Core-excited States Research Articles

Vibronic Coupling Effects in Resonant Auger Spectra of H2O

We present a theoretical investigation of the resonant Auger effect in gas-phase water. As in our earlier work, the simulation of nuclear dynamics is treated in a one-step picture, because excitation and decay events cannot be disentangled. Extending this framework, we now account for the vibronic coupling in the cationic final states arising from degeneracies in their potential energy surfaces (PESs). A diabatization of the cationic states permits a correct treatment of non Born-Oppenheimer dynamics leading to a significantly better agreement with experimental results. Moreover, we arrive at a more balanced understanding of the various spectral features that can be attributed to nuclear motion in the core-excited state or to vibronic coupling effects. The nuclear equations of motion have been solved using the multiconfiguration time-dependent Hartree (MCTDH) method. The cationic PESs were recalculated using the coupled electron pair approach (CEPA) whereas previously a multireference configuration interaction method had been employed.

Inner-shell photodetachment: Shape and Feshbach resonances of anions

Inner-shell photoexcitation and photodetachment studies in strongly correlated systems are presented, with the focus on selected shape and Feshbach resonance studies, and inner-shell threshold measurements conducted on anions at the Advanced Light Source. We highlight the observations of simultaneous double-Auger decay from the He−2s2p2P4 hollow ion state and the formation of shape and Feshbach resonances when filling a vacancy in the valence shell with a core electron in oxygen-group elements and transition metal anions. Measurements of inner-shell thresholds have also led to the first measurements of core-excited neutral states in a number of species.

Lifetime broadening of core-excited and -ionized states

Natural line widths are measurable fundamental quantities of quantum states that, in atoms and molecules, characterize the lifetime of the electronic states and therefore carry information about electronic structure and transition matrix elements. For core-levels, their determination is based on various types of spectroscopic measurements such as the photoabsorption or the photoelectron spectroscopy for core-excited and core-ionized states, respectively. The values reported in the literature used to change over the years owing to the improvement of the experimental accuracy available at synchrotron storage rings. We report here, in a broad energy range, the natural line widths for a series of core-levels in atoms (Kr3d, Ar2p, Xe3d) and molecules (S2p in CH3SSCH3, B1s in BF3, Cl2p in HCl, C1s in CO, C2H2, CH4 and CF4, N1s in N2, O1s in O2, CO and CO2), based on ultrahigh resolution total ion yield or X-ray photoelectron spectroscopy (XPS) measurements at the PLEIADES beamline at SOLEIL synchrotron (France). These newest measurements of the lifetime broadening of core levels are compared with previously published experimental results.

Theoretical study of two Ih-symmetry-breaking C60 isomers and their chlorinated species in core-excited and ground states

X-ray photoelectron (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectra, as well as ground-state electronic/geometrical structures of two Ih-symmetry-breaking C60 isomers (C2v- and Cs-symmetry, respectively) and their chlorinated species have been simulated at the density functional theory level. Effective changes in the electronic structure and simulated X-ray spectra have been observed after chlorination. Both spectra show strong isomer dependence, therefore the ‘fingerprints’ in the X-ray spectra provide a valuable way for isomer identification in further experimental and theoretical studies.

Theoretical study on decay processes of the core-excited states in BII

Wavefunctions, energies, radiative decay possibilities and non-radiative (Auger) decay rates of the core-excited 1s(2l)n(2l′)m(n+m=3) states in BII are calculated using relativistic Multiconfiguration Dirac–Fock method. We found that electron correlations effects are very important for energies and radiative decay rates of core-excited states in BII and that both the valence–valence and core–valence electron correlations need to be considered in the calculations. Auger decay rates of quintuplet 1s2p3 5S2 states and 1s2s2p2 5P1, 5P2, 5P3 states are much smaller compared to these of other states, which implies that these states are more stable than the other states in this work. Our results provide theoretical supports to further experimental researches.

Quantum Monte Carlo for the x-ray absorption spectrum of pyrrole at the nitrogen K-edge

Fixed-node diffusion Monte Carlo (FNDMC) is used to simulate the x-ray absorption spectrum of a gas-phase pyrrole molecule at the nitrogen K-edge. Trial wave functions for core-excited states are constructed from ground-state Kohn-Sham determinants substituted with singly occupied natural orbitals from configuration interaction with single excitations calculations of the five lowest valence-excited triplet states. The FNDMC ionization potential (IP) is found to lie within 0.3 eV of the experimental value of 406.1 ± 0.1 eV. The transition energies to anti-bonding virtual orbitals match the experimental spectrum after alignment of IP values and agree with the existing assignments.

