Many-particle excited states in nuclei

Energies, quadrupole moments and shapes of many-particle excited states in ¹²C and ¹⁶O are calculated. The connection of the obtained results with the cluster model is discussed.     

Nuclear quadrupole interaction and chemical bonding. MSXα cluster calculation for Cu2O

The influence of chemical bonding on the ¹⁶Cu and ⁶³Cu quadrupole coupling constants in Cu₂O was studied by means of a cluster calculation based on the multiple scattering X_α, method One-electron wave functions are given in a form which resembles that of usual basis set methods An analysis of the atomic orbital decomposition shows good correspondence between cluster and band structure calculations. For a comparison with X-ray diffraction data, the wave functions were used to compute atomic form factors, structure factors and charge density maps. Satisfactory agreement with experimental data was found for the quadrupole coupling constant and the theoretical structure factors The results indicate an unusual type of bonding in Cu₂O with the absence of strong covalent bonds on the one hand and significant deviations from a simple ionic picture on the other.     

Investigation of electronic structure and associated nuclear quadrupole interaction of adsorbed fluorine atoms on silicon surface

The electronic energy levels and wave functions of fluorine and chlorine atoms adsorbed at the <111> surface of silicon are obtained by the Hartree-Fock procedure applied to clusters simulating the surface structure. Minimising the total energy with respect to the positions of the halogen atoms, the Si-Cl distance in the adsorbed system was found to be in good agreement with that from a recent SEXAFS measurement, there being no corresponding data presently available for fluorine on silicon surface. The calculated¹⁹ f ^* nuclear quadrupole coupling constants using the electronic wave functions for SiH₃F and the larger cluster Si₄H₉F, simulating the surface, differed substantially in contrast to the situation for the Si-F distance, indicating that the quadrupole coupling is a more sensitive indicator of the adequacy of the cluster chosen to represent the surface system. Experimental results for the coupling constants are awaited for comparison with theory.     

Semi-microscopic description of133Xe by GVISR and diagonalization

The properties of low-lying states in¹³³Xe are described by coupling a three-neutronhole cluster to the quadrupole vibrational field. The results obtained by diagonalization of the model Hamiltonian and qualitative predictions based on Generalized Vibrational Intensity and Selection Rules (GVISR) are discussed. The results obtained in the present paper are compatible with recent experimental data on¹³³Xe. Further experimental tests of the model are suggested.     

67Zn Mössbauer study of lattice-dynamical effects and hyperfine interactions in ZnF2

Using the high-resolution 93.3-keV transition in⁶⁷Zn we studied the temperature dependence of the Lamb-Mössbauer factor (LMF), the center shift, and the quadrupole interaction in polycrystalline ZnF₂ in the temperature range between 4.2 and 55.3 K. The LMF is anisotropic within the whole temperature range. The electric quadrupole tensor characterized byeV _zz Q/h=+(8.2±0.1) MHz and η=(0.15±0.02) is independent of temperature. The results on the isomer shift (relative to ZnO) and on the electric quadrupole tensor are in very good agreement with recent theoretical cluster calculations.     

Cluster and symplectic excitations in nuclei

To incorporate two successful microscopic theories, the cluster and the symplectic models, into a single theory we have developed a recursion formula method of calculating overlaps between cluster and symplectic states. This has facilitated the calculation of cluster widths and isoscalar E2 transition rates in a basis including both types of state. A method of calculating interaction matrix elements is briefly discussed. We have calculated the overlaps between the cluster and the symplectic states for the ¹⁶O, ²⁰Ne, and ²⁴Mg systems. Comparison of the E2 transition rates between the cluster and the symplectic states has clearly indicated the complementary role of these excitations. The feasibility of unifying the cluster and the symplectic excitations is expected to be very useful for a number of nuclear structure problems, particularly those associated with both cluster correlation and quadrupole collectivity.     

