| 1. |
- Bolognesi, P., et al.
(author)
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Inner shell excitation, ionization and fragmentation of pyrimidine
- 2010
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In: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 212, s. 012002-
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Journal article (peer-reviewed)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.
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| 2. |
- Couto, Rafael Carvalho, et al.
(author)
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Breaking inversion symmetry by protonation : Experimental and theoretical NEXAFS study of the diazynium ion, N2H
- 2021
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In: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry (RSC). - 1463-9076 .- 1463-9084. ; 23:32, s. 17166-17176
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Journal article (peer-reviewed)abstract
- As an example of symmetry breaking in NEXAFS spectra of protonated species we present a high resolution NEXAFS spectrum of protonated dinitrogen, the diazynium ion N2H+. By ab initio calculations we show that the spectrum consists of a superposition of two nitrogen 1s absorption spectra, each including a π∗ band, and a nitrogen 1s to H+ charge transfer band followed by a weak irregular progression of high energy excitations. Calculations also show that, as an effect of symmetry breaking by protonation, the π∗ transitions are separated by 0.23 eV, only slightly exceeding the difference in the corresponding dark (symmetry forbidden) and bright (symmetry allowed) core excitations of neutral N2. By DFT and calculations and vibrational analysis, the complex π∗ excitation band of N2H+ is understood as due to the superposition of the significantly different vibrational progressions of excitations from terminal and central nitrogen atoms, both leading to bent final state geometries. We also show computationally that the electronic structure of the charge transfer excitation smoothly depends on the nitrogen-proton distance and that there is a clear extension of the spectra going from infinity to close nitrogen-proton distance where fine structures show some, although not fully detailed, similarities. An interesting feature of partial localization of the nitrogen core orbitals, with a strong, non-monotonous, variation with nitrogen-proton distance could be highlighted. Specific effects could be unraveled when comparing molecular cation NEXAFS spectra, as represented by recently recorded spectra of N2+ and CO+, and spectra of protonated molecules as represented here by the N2H+ ion. Both types containing rich physical effects not represented in NEXAFS of neutral molecules because of the positive charge, whereas protonation also breaks the symmetry. The effect of the protonation on dinitrogen can be separated in charge, which extends the high-energy part of the spectrum, and symmetry-breaking, which is most clearly seen in the low-energy π∗ transition.
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| 3. |
- Lindblad, R., et al.
(author)
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X-Ray Absorption Spectrum of the N-2(+) Molecular Ion
- 2020
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In: Physical Review Letters. - : American Physical Society. - 0031-9007 .- 1079-7114. ; 124:20
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Journal article (peer-reviewed)abstract
- The x-ray absorption spectrum of N-2(+) in the K-edge region has been measured by irradiation of ions stored in a cryogenic radio frequency ion trap with synchrotron radiation. We interpret the experimental results with the help of restricted active space multiconfiguration theory. Spectroscopic constants of the l sigma(-1 2)(u)Sigma(+)(u) state, and the two 1 sigma(-1)(u) 3 sigma(-1)(g) 1 pi(y) (II alpha)-I-2 states are determined from the measurements. The charge of the ground state together with spin coupling involving several open shells give rise to double excitations and configuration mixing, and a complete breakdown of the orbital picture for higher lying core-excited states.
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| 4. |
- Alagia, M., et al.
(author)
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Probing the potential energy surface by highresolution x-ray absorption spectroscopy : The umbrella motion of the coreexcited CH3 free radical
- 2007
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In: Physical Review. A. - 1050-2947 .- 1094-1622. ; 76:2, s. 124305-
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Journal article (peer-reviewed)abstract
- A detailed study of the umbrellalike vibration in inner-shell spectroscopy is presented. The high-resolution x-ray absorption spectrum for the lowest lying core excitation of the CH3 free radical was recorded. High quality potential energy surfaces (PES) for the initial and final states of the transition were calculated as a function of the symmetrical stretching and the umbrella deformation coordinates. The strong anharmonicity along the umbrella coordinate in the double-well region of the PES of the core excited state has a strong effect on the bending vibrational progressions. The excellent agreement between the experiment and theory allows an accurate spectroscopic characterization of the vibrational structure of the electronic transition, and the estimation of the umbrella inversion time of 149 fs.
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| 5. |
- Andersson, Egil, 1981-, et al.
