Low-lying Transitions Research Articles

Theoretical modeling of dissociative addition of an H2 molecule to doped aluminum clusters FeAl12 and CoAl12

The potential energy surfaces (PES) of the reactions FeAl12 + Н2 → FeН2Al12 (1) and CoAl12 + Н2 → CoН2Al12 (2) of dissociative addition of an H2 molecule to Fe- and Co-doped aluminum clusters have been calculated by the density functional theory method. Local minima on the PES in the vicinity of low-lying isomers, intermediates, and transition states have been found, and their structural and spectroscopic characteristics and energies have been calculated. The energies of the successive stages of the catalytic cycle have been evaluated, and the channels corresponding to the minimum energy path of the reactions have been studied. Differences between the structural characteristics and energies of key structures in reactions (1) and (2) have been considered. The results are compared with previous calculations of the PES of hydrogenation reactions performed for related clusters doped with nickel and titanium atoms.

Simultaneous description of low-lying positive and negative parity states in spd, sdf and spdf interacting boson model

In order to investigate negative parity states, it is necessary to consider negative parity-bosons additionally to the usual [Formula: see text]- and [Formula: see text]-bosons. The dipole and octupole degrees of freedom are essential to describe the observed low-lying collective states with negative parity. An extended interacting boson model (IBM) that describes pairing interactions among s, p, d and f-boson based on affine [Formula: see text] Lie algebra in the quantum phase transition (QPT) field, such as spd-IBM, sdf-IBM and spdf-IBM, is composed based on algebraic structure. In this paper, a solvable extended transitional Hamiltonian based on affine [Formula: see text] Lie algebra is proposed to describe low-lying positive and negative parity states between the spherical and deformed gamma-unstable shape. Three model of new algebraic solution for even–even nuclei are introduced. Numerical extraction to low-lying energy levels and transition rates within the control parameters of this evaluated Hamiltonian are presented for various [Formula: see text] values. We reproduced the positive and negative parity states and our calculations suggest that the results of spdf-IBM are better than spd-IBM and sdf-IBM in this literature. By reproducing the experimental results, the method based on signature of the phase transition such as level crossing in the lowest excited states is used to provide a better description of Ru isotopes in this transitional region.

Double resonance rotational spectroscopy of CH2D+

Context. Deuterated forms of CH3 + are thought to be responsible for deuterium enrichment in lukewarm astronomical environments. There is no unambiguous detection of CH2 D+ in space to date.Aims. Four submillimetre rotational lines of CH2 D+ are documented in the literature. Our aim is to present a complete dataset of highly resolved rotational lines, including millimetre (mm) lines needed for a potential detection.Methods. We used a low-temperature ion trap and applied a novel IR–mm-wave double resonance method to measure the rotational lines of CH2 D+ . Results. We measured 21 low-lying (J ≤ 4) rotational transitions of CH2 D+ between 23 GHz and 1.1 THz with accuracies close to 2 ppb.

Thorium silicate compound as a solid-state target for production of isomeric thorium-229 nuclei by electron beam irradiation

In this paper, we discuss an idea of the experiment for excitation of the isomeric transition in thorium-229 nuclei by irradiating with electron beam targets with necessary physical characteristics. The chemical composition and bandgap of ThSi10O22 were determined by X-ray photoelectron spectroscopy and reflection electron energy loss spectroscopy. It was found that the energy gap is equal to 7.7 eV and does not change when the target is exposed to a medium energy electron beam for a long time. This indicates that the compound possesses high electron-beam resistance. A quantitative estimation of the output function of isomeric thorium-229 nuclei generated by interaction of nuclei with the secondary electron flow formed by irradiating the solid-state ThSi10O22-based target is given. The estimation shows that ThSi10O22 is a promising thorium-containing target for investigating excitation of the nuclear low-lying isomeric transition in the thorium-229 isotope using medium-energy electrons.

