Low-lying Transitions Research Articles

Two-body weak currents in heavy nuclei

In light and medium-mass nuclei, two-body weak currents from chiral effective field theory account for a significant portion of the phenomenological quenching of Gamow-Teller transition matrix elements. Here we examine the systematic effects of two-body axial currents on Gamow-Teller strength and $\beta$-decay rates in heavy nuclei within energy-density functional theory. Using a Skyrme functional and the charge-changing finite amplitude method, we add the contributions of two-body currents to the usual one-body linear response in the Gamow-Teller channel, both exactly and though a density-matrix expansion. The two-body currents, as expected, usually quench both summed Gamow-Teller strength and decay rates, but by an amount that decreases as the neutron excess grows. In addition, they can enhance individual low-lying transitions, leading to decay rates that are quite different from those that an energy-independent quenching would produce, particularly in neutron-rich nuclei. We show that both these unexpected effects are related to changes in the total nucleon density as the number of neutrons increases.

Determination of quadrupole polarizabilities of the excited states of alkali-metal atoms

The scalar and tensor components of the electric quadrupole (E2) polarizabilities of the first two excited states of all the alkali-metal atoms are determined. To validate the calculations, we have evaluated the ground state E2 polarizabilities of these atoms and compared them with the literature values. We could not find the ground state E2 polarizability value for Fr in the literature to compare with our result. The dominant parts of these quantities are estimated by combining the precisely calculated E2 transition matrix elements of many low-lying transitions with the experimental energies, while the other contributions are estimated using lower-order methods. Our estimated values for the ground states of the above atoms are in good agreement with the literature values suggesting that our estimated E2 polarizabilities for the excited states of the alkali atoms, which were not known earlier except for the Li atom, are also quite accurate. These reported E2 polarizabilities could be useful in guiding many precision measurements in the alkali atoms.

The Potential Energy Profile of the HBr+ + HCl Bimolecular Collision.

Recently, the HBr+ + HCl bimolecular reaction has been exploited by guided ion beam studies to probe the effect of rotational excitations and collision energies on the dynamics of the ion-molecule reactions. The current manuscript employs high-level ab initio calculations and reports the potential energy of pathways leading to various products, including HBr + HCl+, H2Cl+ + Br, H2Br+ + Cl, and H2 + BrCl+. The study shows that the intermediates involved in this reaction are connected by low-lying transition states, thus frequent isomerizations and diverse products are expected. Further, this manuscript screens the performance of 192 different combinations of computationally efficient methods and basis sets in order to identify the optimal quantum chemical method for further dynamics simulations.

A Computational Approach to Nontraditional Intrinsic Luminescence: Vibrationally Resolved Absorption and Fluorescence Spectra of DABCO.

Recently, so-called "nontraditional intrinsic luminescence" has been reported in several macromolecular systems. Although DABCO (1,4-diazabicyclo[2.2.2]octane) is the first system in which the effect was observed, a thorough analysis of the optical properties of the molecule, which would reveal the origin of this mysterious effect, is still pending. We perform an advanced post-Hartree-Fock treatment of the low-lying electronic states of this molecule, which need to be described with care because of their pronounced Rydberg character. We take a deeper look into the low-lying electronic transitions of DABCO targeting the explanation of the complex vibronic structures of its absorption and fluorescence spectra. Two electronic states, the 1E'(n+3pxy) and 1A2″(n+3pz) states, contribute to the absorption spectrum in the 39000-46000 cm-1 spectral range. We also reveal the spectroscopic signature of the 1A2″(n+3pz) state. The analyses of the contributions of individual vibrational normal modes allowed the identification of those giving rise to the complex vibronic structures of the spectra. Fluorescence emission arises from the vibronic coupling of the one-photon forbidden transition between the 1A1'(n+3s) state and the electronic ground state. The spectrum, which can be interpreted in terms of populating a few vibrational normal modes, is shifted toward visible wavelengths mostly due to the forced interaction of the lone pair electrons of the two nitrogen atoms. Our work on DABCO may help to rationalize the luminescence of more complex systems containing tertiary amine groups.

