nb − 200

Abstract. We present UV (hν = 6.42 eV) photoelectron spectra of niobium cluster anions Nb − n in the size range from n = 4 to n = 200. The spectra exhibit a variety of patterns, which can be related to the geometrical structures of the clusters. The charging energies of the larger clusters are in very good agreement with the metallic sphere model. Nevertheless a strong size dependence of the work function is observed, which underlines the special properties of this transition metal. PACS. 33.60.Cv Ultraviolet and vacuum ultraviolet photoelectron spectra -36.40.Cg Electronic and magnetic properties of clusters -73.22.-f Electronic structure of nanoscale materials

Large orbital magnetic moments of small, free cobalt cluster ions Co n with n lt; 9

The size dependent electronic structure and separate spin and orbital magnetic moments of free Co cluster ions have been investigated by x ray absorption and x ray magnetic circular dichroism spectroscopy in a cryogenic ion trap. A very large orbital magnetic moment of per atom was determined for Co, which is one order of magnitude larger than in the bulk metal. Large orbital magnetic moments per atom of amp; 8201; amp; 8201; amp; 8776;1 were also found for Co, Co, and Co. The orbital contribution to the total magnetic moment shows a non monotonic cluster size dependence The orbital contribution increases from a local minimum at n amp; 8201; amp; 8201; amp; 8201; amp; 8201;2 to a local maximum at n amp; 8201; amp; 8201; amp; 8201; amp; 8201;5 and then decreases with increasing cluster size. The 3d spin magnetic moment per atom is nearly constant and is solely defined by the number of 3d holes which shows that the 3d majority spin states are fully occupied, that is, 3d hole spin polarization is 10

Abrupt Change from Ionic to Covalent Bonding in Nickel Halides Accompanied by Ligand Field Inversion

The electronic configuration of transition metal centers and their ligands is crucial for redox reactions in metal catalysis and electrochemistry. We characterize the electronic structure of gas phase nickel monohalide cations via nickel L2,3 edge X ray absorption spectroscopy. Comparison with multiplet charge transfer simulations and experimental spectra of selectively prepared nickel monocations in both ground and excited state configurations are used to facilitate our analysis. Only for [NiF] with an assigned ground state of 3 amp; 928; can the bonding be described as predominantly ionic, while the heavier halides with assigned ground states of 3 amp; 928; or 3 amp; 916; exhibit a predominantly covalent contribution. The increase in covalency is accompanied by a transition from a classical ligand field for [NiF] to an inverted ligand field for [NiCl] , [NiBr] , and [NiI] , resulting in a leading 3d9 L amp; 818; configuration with a ligand hole L amp; 818; and a 3d occupation indicative of nickel I compounds. Hence, the absence of a ligand hole in [NiF] precludes any ligand based redox reactions. Additionally, we demonstrate that the shift in energy of the L3 resonance is reduced compared to that of isolated atoms upon the formation of covalent compound

Mn12_{12}-Acetate Complexes Studied as Single Molecules

The phenomenon of single molecule magnet (SMM) behavior of mixed valent Mn12 coordination clusters of general formula [Mn$^{III}$$_{8}$Mn$^{IV}$$_{4}$O$_{12}$(RCOO)$_{16}$(H$_{2}$O)$_{4}$] had been exemplified by bulk samples of the archetypal [Mn$^{III}$$_{8}$Mn$^{IV}$$_{4}$O$_{12}$(CH$_{3}$COO)$_{16}$(H$_{2}$O)$_{4}$] (4) molecule, and the molecular origin of the observed magnetic behavior has found support from extensive studies on the Mn12 system within crystalline material or on molecules attached to a variety of surfaces. Here we report the magnetic signature of the isolated cationic species [Mn$_{12}$O$_{12}$(CH$_{3}$COO)$_{15}$(CH$_{3}$CN)]$^{+}$ (1) by gas phase X-ray Magnetic Circular Dichroism (XMCD) spectroscopy, and we find it closely resembling that of the corresponding bulk samples. Furthermore, we report broken symmetry DFT calculations of spin densities and single ion tensors of the isolated, optimized complexes [Mn$_{12}$O$_{12}$(CH$_{3}$COO)$_{15}$(CH$_{3}$CN)]$^{+}$ (1), [[Mn$_{12}$O$_{12}$(CH$_{3}$COO)$_{16}$] (2), [Mn$_{12}$O$_{12}$(CH$_{3}$COO)$_{16}$(H$_{2}$O)$_{4}$] (3), and the complex in bulk geometry [Mn$^{III}$$_{8}$Mn$^{IV}$$_{4}$O$_{12}$(CH$_{3}$COO)$_{16}$(H$_{2}$O)$_{4}$] (5). The found magnetic fingerprints – experiment and theory alike – are of a remarkable robustness: The Mn$^{IV}$$_{4}$ core bears almost no magnetic anisotropy while the surrounding MnIII8 ring is highly anisotropic. These signatures are truly intrinsic properties of the Mn$_{12}$ core scaffold within all of these complexes and largely void of the environment. This likely holds irrespective of bulk packing effects