On the nature of the isomeric 6+ state in the heavy doubly-even Te isotopes

The isomeric 6⁺ state in the heavy doubly-even Te isotopes has been studied in the two-proton cluster core coupling model. The lifetime is calculated and compared to the experimental data. Reduced quadrupole transition probabilities for transitions between the first excited 2⁺ state and the ground state are discussed.     

Cluster calculation of electronic structure and hyperfine interactions for α-Fe2O3

The MSX_α cluster technique has been used to study the electronic structure of hematite α-Fe₂O₃, where iron is formally in a 3d₅⁶S state. The calculated energy levels are compared with X-ray emission and photoelectron spectra. The wave functions have been used to compute the charge distribution, as well as hyperfine parameters such as quadrupole coupling constant, isomer shift and magnetic hyperfine field. The results indicate a considerable influence of chemical bonding on these parameters due to charge transfer and covalency. From the calculated field gradient and the measured quadrupole coupling constant a nuclear quadrupole moment for ^57mTe of about 0.11b is deduced. This value is smaller than the most recent estimates of 0.15b based on the ionic model but not as small as the value of 0.082b obtained from first principles calculations on iron dihalides.     

Inelastic electron scattering from 6Li near the α-d threshold

The form factor for the alpha-deuteron continuum in ⁶Li has been measured in the excitation region within 2 MeV of threshold, for momentum transfers q < 1.2 fm^?1. The results are interpreted in terms of an α-d cluster model. The monopole breakup appears to be predominant near threshold, but the influence of the J = 2⁺ (4.31 MeV) quadrupole resonance becomes important a few MeV above threshold. The form factor for the C2 transition to the 3⁺ (2.185 MeV) state is also presented.     

The (τ,d), (d,p) and (d,n) reactions on28Si

The model of a three-particle proton cluster coupled to the quadrupole vibrational field is applied to¹²⁹I. The energy spectrum and spectroscopic factors, as obtained in a (³He,d) stripping reaction, are calculated and compared with experimental data. With the wave functions obtained magnetic-dipole and electric-quadrupole moments are derived.     

Stability of Cf isotopes against alpha and cluster radioactivity

Stability of ^248–254Cf nuclei against alpha and cluster emissions is studied within our Coulomb and proximity potential model (CPPM). It is found that these nuclei are stable against light clusters (except alpha particles) and unstable against heavy cluster (A₂≥40)$(A_2\ge 40)$ emissions. For heavy cluster emissions the daughter nuclei lead to doubly magic ²⁰⁸Pb or the neighbouring one. The effects of quadrupole and hexadecapole deformations of parent nuclei, daughter nuclei and emitted cluster on half lives are also studied. The computed alpha decay half life values (including quadrupole deformation β₂${\beta }_2$) are in close agreement with experimental data. Inclusion of quadrupole deformation reduces the height and width of the barrier (increases the barrier penetrability) and hence half life decreases.     

High-intensity Si cluster ion emission from a silicon target bombarded with large Ar cluster ions

Secondary ions emitted from Si targets were measured with a quadrupole mass spectrometer under large Ar cluster and monomer ion bombardment. Incident ion beams with energies from 7.5 to 25 keV were used and the mean size of the Ar cluster ion was about 1000 atoms/cluster. Si_n⁺ ions with n values up to n = 8 were detected under Ar cluster ion bombardment, whereas Si cluster ions were scarcely detected under Ar monomer ion bombardment. These cluster ion yields showed the power law dependence on the cluster size.     

Facile formation of bare silver clusters in electrospray ionization multi-stage mass spectrometry

We report the observation of bare silver cluster cations obtained by electrospray ionization from a solution containing a metal salt and tryptophan. Multiple step collision-induced dissociation in a quadrupole ion trap of Ag₉⁺ is reported.     