(author)
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Core-valence double photoionization of the CS2 molecule
- 2010
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 133:9, s. 94305-
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Journal article (peer-reviewed)abstract
- Double photoionization spectra of the CS2 molecule have been recorded using the TOF-PEPECO technique in combination with synchrotron radiation at the photon energies h nu=220, 230, 240, 243, and 362.7 eV. The spectra were recorded in the S 2p and C 1s inner-shell ionization regions and reflect dicationic states formed out of one inner-shell vacancy and one vacancy in the valence region. MCSCF calculations were performed to model the energies of the dicationic states. The spectra associated with a S 2p vacancy are well structured and have been interpreted in some detail by comparison to conventional S 2p and valence photoelectron spectra. The lowest inner-shell-valence dicationic state is observed at the vertical double ionization energy 188.45 eV and is associated with a (2p(3/2))(-1)(2 pi(g))(-1) double vacancy. The spectrum connected to the C 1s vacancy shows a distinct line at 310.8 eV, accompanied by additional broad features at higher double ionization energies. This line is associated with a (C 1s)(-1)(2 pi(g))(-1) double vacancy. (C-) 2010 American Institute of Physics. [doi: 10.1063/1.3469812]
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| 6. |
- Carravetta, V., et al.
(author)
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A screened static-exchange potential for core electron excitations
- 2001
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In: Chemical Physics. - 0301-0104 .- 1873-4421. ; 263:03-feb, s. 231-242
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Journal article (peer-reviewed)abstract
- We explore the use of a screened potential for static exchange calculations of core electron excitations, and to which extent this potential can relieve the main limitation of the static exchange method, namely the non-account of the residual screening in the presence of the excited electron. The screened potential is obtained by projecting the target state on a set of polarized orbitals, giving origin to a fully screened potential. Applications are presented and discussed for X-ray absorption spectra of a set of molecules of different size and type. It is found that with the little extra computational effort using the screened potential, the low-energy part of the spectra is generally improved, while the error in the term values are roughly of equal magnitude but of different sign compared to the normal static-exchange calculation. Various aspects of the screened and un-screened static-exchange approximations are analyzed in comparison with other techniques.
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| 7. |
- Carravetta, V., et al.
(author)
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Ab initio calculations of molecular resonant photoemission spectra
- 2000
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 113:18, s. 7790-7798
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Journal article (peer-reviewed)abstract
- An ab initio method for calculations of molecular resonant photoemission (RPE) spectra is described. The method includes a multicenter expansion of both the dipole matrix element-direct emission-and the Hamiltonian matrix element between the resonant state and the autoionizing states-resonant emission. These quantities are relevant for the description of the process both in the two-step model, where the spectrum is computed at the resonance energy only, and in the one-step model where, by a K-matrix approach, the direct-to-resonant interference is taken into account and the electronic line profile is fully described. The resonant two-electron matrix elements are evaluated over the core-excited relaxed orbitals with the outgoing Auger electron orbital expanded on an augmented multicentered Gaussian basis set. Stieltjes imaging is shown to work excellently for such Gaussian basis sets giving correct continuum normalization matrix elements even for RPE electron energies as high as 100-1000 eV. A numerical investigation is carried out for the participator decay of the C 1s --> pi* and O 1s --> pi* states of CO.
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| 8. |
- Carravetta, V., et al.
(author)
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An ab initio method for computing multi-atom resonant photoemission
- 2002
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In: Chemical Physics Letters. - 0009-2614 .- 1873-4448. ; 354:02-jan, s. 100-108
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Journal article (peer-reviewed)abstract
- We present an ab initio method for computing multi-atom resonant photoemission (MARPE) in molecules and use this method to investigate the possibility of having a sizable MARPE effect. The MARPE cross section is obtained by making multi-center expansions of both the dipole matrix element - direct emission - and the hamiltonian matrix element between the resonant state and the autoionizing states - resonant emission. We show by sample calculations that the resonant contribution exhibits a strong distance dependence of the atoms participating in the MARPE process, and that it is energy dependent and element specific for the neighboring atoms. The ratio of resonant MARPE and direct photoionization cross sections is found to amount to less than one percent which probably makes the molecular MARPE effect difficult to apply in practice.
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| 9. |
- Carravetta, V., et al.
(author)
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Computational X-Ray Spectroscopy
- 2011
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In: Computational Strategies for Spectroscopy. - Hoboken, NJ, USA : John Wiley & Sons. - 9780470470176 ; , s. 137-205
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Book chapter (other academic/artistic)
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| 10. |
- Carravetta, V, et al.
(author)
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Excitation-energy-dependent resonant photoemission: C1s-pi* spectra of carbon monoxide
- 1997
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In: PHYSICAL REVIEW A. - : AMER INST PHYSICS. - 1050-2947. ; 56:6, s. 4665-4674
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Journal article (other academic/artistic)abstract
- High-resolution measurements of the x-ray resonant photoemission of CO near the C1s-->pi* transition are presented. The branching ratios for the valence shell ionization close to the resonance how a strong dependence on the photon energy with a clear asym
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