Core Excitation Research Articles

Detailed spectroscopy of doubly magic Sn132

The structure of the doubly magic $^{132}_{50}$Sn$_{82}$ has been investigated at the ISOLDE facility at CERN, populated both by the $\beta^-$decay of $^{132}$In and $\beta^-$-delayed neutron emission of $^{133}$In. The level scheme of $^{132}$Sn is greatly expanded with the addition of 68 $\gamma$-transitions and 17 levels observed for the first time in the $\beta$ decay. The information on the excited structure is completed by new $\gamma$-transitions and states populated in the $\beta$-n decay of $^{133}$In. Improved delayed neutron emission probabilities are obtained both for $^{132}$In and $^{133}$In. Level lifetimes are measured via the Advanced Time-Delayed $\beta\gamma\gamma$(t) fast-timing method. An interpretation of the level structure is given based on the experimental findings and the particle-hole configurations arising from core excitations both from the \textit{N} = 82 and \textit{Z} = 50 shells, leading to positive and negative parity particle-hole multiplets. The experimental information provides new data to challenge the theoretical description of $^{132}$Sn.

Correction of EELS dispersion non-uniformities for improved chemical shift analysis

We outline a simple routine to correct for non-uniformities in the energy dispersion of a post-column electron energy-loss spectrometer for use in scanning transmission electron microscopy. We directly measure the dispersion and its variations by sweeping a spectral feature across the full camera to produce a calibration that can be used to linearize datasets post-acquisition, without the need for reference materials. The improvements are illustrated using core excitation electron energy-loss spectroscopy (EELS) spectra collected from NiO and diamond samples. The calibration is rapid and will be of use in all EELS analysis, particularly in assessments of the chemical states of materials via the chemical shift of core-loss excitations.

Design and synthesis of light-harvesting rotor based on 1,8-naphthalimide units

A novel pH and viscosity fluorescence sensing multicomponent molecular device based on 1,8-naphthalimide fluorophores is synthesized and investigated. The system is designed as a light-harvesting antenna where energy supplier, molecular rotor and molecular “on-off” switcher are integrated. The peripheral 1,8-naphthalimide energy suppliers transfer rapidly the trapped energy to the core molecular rotor through high efficient FRET (99 %). The 4-piperazinyl-1,8-naphthalimide core excitation results in a TICT driven molecular motion. Due to the non-emissive de-excitation nature of the TICT core fluorophore, system shows low yellow-green fluorescence that is a “power-on”/“rotor-on” state. The protonation of the methylpiperazine amine destabilized TICT process thus indicating a “power-on”/“rotor-off” system state.

Shower of γ rays in the decay of the 49/2+ isomeric state in Gd147

Decay of the $49/{2}^{+}$, $0.51\phantom{\rule{0.28em}{0ex}}\ensuremath{\mu}\mathrm{s}$ isomeric state in $^{147}\mathrm{Gd}$ was reinvestigated in the $\ensuremath{\gamma}$-coincidence catcher experiment using the $^{76}\mathrm{Ge}(^{76}\mathrm{Ge},5n$) reaction and the GASP $\ensuremath{\gamma}$ detector array. A previously suggested extraordinary complexity of the decay was confirmed and established down to intensity level better than ${10}^{\ensuremath{-}3}$/decay, involving 400 transitions and 89 levels populated between the isomer and the $^{147}\mathrm{Gd}$ ground state. The earlier measured electron conversion coefficients (ECC) and $\ensuremath{\gamma}$ angular distributions, together with the total ECC extracted presently for low-energy transitions from the intensity balance, allowed to assign spin and parity values to all of the observed levels. Discussion of the level structure is restricted to few important cases clarified by the present investigation in the low-energy part of the decay scheme. Those involved the identification of the new $M2$ and $E2$ branches populating two new levels in the decay of the ${27/2}^{\ensuremath{-}}$ isomer and firm location of positive parity levels arising in the coupling of this isomer with the ${3}^{\ensuremath{-}}$ core excitation. In the upper part it was concluded that the observed complexity of the decay is initiated by fast low-energy $M1$ transitions populating a group of similar structure levels located within 7.8--8.0 MeV excitation energy range. Some statistical features of the observed complex isomeric decay are reviewed and discussed.

Probing Solute-Solvent Interactions of Transition Metal Complexes Using L-Edge Absorption Spectroscopy.