Investigation of quantum phase transitions in the spdf interacting boson model based on dual algebraic structures for the four-level pairing model

The building blocks of the interacting boson model (IBM) are associated with both s and d bosons for positive parity states. An extension of sd−IBM along these models to spdf−IBM can provide the appropriate framework to describe negative parity states. In this paper, a solvable extended transitional Hamiltonian based on the affine Lie algebra is proposed to describe low lying positive and negative parity states between the spherical and deformed gamma-unstable shape. Quantum phase transitions (QPTs) are investigated based on dual algebraic structures for the four-level pairing model. Numerical extraction to low-lying energy levels and transition rates within the control parameters of this evaluated Hamiltonian are presented for various N values. By reproducing the experimental results, the method based on the signatures of the phase transition, such as the expectation value of the boson number operators in the lowest excited states, are used to provide a better description of Ru isotopes in this transitional region.

The first potential energy surfaces for the C6H(-)-H2 and C6H(-)-He collisional systems and their corresponding inelastic cross sections.

Molecular anions have recently been detected in the interstellar and circumstellar media. Accurate modeling of their abundance requires calculations of collisional data with the most abundant species that are usually He atoms and H2 molecules. In this paper, we focus on the collisional excitation of the first observed molecular anion, C6H(-), by He and H2. Theoretical calculations of collisional cross sections rely generally on ab initio interaction potential energy surfaces (PESs). Hence, we present here the first PESs for the C6H(-)-H2 and C6H(-)-He van der Waals systems. The ab initio energy data for the surfaces were computed at the explicitly correlated coupled cluster with single, double, and scaled perturbative triple excitations level of theory. The method of interpolating moving least squares was used to construct 4D and 2D analytical PESs from these data. Both surfaces are characterized by deep wells and large anisotropies. Analytical models of the PESs were used in scattering calculations to obtain cross sections for low-lying rotational transitions. As could have been anticipated, important differences exist between the He and H2 cross sections. Conversely, no significant differences exist between the collisions of C6H(-) with the two species of H2 (para- and ortho-H2). We expect that these new data will help in accurately determining the abundance of the C6H(-) anions in space.

Particle-hole configuration interaction and many-body perturbation theory: Application toHg+

The combination of configuration interaction and many-body perturbation theory methods (CI+MBPT) is extended to non-perturbatively include configurations with electron holes below the designated Fermi level, allowing us to treat systems where holes play an important role. For example, the method can treat valence-hole systems like Ir$^{17+}$, particle-hole excitations in noble gases, and difficult transitions such as the $6s \rightarrow 5d^{-1}6s^2$ optical clock transition in Hg$^+$. We take the latter system as our test case for the method and obtain very good accuracy (~1%) for the low-lying transition energies. The $\alpha$-dependence of these transitions is calculated and used to reinterpret the existing best laboratory limits on the time-dependence of the fine-structure constant.

Thermally activated heavy states and anomalous optical properties in a multiband metal: The case ofSrMnSb2

We report an optical spectroscopic study of $\mathrm{SrMnS}{\mathrm{b}}_{2}$, a low-carrier-density metal. As temperature is decreased, our measurements reveal a large increase in the quasiparticle plasma frequency, which is highly unusual for a metal. This seemingly anomalous behavior can be accounted for using a ``three-band'' model of the multiband electronic structure of $\mathrm{SrMnS}{\mathrm{b}}_{2}$ that includes two conduction bands and one valence band. The second conduction band is assumed to be heavy and its minimum is taken to be close to, but not intersecting, the Fermi level. At finite temperature, quasiparticles are thermally redistributed between the two conduction bands, leading to an increase in the optical effective mass and a decrease in the plasma frequency. The temperature dependence of the low-lying interband optical transitions and the Hall number can also be understood using our model. The phenomenology of such a three-band scenario has not been widely considered to date in optical spectroscopic studies. Our results provide an explanation for the puzzling optical properties that have previously been reported in a number of topical low-carrier-density metals and semimetals and lay a foundation for future optical studies of these materials.