Excitation energies, radiative and autoionization rates, dielectronic satellite lines, and dielectronic recombination rates for excited states of Yb-like W

Energy levels, radiative transition probabilities and autoionization rates for [Cd]4f145p65l′nl, [Cd]4f145p66l″nl, [Cd]4f145p55d2nl, [Cd]4f145p55d6l″nl, [Cd]4f135p65d2nl and [Cd]4f135p65d6l″nl (l′ = d, f, g, l″ = s, p, d, l = s, p, d, f, g and n = 5–7) states of Yb-like tungsten (W4 +) are calculated using the relativistic many-body perturbation theory method (RMBPT code), the multiconfiguration relativistic Hebrew University–Lawrence Livermore Atomic Code (HULLAC code) and the Hartree–Fock relativistic method (COWAN code). Branching ratios relative to the [Cd]4f145p65d, [Cd]4f145p66s and [Cd]4f145p66p thresholds in Tm-like tungsten and intensity factors are calculated for satellite lines, and dielectronic recombination (DR) rate coefficients are determined for the singly excited, as well as non-autoionizing core-excited states in Yb-like tungsten. Contributions from the autoionizing doubly excited states [Cd]4f145p65fnl, [Cd]4f145p66l″nl and core-excited [Cd]4f145p55d2nl, [Cd]4f145p55d6l″nl, [Cd]4f135p65d2nl, [Cd]4f135p65d6l″nl states (with n up to 100), which are particulary important for calculating the total DR rates, are estimated. Synthetic dielectronic satellite spectra from Yb-like W are simulated in a broad spectral range from 200 to 1400 Å. These calculations provide recommended values critically evaluated for their accuracy for a number of W4 + properties useful for a variety of applications including for fusion applications.

Rydberg-valence mixing and interchannel coupling in resonant oxygen1sinelastic x-ray scattering of O2

Rydberg-valence (RV) mixing and doublet-quartet (DQ) interchannel coupling in the core-excited states of triplet open-shell O2 are experimentally and theoretically unveiled through the resonant ine ...

Energy levels, Auger branching ratios, and radiative rates of the core-excited states of B-like carbon

Energy levels, Auger branching ratios, and radiative rates of the core-excited states of B-like carbon are calculated by the saddle-point variation and saddle-point complex-rotation methods. Relativistic and mass polarization corrections are included using first-order perturbation theory. Calculated Auger channel energies and branching ratios are used to identify high-resolution Auger spectrum in the 300-keV C(+) → CH(4) collision experiment. It is found that Auger decay of these five-electron core-excited states gives significant contributions to Auger spectrum in the range of 238-280 eV.

Isomer and β-decay spectroscopy of Tz=1 isotopes below the N=Z=50 shell gap

The RISING setup at the GSI-FRS facility was used to investigate the isomer and beta decays in N~Z~50 Cd, Ag and Pd isotopes. A preliminary analysis of the data has revealed new results on the Tz=1, 94Pd, 96Ag and 98Cd isotopes. In 94Pd a new high-spin isomer was observed, whilst in 96Ag 3 new isomeric states were identified, including core-excited states. In 98Cd a new high-energy isomeric γ-ray transition is observed, thus enabling us to confirm the previous spin assignment for the core-excited 12+ isomer.

Observation of transitions involving core-excited states in Ar III and Ar IV and high-lying singly excited states in Ar I–Ar IV

In this paper we present a comprehensive analysis of beam-foil and beam-gas excited spectrum of argon observed in small wavelength region, 2965–3090 Å, using Ar +/2+ ions in the energy range 200–650 keV. The comparison of beam-foil spectrum (BFS) at different incident beam energy and with that of beam-gas spectrum (BGS), one can find solution for blending problem in beam-foil spectroscopy. Many new transitions were identified on the basis of calculated wavelength from the accurately known energy levels of Ar I, Ar II, Ar III and Ar V. The transitions of Ar I originate from highly excited states (10 p to 17 p and 10 f to 17 f). Based on TOPbase estimates using close-coupling approximation five transitions involving core-excited states and nine transitions originating from highly excited states in the spectrum of Ar III and Ar IV were also identified. Radiative life time of two core excited quintet states (3 s3 p 4( 4P)6d 5D and 3 s3 p 4( 4P)8 f 5F o) of Ar III were measured and found to be in ‘good’ agreement with that of the calculated value using close-coupling approximation.