The Line Variability of 78 Vir (A2p SrCrEu) in the Mid-ultraviolet

Abstract Line variations are studied in the mid-ultraviolet spectra of 78 Vir collected with IUE between phases of far-UV (FUV) maximum and FUV minimum. The mid-UV spectrum of 78 Vir is populated with low-lying transitions of iron-peak elements. The lines of Cr ii and Ca ii are found to be stronger at phase of FUV minimum, while the lines of Ni ii probably vary in anti phase with those of chromium and calcium. Those of Fe ii and Mn ii do not show convincing variations between the two phases. The analysis of these mid-ultraviolet spectra suggests that iron and manganese are uniformly distributed over the surface of 78 Vir, while chromium, calcium and nickel are not but more ultraviolet lines should be studied to confirm this.

Level Structure and Properties of Open f-Shell Elements

Open f-shell elements still constitute a great challenge for atomic theory owing to their (very) rich fine-structure and strong correlations among the valence-shell electrons. For these medium and heavy elements, many atomic properties are sensitive to the correlated motion of electrons and, hence, require large-scale computations in order to deal consistently with all relativistic, correlation and rearrangement contributions to the electron density. Often, different concepts and notations need to be combined for just classifying the low-lying level structure of these elements. With Jac, the Jena Atomic Calculator, we here provide a toolbox that helps to explore and deal with such elements with open d- and f-shell structures. Based on Dirac’s equation, Jac is suitable for almost all atoms and ions across the periodic table. As an example, we demonstrate how reasonably accurate computations can be performed for the low-lying level structure, transition probabilities and lifetimes for Th2+ ions with a 5f6d ground configuration. Other, and more complex, shell structures are supported as well, though often for a trade-off between the size and accuracy of the computations. Owing to its simple use, however, Jac supports both quick estimates and detailed case studies on open d- or f-shell elements.

Toward More Efficient Organic Semiconductors: The Relationship between Morphology, Charge Transport, and Photophysical Properties

In recent years, materials science has been facing an electronics revolution due to the fast progress in the development of organic semiconductors. In this context, the current study attempts to tune the photophysical properties of polycyclic aromatic hydrocarbons (PAHs) by engineering their morphology and size. Four families of PAHs─oligoacenes, circulenes, triangulenes, and sunflowers─are considered to investigate the evolution of their optoelectronic properties with size and morphology. Based on the calculated properties, several quantitative structure–property relationships have been established for the prediction of reorganization energy. It is found that morphologies like circulene and triangulene that are less aromatic and have smaller energy gaps achieve smaller reorganization energy and thus are better candidates for charge transport materials. For all considered PAHs, the excitation energies of the strongest low-lying electronic transition (β peak) span in the visible region and demonstrate the size tunability. The β peak of some oligoacene and triangulene families exhibits plasmonic behavior that is important for the fabrication of lighter optical devices compared with metallic systems. We have also explored the effect of sulfur heteroatoms on the structural, electronic, and optical properties of circulene and sunflower morphologies.

The Distribution of Iron over the Surface of 21 Com

Abstract Comparison of high dispersion LWP spectra obtained in 24 hr of monitoring of 21 Com with IUE in April 1991 reveals variations of low excitation Fe ii lines at phase 0.65 of the rotational period (FUV minimum flux) compared to phase 0.18 (FUV maximum flux). All 10 Fe ii lines of UV Multiplets 1 and 62 are consistently stronger at phase 0.65, which shows that the FUV flux minimum can be partially explained to an increase of the disk averaged iron abundance. In contrast, comparison of optical high resolution high signal-to-noise ELODIE spectra of 21 Com taken in 2004 April and separated by about half of the 2 days rotational period, reveals no significant variations of the Fe ii and Fe i lines. The lines monitored in the mid UV are strong low-lying transitions which are probably more sensitive to small abundance gradients over the surface of 21 Com.

Raman scattering study of the rare-earth binary ferroborate Nd0.75Dy0.25Fe3(BO3)4 single crystal