Structure of87Y

The structure of⁸⁷Y nucleus is described within the framework of the extended clusterphonon model taking into account the coupling of single-proton and two-proton-oneproton-hole configurations with quadrupole and octupole vibrations of the underlying⁸⁶Sr core. The energy levels and electromagnetic transitions are calculated and compared with available experimental information. Attempts to consider the⁸⁶Sr as the vibrating core were also made. In this case the cluster was extended to include two-neutron-hole states too. No results are shown since the calculations failed to explain the experimental data.     

Ab initio dipole polarizabilities and quadrupole moments of the lowest excited states of atomic Yb

The finite-field scalar relativistic coupled cluster and configuration interaction, as well as the spin-orbit configuration interaction, methods are employed to calculate the static dipole polarizabilities for the lowest (6s ²) ¹S, (6s6p) ^1,3^{1,3}P^o and (6s5d) ^1,3^{1,3}D excited states of atomic ytterbium. The results agree very well with existing experimental and best theoretical data and refine the set of ytterbium polarizabilities known so far. Similar techniques are used to calculate the electric quadrupole moments for the ^1,3^{1,3}P^o and ^1,3^{1,3}D states.     

Theoretical investigations of the EPR parameters and local structure for Cu2+ in BeO

The electron paramagnetic resonance (EPR) parameters (the anisotropic g factors, the hyperfine structure parameters and the quadrupole coupling constant Q) and local structure for Cu²⁺ in BeO are theoretically investigated from the perturbation formulas of these parameters for a 3d⁹ ion under trigonally distorted tetrahedra. The ligand orbital and spin-orbit coupling contributions are included in the basis of the cluster approach, in view of the strong covalency of the [CuO₄]^6? cluster. From the calculations, the impurity Cu²⁺ is suggested not to occupy exactly the ideal Be²⁺ site but to suffer a slight inward displacement (≈0.024 Å) toward the ligand triangle along the C₃ axis. The theoretical EPR parameters show good agreement with the experimental data.     

Self-consistent field cluster study of the origin of hyperfine interaction and isomer shift in57mFe2O3)

We have carried out a first-principle Unrestricted Hartree-Fock (UHF) investigation of the hyperfine field, isomer shift and nuclear quadrupole interaction in Fe₂O₃. The cluster used in our work involved Fe⁺³ and its six nearest O^–2 neighbours. The Madelung contribution to the potential due to the rest of the ions in the lattice was explicitly incorporated. Our results will be compared with experiment and the results of a recent investigation using an approximation (MSXExtended Scheme) to the procedure.     

A new cluster model interpretation of the anomalous lifetime of <Superscript>14</Superscript>C

A new cluster model solution to the long-standing nuclear structure problem of describing the anomalously long lifetime of ¹⁴C is presented. Related beta-decay data for ¹⁴O to states in ¹⁴N, gamma-decay data between low-lying positive parity states in ¹⁴N and the elastic and inelastic magnetic dipole electron scattering from ¹⁴N data are all shown to be very accurately described by the model. The shapes of the beta spectra for the A = 14 system are also well reproduced by the model. The model invokes four-nucleon tetrahedral symmetric spatial correlations arising from three- and four-nucleon interactions, which yields a high degree of SU(4) singlet structure for the clusters and a tetrahedral intrinsic shape for the doubly magic ¹⁶O ground state. The large quadrupole moment of the ¹⁴N ground state is obtained here for the first time and arises because of the almost 100% d-wave deuteron-like-hole cluster structure inherent in the model.     

Cluster Folding and Coupled-Channels Analysis of 16O + 16O Elastic and Inelastic Scattering

The data of elastic and inelastic ¹⁶O?+?¹⁶O scattering to the lowest 0⁺, 2⁺, and 3^? excitation channels of ¹⁶O in the energy range of 250–1,120 MeV is reanalyzed using the derived double folding cluster (DFC1) potential in the framework of the coupled-channels (CC) mechanism. The potential provides a very good agreement with the experimental data for the elastic and inelastic scattering angular distribution. Furthermore, the deduced deformation length and quadrupole and octupole deformation parameter are quite consistent with the corresponding electromagnetic measurements.