Ultra-fast intramolecular vibronic coupling revealed by RIXS and RPES maps of an aromatic adsorbate on TiO2(110).

Two-dimensional resonant inelastic x-ray scattering (RIXS) and resonant photoelectron spectroscopy (RPES) maps are presented for multilayer and monolayer coverages of an aromatic molecule (bi-isonicotinic acid) on the rutile TiO2(110) single crystal surface. The data reveal ultra-fast intramolecular vibronic coupling upon core excitation from the N 1s orbital into the lowest unoccupied molecular orbital (LUMO) derived resonance. In the RIXS measurements, this results in the splitting of the participator decay channel into a purely elastic line which disperses linearly with excitation energy and a vibronic coupling channel at constant emission energy. In the RPES measurements, the vibronic coupling results in a linear shift in binding energy of the participator channel as the excitation is tuned over the LUMO-derived resonance. Localisation of the vibrations on the molecule on the femtosecond time scale results in predominantly inelastic scattering from the core-excited state in both the physisorbed multilayer and the chemisorbed monolayer.

Ligand identification in titanium complexes using X-ray valence-to-core emission spectroscopy.

The identification of ligands in metalloorganic complexes is crucial for understanding many important biological and chemical systems. Nonresonant Kß valence-to-core X-ray emission spectroscopy (XES) has been demonstrated as a ligand identification technique which is complementary to other spectroscopies, such as X-ray absorption. In this study we show the Kß valence-to-core XES alongside the Ti K-edge X-ray absorption near edge structure spectra for a series of chemically relevant low-symmetry Ti organometallic complexes. The spectra are modeled using density functional theory calculations. XES spectra are analyzed in terms of the molecular orbitals probed, in order to understand the effects of bond length, bond nature, orbital hybridization, and molecular symmetry on the observed spectral features.

Electronic structure changes in cobalt phthalocyanine due to nanotube encapsulation probed using resonant inelastic X-ray scattering.

The electronic structure of cobalt phthalocyanine (CoPc) changes upon encapsulation inside multi-walled carbon nanotubes (CoPc@MWNT), as detected in this research using Co-K-edge X-ray absorption near-edge structure spectroscopy (XANES) and Co-Kalpha(1) resonant inelastic X-ray scattering (RIXS). The CoPc molecules are no longer planar once inside the nanotubes, and the molecular symmetry is found to change upon encapsulation from D(4h) to C(4v) symmetry. This change of symmetry increases the amount of p-d orbital mixing, which is seen in the spectra as a change in peak intensity. Energy shifts are also seen between CoPc and CoPc@MWNT, showing that Co in the encapsulated species is more oxidized due to electron donation from the phthalocyanine molecule to the surrounding nanotube. Trends seen in the spectra between CoPc and CoPc@MWNT can be calculated using density functional theory (DFT), which shows the molecular orbitals involved in different spectral features.

High energy resolution X-ray absorption spectroscopy of environmentally relevant lead(II) compounds.

The determination of the chemical environment of Pb in natural samples is a challenge of great importance in environmental and health physics. We report a high energy resolution fluorescence detection (HERFD) X-ray absorption near-edge spectroscopy (XANES) study at the Pb L(3) and L(1) absorption edges to determine the chemical environment of Pb in a series of model and environmentally relevant compounds. HERFD spectroscopy can reveal increased spectral detail due to an apparent reduction in the core hole lifetime broadening. HERFD spectra of model Pb(II) compounds were compared to FEFF 8.4 multiple scattering calculations with reduced peak broadening parameters, and density of state (DOS) simulations, to determine the origins of the spectral features. A pre-edge in the L(3) XANES is revealed which is shown to arise from hybridization between the Pb p and d states. HERFD spectra of Pb(II)-containing environmentally relevant solutions were compared to model spectra and calculations. The results presented in this paper show that the chemical environment of Pb can be identified from spectral features resolved in HERFD spectroscopy at the Pb L(3) edge. The technique provides information that is complementary to conventional extended X-ray absorption fine structure (EXAFS) spectroscopy.

Electrospray deposition of carbon nanotubes in vacuum.