We report comprehensive Raman scattering measurements on a single crystal of binary ferroborate Nd0.75Dy0.25Fe3(BO3)4 in the temperature range of 7–295 K with 532 nm (18797 cm−1) laser excitation. The performed analysis of the polarized Raman spectra revealed the bands assigned to phonon, magnetic, and electronic excitations. The temperature evolution of these quasiparticle excitations has allowed us to ascertain the intricate coupling and interplay between lattice, magnetic, and electronic degrees of freedom. Analysis of the measured Raman spectra made it possible to identify all A1 and E phonon modes predicted by the group-theoretical analysis. The splitting energies between the LO and TO components of the polar E phonons were determined. Below the magnetic ordering temperature of the Fe sublattice, TN, we have revealed a multiple peaked two-magnon excitation. Analyzing the temperature evolution of low-frequency modes in the spectra, we also identified modes that are associated with electronic transitions between the crystal field levels of the Nd3+ ions with ground-state 4I9/2 and of the Dy3+ ions with ground-state 6H15/2 multiplets. In addition to the already known temperatures of magnetic transitions, analysis of the temperature behavior of low-frequency phonon and electronic excitations made it possible to establish a temperature T* = 100 K, presumably associated with local distortions of the crystal lattice. The presence of this temperature is confirmed by our ultrasonic study. A group of intense bands observed in the frequency range 1700–2200 cm−1 has been associated with the mixed low-lying electronic Raman transitions 4I9/2→4I11/2 and the high-energy luminescence ones 4G5/2+2G7/2→4I9/2 for the Nd3+ ion.

Spectroscopic studies of solid Ar condensed on a gold surface

We have studied condensed solid argon films on a gold surface, measuring its reflected electron energy-loss spectra (REELS) and X-ray photoelectron spectra (XPS) at the temperatures 5 K and 20 K. We have also reproduced theoretically the REELS considering both face-centered cubic (FCC) and hexagonal close-packed (HCP) lattices of solid argon using density-functional theory (DFT), GW approximation, random-phase approximation (RPA) and the Bethe–Salpeter equation (BSE). In the calculations the local-field dielectric screening, quasiparticles, and excitonic effects have been taken into account. The excitonic peaks in the REELS appear at ∼12 eV whereas the calculated electronic bandgap amounts to 14.3–14.4 eV for both the FCC and HCP lattices. Our results show that a solid argon film is a wide-gap dielectric best suited for implanting Th and studying its low-lying nuclear transition in 229Th with the energy of 8.2 eV.

Unravelling the electronic nature of the radiative cooling of cobalt clusters

In this paper, we report on the experimental determination of photon emission rates of laser-excited cobalt clusters, ${{\mathrm{Co}}_{n}}^{+}$ ($n=5--23$), deduced from fragmentation mass spectrometry and metastable decay fractions. The rates are so high that they can only be ascribed to recurrent fluorescence (RF), a process where emitting states are populated by inverse internal conversion, followed by photon emission. Cooling via electronic states is confirmed by quantitative agreement with calculated rates using the low-lying electronic transitions predicted by time-dependent density functional theory calculations for $n=5--10$, which are performed considering all electrons and including relativistic effects implicitly. The outstanding agreement between experiment and theory provides clear evidence that the clusters radiate via electronic states, being a consistent theoretical and experimental study invoking RF.

Constraints on the presence of platinum and gold in the spectra of the kilonova AT2017gfo

ABSTRACT Binary neutron star mergers are thought to be one of the dominant sites of production for rapid neutron capture elements, including platinum and gold. Since the discovery of the binary neutron star merger GW170817, and its associated kilonova AT2017gfo, numerous works have attempted to determine the composition of its outflowing material, but they have been hampered by the lack of complete atomic data. Here, we demonstrate how inclusion of new atomic data in synthetic spectra calculations can provide insights and constraints on the production of the heaviest elements. We employ theoretical atomic data (obtained using $\small {\rm GRASP}^{0}$) for neutral, singly and doubly ionized platinum and gold, to generate photospheric and simple nebular phase model spectra for kilonova-like ejecta properties. We make predictions for the locations of strong transitions, which could feasibly appear in the spectra of kilonovae that are rich in these species. We identify low-lying electric quadrupole and magnetic dipole transitions that may give rise to forbidden lines when the ejecta becomes optically thin. The strongest lines lie beyond 8000 Å, motivating high quality near-infrared spectroscopic follow-up of kilonova candidates. We compare our model spectra to the observed spectra of AT2017gfo, and conclude that no platinum or gold signatures are prominent in the ejecta. From our nebular phase modelling, we place tentative upper limits on the platinum and gold mass of ≲ a few 10−3 M⊙, and ≲ 10−2 M⊙, respectively. This work demonstrates how new atomic data of heavy elements can be included in radiative transfer calculations, and motivates future searches for elemental signatures.

