184 research outputs found
Adsorption geometry and electronic structure of iron phthalocyanine on Ag surfaces: A LEED and photoelectron momentum mapping study
We present a comprehensive study of the adsorption behavior of iron
phthalocyanine on the low-index crystal faces of silver. By combining
measurements of the reciprocal space by means of photoelectron momentum mapping
and low energy electron diffraction, the real space adsorption geometries are
reconstructed. At monolayer coverage ordered superstructures exist on all
studied surfaces containing one molecule in the unit cell in case of Ag(100)
and Ag(111), and two molecules per unit cell for Ag(110). The azimuthal tilt
angle of the molecules against the high symmetry directions of the substrate is
derived from the photoelectron momentum maps. A comparative analysis of the
momentum patterns on the substrates with different symmetry indicates that both
constituents of the twofold degenerate FePc lowest unoccupied molecular orbital
are occupied by charge transfer from the substrate at the interface
Adsorption of cytosine and aza derivatives of cytidine on Au single crystal surfaces
The adsorption of cytosine on the Au(111) and Au(110) surfaces has been
studied using both aqueous deposition and evaporation in vacuum to prepare the
samples. Soft X-ray photoelectron spectroscopy (XPS) and near edge X-ray
absorption fine structure spectroscopy (NEXAFS) were used to determine the
electronic structure and orientation of the adsorbates. In addition, three
derivatives of cytosine, 6-azacytosine, 6-azacytidine and 5- azacytidine, were
studied. Monolayer films of the latter three samples were adsorbed on Au(111)
from aqueous solution, and the nature of bonding was determined. Spectra have
been interpreted in the light of published calculations of free cytosine
molecules and new ab initio calculations of the other compounds. Surface core
level shifts of Au 4f imply that all of these compounds are chemisorbed.
Cytosine adsorbs as a single tautomer, but in two chemical states with
different surface-molecule bonding. For deposition in vacuum, a flat-lying
molecular state bonded through the N(3) atom of the pyrimidine ring dominates,
but a second state is also present. For deposition from solution, the second
state dominates, with the molecular plane no longer parallel to the surface.
This state also bonds through the N(3) atom, but in addition interacts with the
surface via the amino group. Two tautomers of 6-azacytosine were observed, and
they and 6-azacytidine adsorb with similar geometries, chemically bonding via
the azacytosine ring. The ribose ring does not appear to perturb the adsorption
of azacytidine compared with azacytosine. The azacytosine ring is nearly but
not perfectly parallel to the surface, like 5-azacytidine, which adsorbs as an
imino tautomer. ...Comment: 40 pages, 3 tables and 8 figure
Kink far below the Fermi level reveals new electron-magnon scattering channel in Fe
Many properties of real materials can be modeled using ab initio methods
within a single-particle picture. However, for an accurate theoretical
treatment of excited states, it is necessary to describe electron-electron
correlations including interactions with bosons: phonons, plasmons, or magnons.
In this work, by comparing spin- and momentum-resolved photoemission
spectroscopy measurements to many-body calculations carried out with a newly
developed first-principles method, we show that a kink in the electronic band
dispersion of a ferromagnetic material can occur at much deeper binding
energies than expected (E_b=1.5 eV). We demonstrate that the observed spectral
signature reflects the formation of a many-body state that includes a photohole
bound to a coherent superposition of renormalized spin-flip excitations. The
existence of such a many-body state sheds new light on the physics of the
electron-magnon interaction which is essential in fields such as spintronics
and Fe-based superconductivity.Comment: 6 pages, 2 figure
An experimental and theoretical study of the resonant Auger spectrum of the ethene molecule
Resonant Auger spectra of the ethene molecule excited at energies across the
C1s → π* energy band are reported. Our measurements address the unexpected
variation of the intensity of the A state with respect to the other singly ionized
valence states. An approach, based on group theory and calculations using
Coulomb 4-center integrals, is proposed to explain the behaviour of the intensity
of the ground state and excited states of the ion upon resonant excitation. The
new method provides a calculationally inexpensive route to predict relative
intensities of different resonant Auger bands in polyatomic molecules, without
the need for an exhaustive knowledge of the potential energy surfaces of the
electronic states involved
Conservation of Nickel Ion Single-Active Site Character in a Bottom-Up Constructed π-Conjugated Molecular Network
On-surface chemistry holds the potential for ultimate miniaturization of functional devices. Porphyrins are promising building-blocks in exploring advanced nanoarchitecture concepts. More stable molecular materials of practical interest with improved charge transfer properties can be achieved by covalently interconnecting molecular units. On-surface synthesis allows to construct extended covalent nanostructures at interfaces not conventionally available. Here, we address the synthesis and properties of covalent molecular network composed of interconnected constituents derived from halogenated nickel tetraphenylporphyrin on Au(111). We report that the π-extended two-dimensional material exhibits dispersive electronic features. Concomitantly, the functional Ni cores retain the same single-active site character of their single-molecule counterparts. This opens new pathways when exploiting the high robustness of transition metal cores provided by bottom-up constructed covalent nanomeshes
Experimental Verification of the Chemical Sensitivity of Two-Site Double Core-Hole States Formed by an X-ray FEL
We have performed X-ray two-photon photoelectron spectroscopy (XTPPS) using
the Linac Coherent Light Source (LCLS) X-ray free-electron laser (FEL) in order
to study double core-hole (DCH) states of CO2, N2O and N2. The experiment
verifies the theory behind the chemical sensitivity of two-site (ts) DCH states
by comparing a set of small molecules with respect to the energy shift of the
tsDCH state and by extracting the relevant parameters from this shift.Comment: 11 pages, 2 figure
Ferrous to Ferric Transition in Fe-Phthalocyanine Driven by NO2 Exposure
Due to its unique magnetic properties offered by the open-shell electronic structure of the central metal ion, and for being an effective catalyst in a wide variety of reactions, iron phthalocyanine has drawn significant interest from the scientific community. Nevertheless, upon surface deposition, the magnetic properties of the molecular layer can be significantly affected by the coupling occurring at the interface, and the more reactive the surface, the stronger is the impact on the spin state. Here, we show that on Cu(100), indeed, the strong hybridization between the Fe d-states of FePc and the sp-band of the copper substrate modifies the charge distribution in the molecule, significantly influencing the magnetic properties of the iron ion. The FeII ion is stabilized in the low singlet spin state (S=0), leading to the complete quenching of the molecule magnetic moment. By exploiting the FePc/Cu(100) interface, we demonstrate that NO2 dissociation can be used to gradually change the magnetic properties of the iron ion, by trimming the gas dosage. For lower doses, the FePc film is decoupled from the copper substrate, restoring the gas phase triplet spin state (S=1). A higher dose induces the transition from ferrous to ferric phthalocyanine, in its intermediate spin state, with enhanced magnetic moment due to the interaction with the atomic ligands. Remarkably, in this way, three different spin configurations have been observed within the same metalorganic/metal interface by exposing it to different doses of NO2 at room temperature
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