1. Infrared spectra in amorphous aluminaLuigi Giacomazzi, Nikita S Shcheblanov, Mikhail E Povarnitsyn, Yanbo Li, Andraž Mavrič, Barbara Zupančič, Jože Grdadolnik, Alfredo Pasquarello, complete scientific database of research data Abstract: We present a combined study based on experimental measurements of infrared (IR) dielectric function and first-principles calculations of IR spectra and vibrational density of states (VDOS) of amorphous alumina (am-Al₂O₃). In particular, we show that the main features of the imaginary part of the dielectric function ε₂(ω) at ~380 and 630 cm-¹ are related to the motions of threefold coordinated oxygen atoms, which are the vast majority of oxygen atoms in am-Al₂O₃. Our analysis (involving three model structures) provides an alternative point of view with respect to an earlier suggested assignment of the vibrational modes, which relates them to the stretching and bending vibrational modes of AlOₙ (n = 4, 5, and 6) polyhedra. Our assignment is based on the additive decomposition of the VDOS and ε₂(ω) spectra, which shows that: (i) the band at ~380 cm-¹ features oxygen motions occurring in a direction normal to the plane defined by the three nearest-neighbor aluminum atoms, i.e. out-of-plane motions of oxygen atoms; (ii) Al-O stretching vibrations (i.e. in-plane motions of oxygen atoms) appear at frequencies above ~500 cm-¹, which characterize the vibrational modes underlying the band at ~630 cm-¹. Keywords: amorphous alumina, infrared spectra, first-principles calculations Published in RUNG: 15.09.2023; Views: 1503; Downloads: 10 Link to file This document has many files! More... |
2. Carbon and Nitrogen K-Edge NEXAFS Spectra of Indole, 2,3-Dihydro-7-azaindole, and 3-FormylindoleAurora Ponzi, Elisa Bernes, Daniele Toffoli, Giovanna Fronzoni, Carlo Callegari, Alessandra Ciavardini, Michele Di Fraia, Robert Richter, Kevin Charles Prince, Hanan Sa'adeh, Michele Devetta, Davide Faccialà, Caterina Vozzi, Lorenzo Avaldi, Paola Bolognesi, Mattea Castrovilli, Daniele Catone, Marcello Coreno, Oksana Plekan, 2021, original scientific article Keywords: Carbon and nitrogen, General description, Hybrid functional, Near-edge X-ray absorption fine structure spectrum, Relevant features, Rydberg character, Theoretical spectra, Time dependent density functional theory Published in RUNG: 19.01.2022; Views: 2613; Downloads: 0 This document has many files! More... |
3. Vibrational spectra of am-Al 2 O 3 : tuning a parametric model. : Written report: in fulfillment of Diploma Seminar 1FAF29 requirementsKurtović Jasmin, 2021, research project (high school) Abstract: The present diploma seminar work has been dedicated first to the implementation of a procedure
to calculate the vibrational density of states of two structural models of vitreous SiO 2 (v-
SiO 2 ) and amorphous Al 2 O 3 (am-Al 2 O 3 ) models, and next to tune a parametric model for the
calculation of the infrared (IR) spectra of am-Al 2 O 3 , in particular of the imaginary part of
the dielectric function. The ground state of both structural models is obtained by relaxing
the atomic structure by using the conjugate gradient method as implemented in the LAMMPS
code. Vibrational frequencies and modes are obtained, in the harmonic approximation, by
diagonalizing the dynamical matrices calculated for the given v-SiO 2 and am-Al 2 O 3 structural
models. Dynamical matrices are obtained through a finite differences approach and vibrational
density of states are plotted by applying Gaussian broadening. The calculation of the dielectric
function requires the knowledge of the vibrational frequencies as well as the knowledge of the
dynamical (or Born) charge tensors related to the atoms of the am-Al 2 O 3 structural model.
For the latter model, a parametrization of the ab-initio Born charge tensors has been carried
out with the purpose to allow for the fast calculation of the IR spectrum of any other am-
Al 2 O 3 model without the need to calculate for it the Born charge tensors using expensive ab-
initio methods. The parametrization of Born charge tensors takes into account, for aluminium
atoms, only of the isotropic charge which depends on coordination number and average Al-
O bond length of aluminium atoms, while for oxygen atoms coordinated to three Al atoms
(75%), dynamical charges are parametrized also by the area bounded by aluminium atoms
nearest neighbours of the 3-coordinated oxygen atom. Moreover for analyzing the dynamical
charge tensors of 3-coordinated oxygen atoms a decomposition in terms of the representations
of the spatial rotations was used. The IR spectrum obtained by means of the above described
parametrization provides a good approximation to the IR spectrum obtained by using the ab-
initio calculated dynamical charges, as it differs from it, on average, by around 5.1% which
is much better (12%) than using a average isotropic charge model (i.e. diagonal Born charge
tensors where each diagonal element is one third of the average isotropic charge). Keywords: Vibrational spectra, am-Al 2 O 3, parametric model Published in RUNG: 15.09.2021; Views: 2785; Downloads: 0 This document has many files! More... |
4. Effects of the molecular potential on coexcitations of valence electrons in the K-shell photoeffect of 3p and 4p elementsRobert Hauko, Jana Padežnik Gomilšek, Alojz Kodre, Iztok Arčon, Giuliana Aquilanti, 2019, original scientific article Abstract: Photoabsorption spectra of gaseous hydrides of 3p (PH3, H2S, HCl) and 4p elements (GeH4, AsH3 , H2Se,
HBr) are measured in the energy region within 50 eV above the K edge, to study coexcitations of valence
electrons by photoeffect in the K shell. The analysis of the valence coexcitations is extended to Ar, Kr, and
SiH4. Relative probabilities and energies of states in the individual coexcitation channels are recovered by
modeling the spectral features with a minimal ansatz based on the features in the contiguous noble gas. The
extracted parameters are compared to the results of theoretical calculations for molecules (ORCA code) and
free atoms (Hartree-Fock code). The experimental results confirm that the valence coexcitations in the 3p and
4p hydride molecules can be satisfactorily described by a two-step process, with the shake of the outer electron
following the excitation of the core electron. The total probability—relative to the K-edge jump—of the shake-up
processes shows a steady decrease from 19% in Si to 14% in Cl, and from 15% in Ge to 12% in Br. The
experimental values for Ar (12%) and Kr (10%) are in accord with the trend. The dominant contribution is
the transition to quasiatomic orbitals, in contrast with the deeper coexcitation channels in hydride molecules
where transition to molecular orbitals prevails. Keywords: X-ray absorption spectra, gaseous hydrides, 3p K-edge spectra, DFT Published in RUNG: 05.09.2019; Views: 3491; Downloads: 0 This document has many files! More... |
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8. Vibrational properties of vitreous GeSe2 with the Becke–Lee–Yang–Parr density functionalLuigi Giacomazzi, Carlo Massobrio, Alfredo Pasquarello, 2011, original scientific article Keywords: GeSe2, glass, vibrational spectra, BLYP, Raman, first-principles, DFT, ab-initio Published in RUNG: 15.10.2018; Views: 3947; Downloads: 0 This document has many files! More... |
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