Nuclear dynamics in the core-excited state of aqueous ammonia probed by resonant inelastic soft x-ray scattering

The electronic structure of aqueous NH${}_{3}$ and ND${}_{3}$ has been investigated using resonant inelastic soft x-ray scattering. Spectral features of different processes involving nuclear dynamics in the core-excited state can be identified. When exciting into the lowest core-excited state, we find a strong isotope effect and clear evidence for ultrafast proton dynamics. Furthermore, a strong vibronic coupling is observed and, in the case of aqueous NH${}_{3}$, a vibrational fine structure can be resolved.

Low-energy electron collisions with thioformaldehyde

The elastic integral, differential, and momentum transfer cross sections and the excitation cross sections from the ground state to the first three low-lying electron excited states for low-energy (0--10 eV) electron collisions with thioformaldehyde (H${}_{2}$CS) molecule are calculated using the $R$-matrix method. Eighteen target states are included in the close-coupling formalism. Three core-excited shape resonant states (${}^{2}$${B}_{2}$,${}^{2}$${B}_{1}$,${}^{2}$${A}_{1}$) and three Feshbach resonant states (${}^{2}$${B}_{2}$,${}^{2}$${A}_{1}$,${}^{2}$${B}_{1}$) are determined for the ($N$ + 1)-electron H${}_{2}$CS${}^{\ensuremath{-}}$ system in the low-energy range. The dissociative electron attachment processes at the first two low-lying core-excited shape resonant states are proven by performing a series of scattering calculations, indicating the S${}^{\ensuremath{-}}$(${}^{2}P$) yields together with the CH${}_{2}$ fragment at three different states.

Core-level positive-ion and negative-ion fragmentation of gaseous and condensed HCCl3 using synchrotron radiation

We investigated the dissociation dynamics of positive-ion and negative-ion fragments of gaseous and condensed HCCl(3) following photoexcitation of Cl 2p electrons to various resonances. Based on ab initio calculations at levels HF/cc-pVTZ and QCISD/6-311G∗, the first doublet structures in Cl L-edge x-ray absorption spectrum of HCCl(3) are assigned to transitions from the Cl (2P(3/2),(1/2)) initial states to the 10a(1)(∗) orbitals. The Cl 2p → 10a(1)(∗) excitation of HCCl(3) induces a significant enhancement of the Cl(+) desorption yield in the condensed phase and a small increase in the HCCl(+) yield in the gaseous phase. Based on the resonant photoemission of condensed HCCl(3), excitations of Cl 2p electrons to valence orbitals decay predominantly via spectator Auger transitions. The kinetic energy distributions of Cl(+) ion via the Cl 2p → 10a(1)(∗) excitation are shifted to higher energy ∼0.2 eV and ∼0.1 eV relative to those via the Cl 2p → 10e(∗) excitation and Cl 2p → shape resonance excitation, respectively. The enhancement of the yields of ionic fragments at specific core-excited resonance states is assisted by a strongly repulsive surface that is directly related to the spectator electrons localized in the antibonding orbitals. The Cl(-) anion is significantly reinforced in the vicinity of Cl 2p ionization threshold of gaseous HCCl(3), mediated by photoelectron recapture through post-collision interaction.

Symmetry and vibrationally resolved absorption spectra near theN Kedges ofN2O: Experiment and theory

In this study, angle-resolved energetic-ion yield spectra were measured in the N $1s$ excitation region of ${\mathrm{N}}_{2}$O. A Franck-Condon analysis based on ab initio two-dimensional potential energy surfaces of the core-excited Rydberg states, which were calculated by the symmetry-adapted cluster-configuration interaction method, reproduced observed vibrational excitations specific to the individual Rydberg states well and enabled quantitative assignments. Geometric changes in the terminal nitrogen ${\mathrm{N}}_{\mathrm{t}}$ $1s$ and the central nitrogen ${\mathrm{N}}_{\mathrm{c}}$ $1s$ excited states with respect to the $3p$\ensuremath{\pi}, $3p$\ensuremath{\sigma}, and $4s$\ensuremath{\sigma} transitions were analyzed. The coupling of these valence and Rydbergs states was examined based on the second moment analysis. Irregular Rydberg-state behavior in the ${\mathrm{N}}_{\mathrm{c}}$ $1{s}^{\ensuremath{-}1}$ $4s$\ensuremath{\sigma} state was observed.