Experimental and theoretical near edge x ray absorption fine structure studies of NO

Experimental near edge x ray absorption fine structure NEXAFS spectra of the nitrosonium NO ion are presented and theoretically analyzed. While neutral NO has an open shell, the cation is a closed shell species, which for NEXAFS leads to the simplicity of a closed shell spectrum. Compared to neutral NO, the electrons in the cation experience a stronger Coulomb potential, which introduces a shift of the ionization potential towards higher energies, a depletion of intensity in a large interval above the amp; 960; amp; 8727; resonance, and a shift of the amp; 963; amp; 8727; resonance from the continuum to below the ionization threshold. NEXAFS features at the nitrogen and oxygen K edges of NO are compared, as well as NEXAFS features at the nitrogen edges of the isoelectronic closed shell species NO , N2, and N2

Intramolecular hydrogen transfer in DNA induced by site selective resonant core excitation

We present experimental evidence for soft X ray induced intramolecular hydrogen transfer in the protonated synthetic tri oligonucleotide d FUAG in the gas phase FU fluorouracil . The trinucleotide cations were stored in a cryogenic ion trap and exposed to monochromatic synchrotron radiation. Photoionization and photofragmentation product ion yields were recorded as a function of photon energy. Predominanly glycosidic bond cleavage leading to formation of nucleobase related fragments is observed. In most cases, glycosidic bond cleavage is accompanied by single or double hydrogen transfer. The combination of absorption site sensitive soft X ray spectroscopy with fragment specific mass spectrometry allows to directly relate X ray absorption site and fragmentation site. We observe pronounced resonant features in the competition between single and double hydrogen transfer towards nucleobases. A direct comparison of experimental data with time dependent density functional theory calculations, using short range corrected hybrid functionals, reveal that these hydrogen transfer processes are universal and not limited to population of particular excited states localized at the nucleobases. Instead, hydrogen transfer can occur upon X ray absorption in any nucleobase and in the DNA backbone. Resonances seem to occur because of site selective suppression of hydrogen transfer channels. Furthermore, non covalent interactions of the optimized ground state geometries were investigated to identify intramolecular hydrogen bonds along which hydrogen transfer is most likel

Experimental determination of the energy difference between competing isomers of deposited, size-selected gold nanoclusters

The equilibrium structures and dynamics of a nanoscale system are regulated by a complex potential energy surface (PES). This is a key target of theoretical calculations but experimentally elusive. We report the measurement of a key PES parameter for a model nanosystem: size-selected Au nanoclusters, soft-landed on amorphous silicon nitride supports. We obtain the energy difference between the most abundant structural isomers of magic number Au561 clusters, the decahedron and face-centred-cubic (fcc) structures, from the equilibrium proportions of the isomers. These are measured by atomic-resolution scanning transmission electron microscopy, with an ultra-stable heating stage, as a function of temperature (125–500 °C). At lower temperatures (20–125 °C) the behaviour is kinetic, exhibiting down conversion of metastable decahedra into fcc structures; the higher state is repopulated at higher temperatures in equilibrium. We find the decahedron is 0.040 ± 0.020 eV higher in energy than the fcc isomer, providing a benchmark for the theoretical treatment of nanoparticles

Atomic-resolution imaging of surface and core melting in individual size-selected Au nanoclusters on carbon

Experimental and theoretical data sets for the Nature Communications paper "Atomic-resolution imaging of surface and core melting in individual size-selected Au nanoclusters on carbon". Data structure and formats are described in the README.docx file