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ABSTRACT: 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. ...
05/2013;
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ABSTRACT: A sizable enhancement of the circular dichroism in photoelectron spectroscopy has been measured and computed for the metal complex Δ-cobalt(III) tris-acetylacetonate highest occupied molecular orbital state in the region of the Co 3p→3d Fano resonance. In the resonance the dichroism reaches the maximum value of 5% and even changes its sign as compared to the direct photoionization channel. We ascribe this enhancement to electron correlation processes, namely, with the coupling between discrete excitations and the continuum, which is correctly described in the time dependent density functional theory (TDDFT) framework. These findings open new physical aspects of photoelectron circular dichroism that now can be interpreted not only via the simple direct ionization, but also through more complex electron correlation processes.
Physical Review Letters 02/2012; 108(8):083001. · 7.37 Impact Factor
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P. O'Keeffe, V. Feyer,
P. Bolognesi,
M. Coreno,
C. Callegari,
G. Cautero,
A. Moise,
K. C. Prince,
R. Richter,
R. Sergo, [......],
M. Devetta,
T. Mazza,
P. Piseri,
V. L,
amayev,
R. Katzy,
F. Stienkemeier,
Y. Ovcharenko,
T. Moller,
L. Avaldi
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ABSTRACT: The design and evaluation of a velocity map imaging spectrometer specifically optimised for experiments
at the FERMI free electron laser source are described. Optimisation of the set-up for the detection of high
energy electrons (experimentally calibrated up to 45 eV), reduction of scattered light and characterisation
of the pulsed gas source are outlined. The instrument has been tested using synchrotron light at
the GasPhase beamline at Elettra, the Italian synchrotron source, and the results are presented
Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms 01/2012; 284:69. · 1.21 Impact Factor
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Surface Science 01/2012; 606:435. · 1.99 Impact Factor
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ABSTRACT: Photoabsorption and S 2p photoionization of the SF(6) molecule have been studied experimentally and theoretically in the excitation energy range up to 100 eV above the S 2p ionization potentials. In addition to the well-known 2t(2g) and 4e(g) shape resonances, the spin-orbit-resolved S 2p photoionization cross sections display two weak resonances between 200 and 210 eV, a wide resonance around 217 eV, a Fano-type resonance around 240 eV, and a second wide resonance around 260 eV. Calculations based on time-dependent density functional theory allow us to assign the 217-eV and 260-eV features to the shape resonances in S 2p photoionization. The Fano resonance is caused by the interference between the direct S 2p photoionization channel and the resonant channel that results from the participator decay of the S 2s(-1)6t(1u) excited state. The weak resonances below 210-eV photon energy, not predicted by theory, are tentatively suggested to originate from the coupling between S 2p shake-up photoionization and S 2p single-hole photoionization. The experimental and calculated angular anisotropy parameters for S 2p photoionization are in good agreement.
The Journal of chemical physics 05/2011; 134(17):174311. · 3.09 Impact Factor
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ABSTRACT: The S 2p Auger spectrum of SF(6) has been studied in the region of the 2t(2g) and 4e(g) resonances. The partial Auger spectra due to the ionization of the 2p spin-orbit components and of a shake-up satellite state have been measured selectively by tuning the photon energy and using the Auger electron-photoelectron coincidence technique. A detailed analysis of the Auger spectrum has also been performed using the Green's function-based second-order algebraic diagrammatic construction method.
The Journal of chemical physics 03/2011; 134(9):094308. · 3.09 Impact Factor
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Physical Review a. 01/2011; 84(5).
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Journal of Chemical Physics. 01/2011; 134.
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ABSTRACT: We present a combined experimental and theoretical investigation of the valence electronic structure of porphyrin-derived molecules. The valence photoemission spectra of the free-base tetraphenylporphyrin and of the octaethylporphyrin molecule were measured using synchrotron radiation and compared with theoretical spectra calculated using the GW method and the density-functional method within the generalized gradient approximation. Only the GW results could reproduce the experimental data. We found that the contribution to the orbital energies due to electronic correlations has the same linear behavior in both molecules, with larger deviations in the vicinity of the HOMO level. This shows the importance of adequate treatment of electronic correlations in these organic systems.