Terahertz conductivity ofSr1−xCaxRuO3

We investigate the optical conductivity of Sr$_{1-x}$Ca$_x$RuO$_3$ across the ferromagnetic to paramagnetic transition that occurs at $x=0.8$. The thin films were grown by metalorganic aerosol deposition with $0 \leq x \leq 1$ onto NdGaO$_3$ substrates. We performed THz frequency domain spectroscopy in a frequency range from 3~cm$^{-1}$ to 40~cm$^{-1}$ (100~GHz to 1.4~THz) and at temperatures ranging from 5~K to 300~K, measuring transmittivity and phase shift through the films. From this we obtained real and imaginary parts of the optical conductivity. The end-members, ferromagnetic SrRuO$_3$ and paramagnetic CaRuO$_3$, show a strongly frequency-dependent metallic response at temperatures below 20~K. Due to the high quality of these samples we can access pronounced intrinsic electronic contributions to the optical scattering rate, which at 1.4~THz exceeds the residual scattering rate by more than a factor of three. Deviations from a Drude response start at about 0.7~THz for both end-members in a remarkably similar way. For the intermediate members a higher residual scattering originating in the compositional disorder leads to a featureless optical response, instead. The relevance of low-lying interband transitions is addressed by a calculation of the optical conductivity within density functional theory in the local density approximation (LDA).

Optical absorption spectrum of the N3 solar cell sensitizer by second-order multireference perturbation theory

Here we report second-order n-electron valence state perturbation theory excited state calculations on the low-lying transitions of the [Ru(4,4′-COOH-2,2′-bpy)2(NCS)2] complex, better known as the N3 dye, both in vacuo and water solution. The present results provide unprecedented insights into the crucial role of the solvation effects, previously reported at density functional theory (DFT) and time-dependent DFT (TDDFT) level of theory, to adequately reproduce the optical absorption spectrum. We find that the NEVPT2 results in gas phase, even if slightly blueshifted, already provide a reliable description and assignation of the electronic structure and low-lying excited states of N3, whereas these are achieved in a DFT/TDDFT approach only by means of the solvent, which properly polarizes the ground-state frontier molecular orbitals. Then, inclusion of the solvent at the NEVPT2 level does not substantially modify the ground-state electronic structure as well as the excited states picture found in vacuo, solely yielding the expected spectral blueshift.

A microscopic approach based on particle-vibration coupling: application to charge-exchange transitions and multiplets in odd nuclei

In this contribution, we shall describe a formalism that goes beyond the simple time-dependent mean field and is based on particle-vibration coupling (PVC). Such a formalism has been developed with the idea of being self-consistent. It makes use of Skyrme effective forces, and has been used for several applications. We will focus on charge-exchange transitions, namely we will show that our model describes well both the Gamow-Teller giant resonance width, and the low-lying transitions associated with β -decay. In this latter case, including PVC produces a significant improvement of the half-lives obtained at mean-field level, and leads to a good agreement with experimental data. We will end by discussing particle-phonon multiplets in odd nuclei.

Transition probabilities in neutron-richSe84,86

Reduced quadrupole transition probabilities for low-lying transitions in neutron-rich Se84,86 are investigated with a recoil distance Doppler shift (RDDS) experiment. The experiment was performed at the Istituto Nazionale di Fisica Nucleare (INFN) Laboratori Nazionali di Legnaro using the Cologne Plunger device for the RDDS technique and the AGATA Demonstrator array for the γ-ray detection coupled to the PRISMA magnetic spectrometer for an event-by-event particle identification. In Se86 the level lifetime of the yrast 21+ state and an upper limit for the lifetime of the 41+ state are determined for the first time. The results of Se86 are in agreement with previously reported predictions of large-scale shell-model calculations using Ni78-I and Ni78-II effective interactions. In addition, intrinsic shape parameters of lowest yrast states in Se86 are calculated. In semimagic Se84 level lifetimes of the yrast 41+ and 61+ states are determined for the first time. Large-scale shell-model calculations using effective interactions Ni78-II, JUN45, jj4b, and jj4pna are performed. The calculations describe B(E2;21+→01+) and B(E2;61+→41+) fairly well and point out problems in reproducing the experimental B(E2;41+→21+).2 MoreReceived 23 November 2015DOI:https://doi.org/10.1103/PhysRevC.92.064322©2015 American Physical Society