The P-branch high-lying rovibrational transition spectral lines of BiLi molecule

Starting from the classical expression of the rovibrational transition spectra of diatomic molecule, the Difference Converging Method (DCM) is used again to derive a new convenient and easy-to-use analytical formula for predicting the P-branch transitional spectral lines in the highly rovibrational excited states of diatomic molecule. Based on the limited experimental low-lying transition spectra the emission spectral lines up to the rotational quantum number of (0–0) band in the transition systems of BiLi molecule are predicted and studied. The accuracy of prediction is analyzed in detail by using the prediction error Δδ, which gives a quantitative conclusion for the reliability of prediction data. Compared the data obtained by DCM method with experimental ones, and analyzed the in deeply, the results show that the DCM method can use 11 known P-branch emission spectral lines to reliably reproduce the experimental values and accurately predict the high-excited transition spectral lines which may be difficult obtained in the experiment. It provides a new method to solve the problem of lack of high excited state transition spectra of diatomic molecules and the application of molecular spectrum in other fields.

Calculation of nuclear structure in N = 90 isotones

The behavior of nuclear structures with an increase in atomic number from $${}^{150}$$ Nd to $${}^{158}$$ Er was investigated in this study. $${}^{152}$$ Sm and $${}^{154}$$ Gd are typical nuclei lying between the rotational and vibrational states, which can be explained by a slight breaking of the SU(3) symmetry in the U(5)-direction via the interacting boson model (IBM). Furthermore, $$N=90$$ isotones lie in the path of this symmetry-breaking phase transition. Moreover, the nuclear structure of $${}^{150}$$ Nd can be explained using X(5) symmetry. However, because the $${}^{156}$$ Dy and $${}^{158}$$ Er nuclei are not fully symmetrical, they can be represented by adding a perturbed term to express symmetry breaking. We identified the tendency of change in the nuclear structure using the following three calculation steps. First, the structures of $${}^{152}$$ Sm and $${}^{154}$$ Gd were described using the matrix elements of the Hamiltonian and the electric quadrupole operator between the basis states of the SU(3) limit in the IBM. Second, the low-lying energy levels and E2 transition ratios corresponding to the observable physical values were calculated by adding a perturbed term with the first-order Casimir operator of the U(5) limit to the SU(3) Hamiltonian in the IBM. We compared the calculation results with the experimental data of $$N=90$$ isotones. Finally, the potential of the Bohr Hamiltonian was represented by a harmonic oscillator; consequently, the structure of $$N=90$$ isotones could be expressed in the closed form by an approximate separation of variables. These nuclear structure prediction results were applied to elucidate the low-lying energy and E2 transitions in isotones.

Low-lying electric and magnetic dipole strengths in Pb207

Low-lying dipole transitions in $^{207}\mathrm{Pb}$ were measured via nuclear photon scattering using a quasimonochromatic, linearly polarized photon beam. The spins and/or parities of the states observed in $^{207}\mathrm{Pb}$ were determined from the intensity asymmetry of resonantly scattered $\ensuremath{\gamma}$ rays with respect to the polarization plane of the incident photon beam. The electric ($E1$) and magnetic ($M1$) dipole strengths were obtained for excitation energies from 5.49 to 6.75 MeV. The present experimental results, combined with $(\ensuremath{\gamma},n)$ data from the literature, were used to investigate the $E1$ and $M1$ photoabsorption cross sections near the neutron separation energy by comparison with predictions from the particle-vibration coupling with the quasiparticle random-phase approximation model.

Autoelectronic emission and charge relaxation of thorium ions implanted into a thin-film silicon oxide matrix

An ensemble of thorium-229 ions embedded by pulsed laser implantation into a matrix of a broadband dielectric-silicon oxide is studied. The results of the experimental investigation of thorium ions’ lifetime as charged components of the Th+/SiO2/Si system formed immediately after laser implantation are presented. The modelled theoretical description takes into account the instantaneous charging of the surface via laser implantation followed by charge relaxation due to the effect of autoelectronic emission that leads to time dependent partial neutralization of the surface charge by tunneling electrons. It was found that the lifetime of Th+ ions on the SiO2 surface can exceed 10 s, which would be an attractive opportunity for studying the nuclear low-lying isomeric transition in the thorium-229 isotope.