Physical Chemistry Chemical Physics 09/2010; 12(36):10812-7. · 3.57 Impact Factor
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ABSTRACT: The inner shell excitation of pyrimidine and some halogenated pyrimidines near the C and N K-edges has been investigated experimentally by near edge x-ray absorption fine structure spectroscopy and theoretically by density functional theory calculations. The selected targets, 5-Br-pyrimidine, 2-Br-pyrimidine, 2-Cl-pyrimidine, and 5-Br-2-Cl-pyrimidine, allow the effects of the functionalization of the pyrimidine ring to be studied either as a function of different halogen atoms bound to the same molecular site or as a function of the same halogen atom bound to different molecular sites. The results show that the individual characteristics of the different spectra of the substituted pyrimidines can be rationalized in terms of variations in electronic and geometrical structures of the molecule depending on the localization and the electronegativity of the substituent.
The Journal of chemical physics 07/2010; 133(3):034302. · 3.09 Impact Factor
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P Bolognesi,
P O'Keeffe, V Feyer,
O Plekan,
K Prince,
M Coreno,
G Mattioli,
A Amore Bonapasta,
W Zhang,
V Carravetta,
Y Ovcharenko,
L Avaldi
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ABSTRACT: The inner shell excitation and ionisation of pyrimidine have been studied at the carbon K edge by near-edge X ray absorption fine structure (NEXAFS) and X ray photoelectron (XPS) spectroscopies. The theoretical predictions of density functional theory (DFT) provide a satisfactory assignment of the complex spectra of this polyatomic molecule. The fragmentation following the C(1s−1)π* excitation has been investigated by resonant Auger electron-ion coincidence spectroscopy, which allows a site and state selective study.
Journal of Physics Conference Series 04/2010; 212(1):012002.
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ABSTRACT: We investigate the valence electronic properties in the gas
phase of the indene molecule, which is one of the simplest
polycyclic aromatic hydrocarbons, with photoemission spectroscopy
using synchrotron light and through first-principles
calculations using a many-body perturbation theory GW
approach. We found an excellent agreement between theory
and experiment. This allows us to assign to the peaks appearing
in the photoemission spectrum the calculated molecular
orbitals.
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physica status solidi (b) 01/2010; · 1.32 Impact Factor
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ABSTRACT: The inner shell ionization of pyrimidine and some halogenated pyrimidines has been investigated experimentally by X-ray photoemission spectroscopy (XPS) and theoretically by density functional theory (DFT) methods. The selected targets-5-Br-pyrimidine, 2-Br-pyrimidine, 2-Cl-pyrimidine, and 5-Br-2-Cl-pyrimidine-allowed the study of the effect of the functionalization of the pyrimidine ring by different halogen atoms bound to the same molecular site, or by the same halogen atom bound to different molecular sites. The theoretical investigation of the inductive and resonance effects in the C(1s) ionization confirms the soundness of the resonance model for a qualitative description of the properties of an aromatic system. Moreover, the combination of the experimental results and the theoretical analysis provides a detailed description of the effects of the halogen atom on the screening of a C(1s) hole in the aromatic pyrimidine ring.
The Journal of Physical Chemistry A 12/2009; 113(48):13593-600. · 2.95 Impact Factor
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ABSTRACT: The Mg K edge photoabsorption spectrum and the B 1s, Mg 1s, Mg 2p and valence band photoemission spectra of polycrystalline magnesium diboride have been measured. The photoabsorption spectra of the diboride and the oxide, which is present as an impurity, were separated by measuring the Auger electron partial yield at electron energies characteristic of each phase. The spectra are consistent with published calculations of the density of unoccupied p symmetry states. Better agreement is obtained with calculations for the ground state of the system than with ones for the excited state. Valence band photoemission spectra were measured at photon energies corresponding to core resonances, but, within the signal to noise level of the spectra, no resonant enhancement was observed. This is consistent with the delocalized nature of the valence band.
Journal of Physics Condensed Matter 10/2009; 21(40):405701. · 2.55 Impact Factor
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ABSTRACT: The triple differential cross sections (TDCS) for the photodouble ionization of He, Ne, Ar, and Xe leading to the He2+(1s01Se), Ne2+(2s02p61Se), Ar2+(3s03p61Se), and Xe2+(5s05p61Se) states have been measured at about 20 eV above their respective thresholds with the two photoelectrons equally sharing the excess energy. The experimental data are analyzed using a parametrization recently proposed [ J. Phys. B 41 245205 (2008)] which takes into account experimental uncertainties. The parametrization provides a satisfactory representation of the shape of the measured TDCS. The study of the behavior of the gerade amplitude of the TDCS in the different targets gives hints on the dependence of the electron correlation with the principal quantum number n of the ionized ns orbital.