Reanalysis of the Aqueous Spectrum of the Neptunyl(V) [NpO2(+)] Ion.

The actinyl ions, consisting of the dioxoactinde(VI) and dioxoactinide(V) ions, are unique in the periodic table. However, very few quantitative studies of the spectra of these ions have been performed. In this paper the analysis of the optical spectrum of the aqueous neptunyl(V) [NpO2(+)] ion in 1 M HClO4 is reexamined. The species in solution is assumed to be the linear NpO2(+) ion surrounded in the equatorial plane by five oxygen atoms from five H2O molecules. The neptunyl(V) ion has the 5f(2) open-shell configuration, and the low-lying optical transitions (up to ∼20 000 cm(-1)) observed are primarily from the two 5f electrons occupying the 5fφ and 5fδ orbitals. The conventional parametric theory used for f(n) systems is applied to these low-lying transitions utilizing the intensity calculations that Matiska et al.1 have performed for this ion and the data reported by Eisenstein and Pryce.2 Possible ratios for the Slater electrostatic repulsion parameters are obtained from the data for the isoelectronic ion U(4+) (5f(2)) in various host crystals. The results are consistent with earlier crystal field analyses of the 5f(1) neptunyl(VI) [NpO2(2+)] ion.

A fresh look at the photoelectron spectrum of bromobenzene: A third-order non-Dyson electron propagator study.

The valence-shell ionization spectrum of bromobenzene, as a representative halogen substituted aromatic, was studied using the non-Dyson third-order algebraic-diagrammatic construction [nD-ADC(3)] approximation for the electron propagator. This method, also referred to as IP-ADC(3), was implemented as a part of the Q-Chem program and enables large-scale calculations of the ionization spectra, where the computational effort scales as n(5) with respect to the number of molecular orbitals n. The IP-ADC(3) scheme is ideally suited for investigating low-lying ionization transitions, so fresh insight could be gained into the cationic state manifold of bromobenzene. In particular, the present IP-ADC(3) calculations with the cc-pVTZ basis reveal a whole class of low-lying low-intensity two-hole-one-particle (2h-1p) doublet and quartet states, which are relevant to various photoionization processes. The good qualitative agreement between the theoretical spectral profile for the valence-shell ionization transitions generated with the smaller cc-pVDZ basis set and the experimental photoelectron spectrum measured at a photon energy of 80 eV on the PLÉIADES beamline at the Soleil synchrotron radiation source allowed all the main features to be assigned. Some theoretical aspects of the ionization energy calculations concerning the use of various approximation schemes and basis sets are discussed.

Theoretical modeling of dissociative addition of an H2 molecule to Ni-doped aluminum and magnesium–aluminum clusters NiAl12 and NiMg5Al7