In order to tailor solution-phase chemical reactions involving transition metal complexes, it is critical to understand how their valence electronic charge distributions are affected by the solution environment. Here, solute–solvent interactions of a solvatochromic mixed-ligand iron complex were investigated using X-ray absorption spectroscopy at the transition metal L2,3-edge. Due to the selectivity of the corresponding core excitations to the iron 3d orbitals, the method grants direct access to the valence electronic structure around the iron center and its response to interactions with the solvent environment. A linear increase of the total L2,3-edge absorption cross section as a function of the solvent Lewis acidity is revealed. The effect is caused by relative changes in different metal–ligand-bonding channels, which preserve local charge densities while increasing the density of unoccupied states around the iron center. These conclusions are corroborated by a combination of molecular dynamics and spectrum simulations based on time-dependent density functional theory. The simulations reproduce the spectral trends observed in the X-ray but also optical absorption experiments. Our results underscore the importance of solute–solvent interactions when aiming for an accurate description of the valence electronic structure of solvated transition metal complexes and demonstrate how L2,3-edge absorption spectroscopy can aid in understanding the impact of the solution environment on intramolecular covalency and the electronic charge distribution.

Relativistic local hybrid functionals and their impact on 1s core orbital energies.

In this work, we report the first relativistic density functional theory calculations using relativistic local hybrid functionals. Besides outlining the construction of relativistic local hybrid exchange within a two-component-relativistic framework based on the picture-change transformation of the density matrix and a recently developed relativistic iso-orbital indicator, we investigate the influence of two-electron-relativistic effects, using relativistic functional ingredients in local hybrid functionals, and the choice of the exchange-correlation functional on atomic 1s core orbital energies of light and heavier elements. Finally, we discuss the applicability of relativistic 1s core orbital shifts for the relativistic correction of non-relativistic 1s core excitation energies.

EOM-CC methods with approximate triple excitations applied to core excitation and ionisation energies

A number of iterative and perturbative approximations to the full equation-of-motion coupled cluster method with single, double, and triple excitations (EOM-CCSDT) are evaluated in the context of calculating the K-edge core-excitation and core-ionisation energies of several small molecules. Several of these methods are found to accurately reproduce the full EOM-CCSDT energies well, in particular the EOM-CCSD* method which, when using the core-valence separation (CVS) ansatz, scales rigorously with the sixth power of molecular size. These results highlight that very accurate core excitation and ionisation energies for molecules with first-row atoms can be computed at a cost not much larger than that for EOM-CCSD.

Quasirelativistic two-component core excitations and polarisabilities from a damped-response formulation of the Bethe–Salpeter equation

A damped-response formalism in the GW approximation to the Bethe–Salpeter equation (BSE) is presented and implemented. It is based on a quasirelativistic two-component (2c) approach that includes scalar-relativistic and spin–orbit effects derived from the one-electron Dirac equation. A generalised solver, which also allows to calculate static, dynamic and damped-response polarisabilites, is discussed in detail. Throughout our implementation, the resolution-of-the-identity approximation is employed to reduce the computational effort. The performance of 2c GW-BSE is benchmarked against experimental and high-level ab initio data and compared to standard density-functional theory approaches, including modern local hybrid functionals for which a proper non-collinear kernel for Kramers-restricted systems is reported.

Electron-ion recombination rate coefficients of carbon-like Ar12+

Electron-ion recombination of carbon-like Ar12+ forming Ar11+ has been investigated for the first time by using the cooler storage ring CSRm at the Institute of Modern Physics in Lanzhou, China. The absolute recombination rate coefficients are derived from the measurement in the electron-ion collision energy range of 0–50 eV, covering dielectronic recombination (DR) resonances associated with 2s22p2 and 2s2p3 (Δn = 0) core excitations. Theoretical results are obtained by employing FAC code and compared with the experimental recombination spectrum. An overall agreement is found in resonance energy positions between merged-beam and calculated spectra. Temperature dependent rate coefficients are derived from the measured DR spectrum by convoluting it with a Maxwell–Boltzmann energy distribution and compared with the calculations from the literature. In the presented temperature range 103–107 K, the experimentally derived rate coefficients agree with the theoretical results of Gu (2003 Astrophys. J. 590 1131) within the experimental uncertainties. The calculation by Zatsarinny et al (2004 Astron. Astrophys. 417 1173) underestimates the rate coefficients in the temperature range 103–5 × 104 K. The combination of the experimental results and theoretical calculation provides a benckmark for Ar12+ recombination data used in astrophysical modeling.