Here we report on a novel and effective technique for the deposition of carbon nanotubes onto surfaces in vacuum directly from a liquid suspension. The technique, based on in-vacuum electrospray ionization, has the potential to bridge the gap between high resolution techniques requiring ultra-high vacuum conditions, and non-volatile molecules and nanostructures such as carbon nanotubes. Atomic force microscopy of double-walled nanotubes deposited onto silicon surfaces in vacuum show individual nanotubes and low density bundles.

Element substitution by living organisms: the case of manganese in mollusc shell aragonite.

Determining the manganese concentration in shells of freshwater bivalves provides a unique way to obtain information about climate and environmental changes during time-intervals that pre-date instrumental data records. This approach, however, relies on a thorough understanding of how manganese is incorporated into the shell material -a point that remained controversial so far. Here we clarify this issue, using state-of-the-art X-ray absorption and X-ray emission spectroscopy in combination with band structure calculations. We verify that in the shells of all studied species manganese is incorporated as high-spin Mn(2+), i.e. manganese always has the same valence as calcium. More importantly, the unique chemical sensitivity of valence-to-core X-ray emission enables us to show that manganese is always coordinated by a CO3-octahedron. This, firstly, provides firm experimental evidence for manganese being primarily located in the inorganic carbonate. Secondly, it indicates that the structure of the aragonitic host is locally altered such that manganese attains an octahedral, calcitic coordination. This modification at the atomic level enables the bivalve to accommodate many orders of magnitude more manganese in its aragonitic shell than found in any non-biogenic aragonite. This outstanding feature is most likely facilitated through the non-classical crystallization pathway of bivalve shells.

A molecular approach to Cu doped ZnO nanorods with tunable dopant content.

A novel molecular approach to the synthesis of polycrystalline Cu-doped ZnO rod-like nanostructures with variable concentrations of introduced copper ions in ZnO host matrix is presented. Spectroscopic (PLS, variable temperature XRD, XPS, ELNES, HERFD) and microscopic (HRTEM) analysis methods reveal the +II oxidation state of the lattice incorporated Cu ions. Photoluminescence spectra show a systematic narrowing (tuning) of the band gap depending on the amount of Cu(II) doping. The advantage of the template assembly of doped ZnO nanorods is that it offers general access to doped oxide structures under moderate thermal conditions. The doping content of the host structure can be individually tuned by the stoichiometric ratio of the molecular precursor complex of the host metal oxide and the molecular precursor complex of the dopant, Di-aquo-bis[2-(methoxyimino)-propanoato]zinc(II) 1 and -copper(II) 2. Moreover, these keto-dioximato complexes are accessible for a number of transition metal and lanthanide elements, thus allowing this synthetic approach to be expanded into a variety of doped 1D metal oxide structures.

Adsorption and charge transfer dynamics of bi-isonicotinic acid on Au(111).

The interaction of bi-isonicotinic acid (4,4(')-dicarboxy-2,2(')-bipyridine) with the Au(111) surface has been investigated using electron spectroscopic techniques. Near edge x-ray absorption fine structure (NEXAFS) spectra show that monolayers of the molecule lie flat to the surface and also reveal that the monolayer is sensitive to the preparation conditions employed. Core level x-ray photoelectron spectroscopy (XPS) shows that the adsorbed molecule does not undergo deprotonation upon adsorption. The "core-hole clock" implementation of resonant photoemission has been used to probe the coupling between molecule and substrate. This technique has revealed the possibility of ultrafast backtransfer from the substrate into the molecule upon resonant excitation of a N 1s core level electron. This is supported by a NEXAFS and XPS investigation of energy level alignments in the system.

Molecular ordering in isonicotinic acid on rutile TiO2(110) investigated with valence band photoemission.

The adsorption of isonicotinic acid on rutile TiO(2)(110) has been investigated using synchrotron-based valence band photoemission. Structural ordering in multilayer films of the molecules is found to give rise to a strong angular dependence in the valence band intensities when measured using linearly polarized radiation. Molecular ordering in this case is proposed to be induced by intermolecular hydrogen bonding which is found to be highly dependent upon the deposition rate of the isonicotinic acid. Through comparison of the experimental data with density functional calculated valence band spectra of hydrogen-bonded isonicotinic acid molecules, we can account for the angular dependence in terms of the spatial distribution of the molecular orbitals.