Phys. Rev. A. 06/2009; 79(6).
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ABSTRACT: The contribution of the dipole and non-dipole terms to the angular distribution of the photoelectrons from the N K shell of randomly oriented N2 molecules has been measured up to 80 eV above the N 1s threshold. The results are compared with all the previous experimental data and theoretical results. Consistently with the predictions of calculations performed in the frame of density functional theory (Toffoli and Decleva 2006 J. Phys. B: At. Mol. Opt. Phys. 39 2681) and in the random phase approximation (Hosaka et al 2006 J. Phys. B: At. Mol. Opt. Phys. 39 L25) the present data show that the non-dipole effects are rather small in the region investigated.
Journal of Physics B Atomic Molecular and Optical Physics 11/2008; 41(22):221002. · 1.88 Impact Factor
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ABSTRACT: The C 1s excitation and ionization of methyloxirane have been studied via photoabsorption, photoemission and photoelectron–Auger electron coincidence experiments with linearly and circularly polarized synchrotron radiation. The measurements with circularly polarized radiation provide a systematic observation of circular dichroism effects in inner shell processes.
Physica Scripta 11/2008; 78(5):058120. · 1.20 Impact Factor
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ABSTRACT: The core level electron excitation and ionization spectra of thymine and adenine have been investigated by photoabsorption and photoemission spectroscopy, and the results interpreted by means of ab initio calculations using the second-order algebraic–diagrammatic construction (ADC(2)) method for the polarization propagator and the fourth-order ADC method (ADC(4)) for the one-particle Green’s function. The photoabsorption spectra are dominated by transitions from core levels to unoccupied p states, but also show clear structures due to Rydberg transitions. The calculated spectra are in good agreement with the experimental results, and many of the observed structures are assigned.
Chemical Physics 01/2008; 347:306. · 1.90 Impact Factor
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ABSTRACT: Photoionization techniques, such as photoelectron spectroscopy (PES) and photoionization mass spectrometry (PIMS), are well-established and powerful methods for studying the spectroscopy of isolated bio-organic molecules and their fate under vacuum ultraviolet (VUV) irradiation. Measuring the energy selected electron leaving a molecular ion in coincidence with other particles, such as ions, can provide even deeper insights into the mechanisms of the interaction of molecules with ionizing radiation. We have thus implemented the electronic state resolved photoelectron photoIon coincidence (ER-PEPICO) technique in our laboratory. Here, we report our newly constructed apparatus, and its application for characterizing fragmentation processes occurring in pyrimidine. Ionization of the two highest molecular orbitals (MOs) of the valence band does not lead to fragmentation of the resulting ion. The third band observed in photoemission is due to the ionization of two MOs, and leads mainly to the formation of the parent ion. The next three electronic states are not resolved experimentally and appear as a single band; their ionization leads to fragments of mass to charge ratio m/e= 53 (C3H3N+), while ionization of deeper lying MOs leads mostly to m/e= 26 (C2H2+). We compare our data with previous non-coincidence photoionization results and describe the problems encountered and their solutions.
Physica Scripta 01/2008; 78:058105. · 1.20 Impact Factor
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ABSTRACT: The beta angular asymmetry and D dichroic asymmetry parameters of the methyl-oxirane highest occupied molecular orbital (HOMO) band have been experimentally investigated with vibrational resolution using synchrotron radiation. A theoretical calculation of the Franck-Condon factors between vibrational ground state and different ionic vibrational states, in the Born-Oppenheimer harmonic approximation, has been performed in order to gain information on the vibrational states mainly involved in the HOMO photoelectron band. The general good agreement between theoretical and experimental results allows a reliable assignment of the major features. The experimental determination of beta and D shows their dependence on the different final vibrational states. This paper reports, for the first time, experimental evidence of the dependence of the dichroic D parameter on the vibrational excitation of the ion.
The Journal of Chemical Physics 10/2007; 127(12):124310. · 3.33 Impact Factor