Shell model description of the core excited level structure of $$^{89}$$Sr nucleus and systematic features of the $$N=51$$ odd-A isotones

Shell-model calculations are performed using NuShellX code in the model space $$\pi $$($$f_{5/2}$$, $$p_{3/2}$$, $$p_{1/2}$$, $$g_{9/2}$$) $$\otimes $$ $$\nu $$($$g_{9/2}$$, $$g_{7/2}$$, $$d_{5/2}$$, $$h_{11/2}$$), which probe the proton core excitation from the interior of $$Z=38$$ semiclosed shell and neutron core excitation from the interior of $$N=56$$ semiclosed shell for the level structure of $$^{89}$$Sr. Our calculations show that the excitation of a single $$d_{5/2}$$ neutron across $$N=56$$ semiclosed shell into the $$h_{11/2}$$ orbit should have great effects on the excited states of $$^{89}$$Sr. In addition, the systematic features of proton core excitation across $$Z=38$$ semiclosed shell into the $$g_{9/2}$$ orbit and neutron core excitation across $$N=56$$ semiclosed shell into the $$g_{7/2}$$, $$s_{1/2}$$, $$d_{3/2}$$, $$h_{11/2}$$ orbits in $$N=51$$ isotones are discussed.

Setting the stage for theoretical x-ray spectra of the H2S molecule with multi-configurational quantum chemical calculations of the energy landscape

In the H2S molecule, the interplay between different core levels can be investigated in great detail in relation to x-ray spectroscopy, which requires a theory for interpretation. Hence, valence and core excitations into the two antibonding molecular orbitals of the H2S molecule have been calculated within a multi-configurational wave function framework. Scanning along the S-H stretching coordinates, we derive potential energy surfaces and transition dipole moments involving the ground state and core and valence excited states. Both valence excitations and the S1s-1 and S2p-1 core excitations show pairs of dissociative and bound electronic states. These pairs of states are nearly degenerate in H2S at the ground state geometry. The close degeneracy together with conical intersections makes H2S an interesting target for x-ray spectroscopy involving ultra-fast dissociation influenced by non-adiabatic transitions and interference. For future investigations with x-ray absorption spectroscopy (XAS) and resonant inelastic x-ray scattering (RIXS), it is valuable to compare H2S with the water molecule, which exhibits state-selective gating to different vibrational modes [R. C. Couto et al., Nat. Commun. 8, 14165 (2017)] in its well-separated O1s-1 core excited states. The dense manifolds of the S2p-1 core excited states will complicate the analysis of Kα edge RIXS, but dynamical effects could be evaluated through detuning and by comparing with L edge XAS. In L edge RIXS, the dynamical effects will be more pronounced due to the longer lifetime of the S2p-1 core excited states compared to the S1s-1 core excited states.

Excitation energies expressed as orbital energies of Kohn-Sham density functional theory with long-range corrected functionals.

A new simple and conceptual theoretical scheme is proposed for estimating one-electron excitation energies using Kohn-Sham (KS) solutions. One-electron transitions that are dominated by the promotion from one initially occupied orbital to one unoccupied orbital of a molecular system can be expressed in a two-step process, ionization, and electron attachment. KS with long-range corrected (LC) functionals satisfies Janak's theorem and LC total energy varies almost linearly as a function of its fractional occupation number between the integer electron points. Thus, LC reproduces ionization energies (IPs) and electron affinities (EAs) with high accuracy and one-electron excitation energies are expressed as the difference between the occupied orbital energy of a neutral molecule and the corresponding unoccupied orbital energy of its cation. Two such expressions can be used, with one employing the orbital energies for the neutral and cationic systems, while the other utilizes orbital energies of just the cation. Because the EA of a molecule is the IP of its anion, if we utilize this identity, the two expressions coincide and give the same excitation energies. Reasonable results are obtained for valence and core excitations using only orbital energies.

Spin–isospin properties of $$\varvec{N=Z}$$ odd–odd nuclei from a core+$$\varvec{pn}$$ three-body model including core excitations

For $$N=Z$$ odd–odd nuclei, a three-body model assuming two valence particles and an inert core can provide insight into pairing correlations in the ground state and spin–isospin excitations. However, since residual core–nucleon interactions can have a significant impact, the inclusion of core excitations in the model is essential for a detailed understanding of both the ground state and spin–isospin properties. To qualitatively understand the effect of core excitation on the ground state and spin–isospin excitations of $$N=Z$$ odd–odd nuclei, we solve the three-body core–nucleon–nucleon problem including core vibrational states described by the random-phase approximation. We pay a particular attention to the magnetic dipole and Gamow–Teller transitions between $$0^+$$ and $$1^+$$ states of $$N=Z$$ nuclei and discuss it in terms of the SU(4) multiplet. We also discuss the effect of coupling to the core vibration on low energy spectra of even–odd nuclei, the spatial distribution, core contributions to the ground state, and possible uncertainties in the present framework.