The potential energy surfaces (PES) of the reactions NiAl12 + H2 → NiH2Al12 (1) and NiMg5Al7 + H2 → NiMg5H2Al7 (2) of dissociative addition of an H2 molecule to nickel-doped aluminum and magnesium–aluminum clusters have been calculated by the density functional theory method. Local minima on the PES in the vicinity of low-lying isomers, intermediates, and transition states have been found, and their structural and spectroscopic characteristics and energies have been calculated. The energies of the early stages of the catalytic cycle involving “adsorption” of an H2 molecule onto the dopant atom, its dissociation into atoms, and surface migration of the H atoms with “vacating” of the dopant Ni atom and its preparation to adding a next H2 molecule have been evaluated. The channels corresponding to the minimum energy path of the reactions have been studied. The calculated energies of reactions are ~13 (1) and ~15 kcal/mol (2). The activation barrier for the first reaction is imperceptible (~1 kcal/mol), and it increases to ~4 kcal/mol for the second reaction. Differences between the structural characteristics and energies of key structures forming in reactions (1) and (2) have been considered. The reactions of magnesium–aluminum clusters are more exothermic than those of aluminum clusters, and the activation barrier of reaction (2) is higher than that of reactions (1), so that reaction (2) should be less reversible and slower than reaction (1). The results are compared with previous calculations of the PES of hydrogenation reactions performed for related clusters doped with titanium atom.

Quantum Monte Carlo methods for nuclear physics

Quantum Monte Carlo methods have proved very valuable to study the structure and reactions of light nuclei and nucleonic matter starting from realistic nuclear interactions and currents. These ab-initio calculations reproduce many low-lying states, moments and transitions in light nuclei, and simultaneously predict many properties of light nuclei and neutron matter over a rather wide range of energy and momenta. We review the nuclear interactions and currents, and describe the continuum Quantum Monte Carlo methods used in nuclear physics. These methods are similar to those used in condensed matter and electronic structure but naturally include spin-isospin, tensor, spin-orbit, and three-body interactions. We present a variety of results including the low-lying spectra of light nuclei, nuclear form factors, and transition matrix elements. We also describe low-energy scattering techniques, studies of the electroweak response of nuclei relevant in electron and neutrino scattering, and the properties of dense nucleonic matter as found in neutron stars. A coherent picture of nuclear structure and dynamics emerges based upon rather simple but realistic interactions and currents.

Reduced transition strengths of low-lying yrast states in chromium isotopes in the vicinity ofN=40

Background: In neutron-rich nuclei around $N=40$ rapid changes in nuclear structure can be observed. While $^{68}\mathrm{Ni}$ exhibits signatures of a doubly magic nucleus, experimental data along the isotopic chains in even more exotic Fe and Cr isotopes---such as excitation energies and transition strengths---suggest a sudden rise in collectivity toward $N=40$.Purpose: Reduced quadrupole transition strengths for low-lying transitions in neutron-rich $^{58,60,62}\mathrm{Cr}$ are investigated. This gives quantitative new insights into the evolution of quadrupole collectivity in the neutron-rich region close to $N=40$.Method: The recoil distance Doppler-shift (RDDS) technique was applied to measure lifetimes of low-lying states in $^{58,60,62}\mathrm{Cr}$. The experiment was carried out at the National Superconducting Cyclotron Laboratory (NSCL) with the SeGA array in a plunger configuration coupled to the S800 magnetic spectrograph. The states of interest were populated by means of one-proton knockout reactions.Results: Data reveal a rapid increase in quadrupole collectivity for $^{58,60,62}\mathrm{Cr}$ toward $N=40$ and point to stronger quadrupole deformations compared to neighboring Fe isotopes. The experimental $B(E2$) values are reproduced well with state-of-the-art shell-model calculations using the LNPS effective interaction. A consideration of intrinsic quadrupole moments and ${\mathrm{B}}_{42}$ ratios suggest an evolution toward a rotational nature of the collective structures in $^{60,62}\mathrm{Cr}$. Compared to $^{58}\mathrm{Cr}$, experimental ${\mathrm{B}}_{42}$ and ${\mathrm{B}}_{62}$ values for $^{60}\mathrm{Cr}$ are in better agreement with the $E(5)$ limit.Conclusion: Our results indicate that collective excitations in neutron-rich Cr isotopes saturate at $N=38$, which is in agreement with theoretical predictions. More detailed experimental data of excited structures and interband transitions are needed for a comprehensive understanding of quadrupole collectivity close to $N=40$. This calls for additional measurements in neutron-rich Cr and neighboring Ti and Fe nuclei.