Thin Ta/Ta oxide core-shell nanoparticle film size-dependent energy structure

The paper presents the research of thin films, consisting of spherical monodisperse Ta particles. For different samples particle sizes varied from 1.3 nm to 5.8 nm. The films were also exposed to the atmosphere for oxidation, which resulted in formation of a core-shell particle structure, where core was metal and shell was Ta oxide (Ta2O5). Dependency of bandgap Egap and valence band offset (VBO) on particle size has been investigated for the structure by using the X-ray photoelectron spectroscopy (XPS) and reflected electron energy loss spectroscopy (REELS) methods. We have found, that Egap decreases from 4.72 eV to 4.47 eV whereas VBO decreases from 3.50 eV to 2.95 eV with increasing the particle size. A new method of a fine regulation of the electronic structure is suggested, particularly for the application in ultraviolet (UV) photodetector design for low-lying nuclear and electron transitions detection.

Local electrochemical deposition of thorium-229 targets and excitation of isomeric state by electron beam

The results of the study of local formation of thin-film coatings based on thorium oxide on SiO2/Si(111) surfaces by electrochemical deposition. It is shown that as a result of electrochemical deposition of thorium atoms from an acetone solution of Th(NO3)4 with the presence of water on the silicon surface, local formation of thorium compounds occurs. Chemical analysis of the composition of the samples obtained by XPS showed the presence of compounds based on thorium, silicon, oxygen and carbon. It is shown that annealing for 6 hours at 600? C in the atmosphere leads to the departure of carbon, and the formed film consists only of thorium and oxygen. It is shown that such a system can be promising for further studies of the nuclear low-lying isomer transition in the 229Th isotope under electron beam irradiation. The method of excitation of isomeric thorium-229 nuclei by irradiation of a solid-state target based on thorium silicate with an electron beam is considered. The process of generating secondary electrons with a significant increase in the multiplication coefficient for electrons with energies in the range from 1 keV to 25 keV is considered. The values of the output function of isomeric nuclei and the excitation cross section of an isomeric transition in inelastic scattering are obtained on the basis of numerical modeling and theoretical estimation.

Local Electrochemical Deposition of Nuclear Materials as a Method for Creating Miniature Solid-State Targets for Precision Measurements

The results of a study of the local formation of thorium oxide-based coatings on the SiO2/Si (111) surface during electrochemical deposition are presented. It was experimentally found that as a result of electrochemical deposition of thorium atoms from an acetone solution of Th(NO3)4 with the presence of water on the silicon surface, local formation of thorium compounds occurs. The results of the analysis of the chemical composition of the surface by the method of local XPS analysis show that these compounds are those based on thorium, silicon, oxygen, and carbon. After annealing for 6 h at 600°C in atmosphere, carbon completely disappears, and the formed cluster film seemingly consists only of thorium and oxygen. It is shown that this system is promising for future research on nuclear low-lying isomeric transition in the 229Th isotope under electron beam irradiation.

Estuarine tidal response to sea level rise: The significance of entrance restriction

Estuarine environments, as dynamic low-lying transition zones between rivers and the open sea, are vulnerable to sea level rise (SLR). To evaluate the potential impacts of SLR on estuarine responses, it is necessary to examine the altered tidal dynamics, including changes in tidal amplification, dampening, reflection (resonance), and deformation. Moving beyond commonly used static approaches, this study uses a large ensemble of idealised estuarine hydrodynamic models to analyse changes in tidal range, tidal prism, phase lag, tidal current velocity, and tidal asymmetry of restricted estuaries of varying size, entrance configuration and tidal forcing as well as three SLR scenarios. For the first time in estuarine SLR studies, data analysis and clustering approaches were employed to determine the key variables governing estuarine hydrodynamics under SLR. The results indicate that the hydrodynamics of restricted estuaries examined in this study are primarily governed by tidal forcing at the entrance and the estuarine length. In addition, SLR increases the average water depth and alters the nodal point location in a seaward direction, thereby significantly affecting tidal wave propagation patterns and reducing the degree of flood domination. In estuaries with restricted entrances, tidal range diminishes drastically in the restricted zone by 20–60%, and the maximum tidal current velocity is higher in the restricted part but lower in the upstream part compared to unrestricted estuaries. For estuary types examined, inundation extent under SLR will likely be greater than the extent expected through simply adjusting existing water levels upwards. The findings also underline that an engineered entrance restriction could regulate tidal wave propagation within an estuary to offset SLR induced tidal range amplification. However, this may pose additional management challenges.