Evolution of Stokes V area asymmetry related to a quiet Sun cancellation observed with GRIS/IFU

A quiet Sun magnetic flux cancellation event at the disk center was recorded using the Integral Field Unit (IFU) mounted on the GREGOR Infrared Spectrograph (GRIS). The GRIS instrument sampled the event in the photospheric Si I 10827 Å spectral line. The cancellation was preceded by a significant rise in line core intensity and excitation temperature, which is inferred from Stokes inversions under local thermodynamic equilibrium (LTE). The opposite polarity features seem to undergo reconnection above the photosphere. We also found that the border pixels neighboring the polarity inversion line of one of the polarities exhibit a systematic variation of area asymmetry. Area asymmetry peaks right after the line core intensity enhancement and gradually declines thereafter. Analyzing Stokes profiles recorded from either side of the polarity inversion line could therefore potentially provide additional information on the reconnection process related to magnetic flux cancellation. Further analysis without assuming LTE will be required to fully characterize this event.

Quantitative electronic stopping power from localized basis set

The electronic stopping power of a swift ion in matter can be obtained from ab initio calculations within time-dependent density functional theory. Most implementations rely today on a plane-wave plus pseudopotential approach, but at the expense of very cumbersome calculations. We show here that localized orbitals, especially with Gaussian-type orbitals, are a valuable alternative. These calculations can yield electronic stopping powers in quantitative agreement with the plane-wave results while maintaining a computational burden that is relatively low. These positive results are possible only when using Gaussian basis sets that were specially designed for the stopping power calculations. With this tool, we investigate the discrepancy between ab initio calculations and experiment at large velocity, the effect of the exchange-correlation functional, and the role of core excitations in the total stopping power. We rule out the widespread centroid path approximation as soon as the core electrons are involved in the process.

Isomer studies in the vicinity of the doubly-magic nucleus 100Sn: Observation of a new low-lying isomeric state in 97Ag

Long-lived isomeric states in 97Ag and 101−109In were investigated with the FRS Ion Catcher at GSI. In the isotope 97Ag, a long-lived (1/2−) isomeric state was discovered, and its excitation energy was determined to be 618(38) keV. This is simultaneously the first discovery of a nuclear isomeric state by multiple-reflection time-of-flight mass spectrometry. The measured excitation energies were compared to large-scale shell-model calculations, which indicated the importance of core excitation around 100Sn. Furthermore, advanced mean-field calculations for the 97Ag nucleus and relevant neighboring nuclei were performed, which have contributed to a better understanding of the repetitive appearance of certain isomeric structures in neighboring nuclei, and which have supported the discovery of the isomeric state in 97Ag in a global shell-evolution scheme.

Dielectronic recombination rate coefficients for F-like nickel

SynopsisAt the main cooler storage ring (CSRm) of IMP, Lanzhou, we have measured merged-beams rate coefficients for dielectronic recombination (DR) of F-like Ni in the energy range 0-160 eV which comprises all DR resonances associated with ΔN = 0 core excitations. We compare our results with calculations by the flexible atomic code (FAC). Good agreement is found between calculated and measured resonance parameters except for the low-energy 2s 2p6 (2S1/2)61 resonances. In addition, we have also derived plasma rate-coefficients from our data for applications in astrophysics.

Cascade and direct processes in multiple Auger decay of core-excited states of N2

SynopsisWe report here a detailed experimental study on multiple Auger decay after core excitation into the π* valence and Rydberg orbitals in N2 by an electron-electron coincidence technique.

Generalized single excitation configuration interaction: an investigation into the impact of the inclusion of non-orthogonality on the calculation of core-excited states.

In this paper, we investigate different non-orthogonal generalizations of the configuration interaction with single substitutions (CIS) method for the calculation of core-excited states. Fully non-orthogonal CIS (NOCIS) has been described previously for species with singlet and doublet ground states, and this paper reports the extension to molecules in their triplet ground state. In addition to NOCIS, we present a novel method, one-center NOCIS (1C-NOCIS), for open-shell molecules which is intermediate between NOCIS and the computationally less demanding static exchange approximation (STEX). We explore this hierarchy of spin-pure methods for core excitations of molecules with singlet, doublet, and triplet ground states. We conclude that, while NOCIS provides the best results and preserves the spatial symmetry of the wavefunction, 1C-NOCIS retains much of the accuracy of NOCIS at a dramatically reduced cost. For molecules with closed-shell ground states, STEX and 1C-NOCIS are identical.

Model for vibronic excitation of CH+ by resonant scattering of electron

SynopsisA new model for theoretical study of vibronic (de-)excitation (including vibrational (de-)excitation) cross sections of CH+ by electron impact was developed based on the multichannel quantum defect theory (MQDT) close-channel-elimination procedure and vibrational-frame-transformation theory of Chang and Fano. The successful application of this model suggested a promising calculation method for vibronic (de-)excitation of diatomic ion core by resonant scattering of electron.