Core excitation contributions to the breakup of the one-neutron halo nucleusBe11on a proton

The effect of the core excitation in the breakup of a one-neutron halo nucleus is studied within two different reaction formalisms, namely, the core excited model and the single-scattering approximation of the three-body Faddeev--Alt-Grassberger-Sandhas equations with target-core potential allowing for the core excitation. As an example, we consider the breakup of $^{11}\mathrm{Be}$ on a proton target at 63.7 MeV/nucleon incident energy and calculate the semi-inclusive cross section in the excitation energy interval ${E}_{x}=3.0$--5.5 MeV (${E}_{\mathrm{rel}}=2.5$--5 MeV) containing the $3/{2}^{+}$ resonance with dominant contribution of the $^{10}\mathrm{Be}$(${2}^{+}$) core excited state. The effect of the core excitation to the breakup cross section integrated around this resonance is found to be very significant. Moreover, when resonant and nonresonant contributions are added, the resulting semi-inclusive cross section is in reasonable agreement with the existing data, demonstrating the relevance of the core excitation mechanism for this observable. The present calculations also show the importance of incorporating the energy dependence of the core-target transition operators in the reaction formalism.

Inelastic X-ray scattering and vibrational effects at the K-edges of gaseous N2, N2O, and CO2

We report non-resonant inelastic X-ray scattering experiments of several gaseous samples in the inner-shell excitation energy range. The experimental near-edge spectra from all the K-edges of N(2), N(2)O, and CO(2) including the momentum transfer dependence are presented. The results are analyzed using density functional theory calculations that accurately reproduce the experimental spectral features. We observe vibrational effects in the measured spectrum and in the calculations the atomic motion is modeled using the Franck-Condon approximation and the linear coupling model. Our findings show that vibrational effects cannot be neglected in the analysis of high resolution inelastic X-ray scattering spectroscopy. The results also support the validity of the transition potential approximation for calculating core excited state potential energy surfaces.

A theoretical study of the near edge X-ray absorption fine structure of amino acids and proteins

The near edge X-ray absorption fine structure (NEXAFS) of amino acids and proteins is studied with time-dependent density functional theory in conjunction with a short-range corrected exchange–correlation functional designed for core-excited states. For glycine, alanine and glycyl-glycine, good agreement between the computed spectra and experiment is obtained at the carbon, nitrogen and oxygen K-edges. The potential of NEXAFS spectroscopy to differentiate between different elements of protein secondary structure is explored. These calculations show that the greatest sensitivity to secondary structure is found at the nitrogen K-edge.

Circularly Polarized X Rays: Another Probe of Ultrafast Molecular Decay Dynamics

Dissociative nuclear motion in core-excited molecular states leads to a splitting of the fragment Auger lines: the Auger-Doppler effect. We present here for the first time experimental evidence for an Auger-Doppler effect following F1s → a(1g)* inner-shell excitation by circularly polarized x rays in SF(6). In spite of a uniform distribution of the dissociating S-F bonds near the polarization plane of the light, the intersection between the subpopulation of molecules selected by the core excitation with the cone of dissociation induces a strong anisotropy in the distribution of the S-F bonds that contributes to the scattering profile measured in the polarization plane.

Resonant Auger electron–photoion coincidence study of the fragmentation dynamics of an acrylonitrile molecule

Monochromatic synchrotron radiation was used to promote K-shell electrons of nitrogen and carbon from the cyano group (C ≡ N) of gaseous acrylonitrile (C2H3–CN) to the unoccupied antibonding π*C ≡ N orbital. Photofragmentation of acrylonitrile molecules following selective resonant core excitations of carbon and nitrogen core electrons to the π*C ≡ N orbital was investigated using the electron-energy-resolved photoelecton–photoion coincidence technique. The fragment ion mass spectra were recorded in coincidence with the resonant Auger electrons, emitted in the decay process of the core-excited states. Singly and triply deuterated samples were used for fragment identification. The results showed the initial core-hole localization to be of minor importance in determining the dissociation pattern of the molecular cation. The participator and spectator Auger transitions produce entirely different fragmentation patterns and the latter indicates that complex nuclear rearrangements take place. It is suggested that the calculated kinetic energy releases are caused by the existence of metastable states, which appear with the opening of the spectator Auger channels.