Gamow-Teller transitions toCa45via theSc45(t,He3+γ)reaction at115 MeV/uand its application to stellar electron-capture rates

Background: Stellar electron-capture reactions on medium-heavy nuclei are important for many astrophysical phenomena, including core-collapse and thermonuclear supernov\ae{} and neutron stars. Estimates of electron-capture rates rely on accurate estimates of Gamow-Teller strength distributions, which can be extracted from charge-exchange reactions at intermediate beam energies. Measured Gamow-Teller transition strength distributions for stable pf-shell nuclei are reasonably well reproduced by theoretical calculations in the shell model, except for lower mass nuclei where admixtures from the sd shell can become important.Purpose: This paper presents a ${\ensuremath{\beta}}^{+}$ charge-exchange experiment on $^{45}\mathrm{Sc}$, one of the lightest pf-shell nuclei. The focus was on Gamow-Teller transitions to final states at low excitation energies, which are particularly important for accurate estimations of electron-capture rates at relatively low stellar densities. The experimental results are compared with various theoretical models.Method: The double-differential cross section for the $^{45}\mathrm{Sc}(t,^{3}\mathrm{He}+\ensuremath{\gamma})$ reaction was measured using the NSCL Coupled-Cyclotron Facility at $115\phantom{\rule{4pt}{0ex}}\mathrm{MeV}/u$. Gamow-Teller contributions to the excitation-energy spectra were extracted by means of a multipole-decomposition analysis. $\ensuremath{\gamma}$ rays emitted due to the deexcitation of $^{45}\mathrm{Ca}$ were measured using GRETINA to allow for the extraction of Gamow-Teller strengths from very weak transitions at low excitation energies.Results: Gamow-Teller transition strengths to $^{45}\mathrm{Ca}$ were extracted up to an excitation energy of 20 MeV, and that to the first excited state in $^{45}\mathrm{Ca}$ at 174 keV was extracted from the $\ensuremath{\gamma}$-ray measurement, which, even though weak, is important for the astrophysical applications and dominates under certain stellar conditions. Shell-model calculations performed in the pf shell-model space with the GXPF1A, KB3G, and FPD6 interactions did not reproduce the experimental Gamow-Teller strength distribution, and a calculation using the quasiparticle random phase approximation that is often used in astrophysical simulations also could not reproduce the experimental strength distribution.Conclusions: Theoretical models aimed at describing Gamow-Teller transition strengths from nuclei in the lower pf shell for the purpose of estimating electron-capture rates for astrophysical simulations require further development. The likely cause for the relatively poor performance of the shell-model theory is the influence of intruder configurations from the sd shell. The combination of charge-exchange experiments at intermediate beam energy and high-resolution $\ensuremath{\gamma}$-ray detection provides a powerful technique to identify weak transitions to low-lying final states that are nearly impossible to identify without the coincidences. Identification of these weak low-lying transitions is important for providing accurate electron-capture rates for astrophysical simulations.

ChemInform Abstract: Quadruple‐Decker Phthalocyanines — One Additional Layer for a Molecule, One Giant Leap for Phthalocyanine Chemistry

AbstractReview: 58 refs.

Local Electrochemical Deposition of Thorium on SiO2/Si(111) Surface

The paper presents the results of the study of local formation of thorium oxide coatings on SiO2/Si(111) surface by electrochemical deposition. It was found that the electrochemical deposition of thorium atoms from thorium nitrate solution of Th(NO3)4 with the presence of water on silicon surface results in local formation of thorium compounds. The results of surface analysis by local XPS indicate that these compounds represent a thorium-, silicon-, oxygen- and carbon-based compounds. After 6hours annealing at 600°C at atmosphere carbon pulled completely, and the cluster formed film consists of oxygen and thorium only. It was shown that this system can be promising for further research of nuclear low-lying isomeric transition in 229Th isotope when irradiated by an electron beam.