1,410 research outputs found
Effects of non-local exchange on core level shifts for gas-phase and adsorbed molecules
Density functional theory calculations are often used to interpret experimental shifts in core level binding energies. Calculations based on gradient-corrected (GC) exchange-correlation functionals are known to reproduce measured core level shifts (CLS) of isolated molecules and metal surfaces with reasonable accuracy. In the present study, we discuss a series of examples where the shifts calculated within a GC-functional significantly deviate from the experimental values, namely the CLS of C 1s in ethyl trifluoroacetate, Pd 3d in PdO and the O 1s shift for CO adsorbed on PdO(101). The deviations are traced to effects of the electronic self-interaction error with GC-functionals and substantially better agreements between calculated and measured CLS are obtained when a fraction of exact exchange is used in the exchange-correlation functional
Molecular double core-hole electron spectroscopy for chemical analysis
We explore the potential of double core hole electron spectroscopy for
chemical analysis in terms of x-ray two-photon photoelectron spectroscopy
(XTPPS). The creation of deep single and double core vacancies induces
significant reorganization of valence electrons. The corresponding relaxation
energies and the interatomic relaxation energies are evaluated by CASSCF
calculations. We propose a method how to experimentally extract these
quantities by the measurement of single and double core-hole ionization
potentials (IPs and DIPs). The influence of the chemical environment on these
DIPs is also discussed for states with two holes at the same atomic site and
states with two holes at two different atomic sites. Electron density
difference between the ground and double core-hole states clearly shows the
relaxations accompanying the double core-hole ionization. The effect is also
compared with the sensitivity of single core hole ionization potentials (IPs)
arising in single core hole electron spectroscopy. We have demonstrated the
method for a representative set of small molecules LiF, BeO, BF, CO, N2, C2H2,
C2H4, C2H6, CO2 and N2O. The scalar relativistic effect on IPs and on DIPs are
briefly addressed.Comment: 35 pages, 6 figures. To appear in J. Chem. Phys
Coulombic Energy Transfer and Triple Ionization in Clusters
Using neon and its dimer as a specific example, it is shown that excited
Auger decay channels that are electronically stable in the isolated monomer can
relax in a cluster by electron emission. The decay mechanism, leading to the
formation of a tricationic cluster, is based on an efficient energy-transfer
process from the excited, dicationic monomer to a neighbor. The decay is
ultrafast and expected to be relevant to numerous physical phenomena involving
core holes in clusters and other forms of spatially extended atomic and
molecular matter.Comment: 5 pages, 1 figure, to be published in PR
Photon angular distribution and nuclear-state alignment in nuclear excitation by electron capture
The alignment of nuclear states resonantly formed in nuclear excitation by
electron capture (NEEC) is studied by means of a density matrix technique. The
vibrational excitations of the nucleus are described by a collective model and
the electrons are treated in a relativistic framework. Formulas for the angular
distribution of photons emitted in the nuclear relaxation are derived. We
present numerical results for alignment parameters and photon angular
distributions for a number of heavy elements in the case of E2 nuclear
transitions. Our results are intended to help future experimental attempts to
discern NEEC from radiative recombination, which is the dominant competing
process
Gas-phase study on uridine: Conformation and X-ray photofragmentation
Fragmentation of RNA nucleoside uridine, induced by carbon 1s core ionization, has been studied. The measurements by combined electron and ion spectroscopy have been performed in gas phase utilizing synchrotron radiation. As uridine is a combination of d-ribose and uracil, which have been studied earlier with the same method, this study also considers the effect of chemical environment and the relevant functional groups. Furthermore, since in core ionization the initial core hole is always highly localized, charge migration prior to fragmentation has been studied here. This study also demonstrates the destructive nature of core ionization as in most cases the C 1s ionization of uridine leads to concerted explosions producing only small fragments with masses ≤43 amu. In addition to fragmentation patterns, we found out that upon evaporation the sugar part of the uridine molecule attains hexagonal formFinancial support from the Academy of Finland, the European COST Action XLIC CM1204 and the EU Transnational Access to Research Infrastructures programme. Computational resources from the FGI project (Finland) are acknowledged. D.T.H. acknowledges the Finnish Cultural Foundation for funding and the MINECO Project No. FIS2013-42002- R. E.R. acknowledges funding from the Swedish Research Council (VR
Experimental methods in chemical engineering: X-ray photoelectron spectroscopy-XPS
X\u2010ray photoelectron spectroscopy (XPS) is a quantitative surface analysis technique used to identify the elemental composition, empiricalformula, chemical state, and electronic state of an element. The kinetic energy of the electrons escaping from the material surface irradiated by anx\u2010ray beam produces a spectrum. XPS identifies chemical species and quantifies their content and the interactions between surface species. It isminimally destructive and is sensitive to a depth between 1\u201310 nm. The elemental sensitivity is in the order of 0.1 atomic %. It requires ultra highvacuum (1
7107 12Pa) in the analysis chamber and measurement time varies from minutes to hours per sample depending on the analyte. XPSdates back 50 years ago. New spectrometers, detectors, and variable size photon beams, reduce analysis time and increase spatial resolution. AnXPS bibliometric map of the 10 000 articles indexed by Web of Science[1]identifies five research clusters: (i) nanoparticles, thin films, and surfaces;(ii) catalysis, oxidation, reduction, stability, and oxides; (iii) nanocomposites, graphene, graphite, and electro\u2010chemistry; (iv) photocatalysis,water, visible light, andTiO2; and (v) adsorption, aqueous solutions, and waste water
Recent advances in electronic structure theory and their influence on the accuracy of ab initio potential energy surfaces
Recent advances in electronic structure theory and the availability of high speed vector processors have substantially increased the accuracy of ab initio potential energy surfaces. The recently developed atomic natural orbital approach for basis set contraction has reduced both the basis set incompleteness and superposition errors in molecular calculations. Furthermore, full CI calculations can often be used to calibrate a CASSCF/MRCI approach that quantitatively accounts for the valence correlation energy. These computational advances also provide a vehicle for systematically improving the calculations and for estimating the residual error in the calculations. Calculations on selected diatomic and triatomic systems will be used to illustrate the accuracy that currently can be achieved for molecular systems. In particular, the F+H2 yields HF+H potential energy hypersurface is used to illustrate the impact of these computational advances on the calculation of potential energy surfaces
High energy photoelectron diffraction: model calculations and future possibilities
We discuss the theoretical modelling of x-ray photoelectron diffraction (XPD)
with hard x-ray excitation at up to 20 keV, using the dynamical theory of
electron diffraction to illustrate the characteristic aspects of diffraction
patterns resulting from such localized emission sources in a multi-layer
crystal.
We show via dynamical calculations for diamond, Si, and Fe that the dynamical
theory well predicts available current data for lower energies around 1 keV,
and that the patterns for energies above about 1 keV are dominated by Kikuchi
bands which are created by the dynamical scattering of electrons from lattice
planes. The origin of the fine structure in such bands is discussed from the
point of view of atomic positions in the unit cell. The profiles and positions
of the element-specific photoelectron Kikuchi bands are found to be sensitive
to lattice distortions (e.g. a 1% tetragonal distortion) and the position of
impurities or dopants with respect to lattice sites. We also compare the
dynamical calculations to results from a cluster model that is more often used
to describe lower-energy XPD. We conclude that hard XPD (HXPD) should be
capable of providing unique bulk-sensitive structural information for a wide
variety of complex materials in future experiments.Comment: 29 pages, 13 figure
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