Showing papers on "Quadrupole published in 1992"

Towards an accurate intermolecular potential for water

Abstract: A new ab initio potential for the water dimer is presented. It is obtained from monomer properties and perturbation theory calculations (IMPT) on the dimer. The long range electrostatic interaction is described by distributed multipoles: charge, dipole and quadrupole on oxygen and charge and dipole on hydrogen. The induction is modelled using polarizabilities on oxygen up to quadrupole. The short range repulsive energy is modelled by an exponential site-site function incorporating site anisotropy. Two models have been used for the dispersion energy: accurate dispersion coefficients for the water dimer with Tang-Toennies damping functions, and a site-site model. Neither is entirely satisfactory. The geometry of the global minimum on the potential surface is in very good agreement with the experimental result. The virial coefficient agrees moderately well with the experimental determination; the discrepancies suggest that the potential is not deep enough.

Multiple-shell structures of laser-cooled 24 Mg + ions in a quadrupole storage ring

Abstract: THE possibility of creating ordered ion beams in high-energy storage rings1,2 by means of electron and laser cooling has opened up a new era in accelerator physics The enhanced luminosity and suppressed momentum spread in such systems create the highest possible phase-space density The first experimental results were obtained by cooling 7Li+ beams to temperatures of a few kelvin or even to sub-kelvin temperatures3,4, and the ordered structures have been studied theoretically5–7 by methods of molecular dynamics Predicted configurations for the lowest ion densities have been observed in low-energy quadrupole storage rings8 and linear traps9 Recently we showed that at slightly higher ion densities helical structures are obtained10 Here we present a series of new experimental results on ordered ion structures in a quadrupole storage ring In order of increasing ion number, a linear chain of ions, a zig-zag structure, helical structures and finally multiple concentric shells could be observed The experimental results agree with molecular dynamics calculations

Threshold pion photoproduction on nucleons

Abstract: Pion photo- and electroproduction of the nucleon at threshold is reviewed in theory and experiment. In this region the leading S-wave amplitudes (E0+) are predicted by low-energy theorems (LET) based on current conservation and chiral symmetry. While these predictions are in agreement with the data for charged pion production, LET cannot explain the reaction p( gamma , pi 0)p over the energy range of the first 10 MeV above threshold. Present calculations involving final state interactions are difficult to reconcile with the requirements of LET and suffer from a strong model dependence due to off-shell formfactors. With the completion of the new electron accelerators with high duty factor and high intensity, new types of coincidence experiments with polarization degrees of freedom will be performed. The authors review the formalism for such reactions and present the multipole decomposition of the various response functions. The new experimental capabilities allow some of the most challenging problems of intermediate energy physics to be attacked, such as the puzzle of neutral pion production at threshold, the electric quadrupole amplitude in the region of the Delta resonance (bag deformation), the Coulomb monopole amplitude near the Roper resonance ('breathing mode'), the production of eta -mesons via the N*(1535) and the production of two pions and more to test consequences of chiral symmetry and the interaction of pions at low energies.

The Nuclear Quadrupole Moments of the 20 First Elements: High-Precision Calculations on Atoms and Small Molecules

Abstract: Abstract The available values of the nuclear quadrupole moments for the elements H - Ca are reviewed and related to recent advances in numerical methods in quantum chemistry. For Li the quantum chemical and nuclear Coulomb scattering values now agree. For Mg, also the muonic result agrees with them. For Na and Al, the new, accurate atomic values deviate from the earlier muonic values

Observation of ordered structures of laser-cooled ions in a quadrupole storage ring.

Abstract: We observed phase transitions and ordered structures of laser-cooled ${\mathrm{Mg}}^{+}$ ions stored in a radio-frequency quadrupole storage ring with a diameter of 115 mm. The ions are confined two dimensionally by a quadrupole field and can move freely along the ring circumference. By cooling the ions with tangentially intersecting laser beams, we observed a linear string of ions aligned along the center line of the quadrupole field. For higher ion densities the linear structure changes into a helical structure. The dynamics of the stored ions was studied.

Site-site Lennard-Jones potential parameters for N2, O2, H2, CO and CO2

Abstract: The classical second-virial coefficients B(T) for identical linear molecules have been calculated exactly by using a potential model that includes site-site Lennard-Jones 12-6 interactions with added dipole and quadrupole interactions. By fitting selected experimental values of B(T) and using up to three different plausible quadrupole moments of the molecules, we have determined site-site Lennard-Jones parameters for N2, O2, H2, CO and CO2. We have also considered, in addition to electrostatic interactions, a limited spherical harmonics expansion of the site-site potential. By using at short range only the isotropic part of this expansion, we have derived parameters that are close for N2, O2 and H2 to the exact site-site parameters. The validity of this approximation in the truncated site-site potential is more questionable for CO and CO2.

Multipole inlet system for ion traps

Abstract: A two dimensional RF multipole field formed by a set of parallel rods (44), e.g. quadrupole rods, services as an inlet for an ion trap (58). The field both accepts and stores ions for the trap, and rejects unwanted ions, while the trap (58) is performing an analysis. This greatly increases the duty cycle of the overall system, increasing the system sensitivity.

Experimental and Theoretical Investigation of the Pre-Peaks at the Ti K-Edge Absorption Spectra in TiO2

Abstract: The available calculated density of states of TiO2 does not predict the very first weak peak at the onset of the Ti K-edge absorption spectra indicating that this feature is not simply due to a one-electron (dipole) transition. In this short paper we present a joint experimental and theoretical study of all the three weak pre-peaks at the Ti K-edge of TiO2. Our interpretation takes into account both dipole and quadrupole transitions as well as the influence of the core hole effect.

Determination of the carbon-13 chemical shift and nitrogen-14 electric field gradient tensor orientations with respect to the molecular frame in a polypeptide

Abstract: With the determination of the nulcear spin interaction tensor orientation for the site of interest in a macromolecule it is now possible to achieve very high resolution structural and dynamic information from uniformly aligned samples of a macromolecule. Here the orientation of both the carbonyl 113 C chemical shift and 14 N electric field gradient tensors are determined with respect to the molecular frame for backbone sites in polypeptides from solid state NMR spectroscopy. 15 N and 14 N dipolar coupled 13 C spectra were analysed. In both cases at a 50 MHz 13 C frequency the presence of the dipolar coupling was plainly seen in powder pattern spectra, and for 14 N coupled spectra the effect of the quadrupole interaction was pronounced

Accurate variational calculations of energies of the 2 (2)S, 2 (2)P, and 3 (2)D states and the dipole, quadrupole, and dipole-quadrupole polarizabilities and hyperpolarizability of the lithium atom.

Abstract: The combined configuration-interaction\char21{}Hylleraas method has been used to calculate the energies for the ground 2 $^{2}$S and the excited 2${\mathrm{}}^{2}$P and 3 $^{2}$D states of the lithium atom. The energies obtained, -7.478 060 1 a.u. for the 2 $^{2}$S state, -7.410 155 4 a.u. and -7.335 523 1 a.u. for the 2 $^{2}$P and 3 $^{2}$D states, respectively, are the lowest nonrelativistic variational upper bounds found so far. Ionization potentials, lifetimes, dipole oscillator strengths and their sums, dynamic dipole, quadrupole and dipole-quadrupole polarizabilities, and the static second hyperpolarizability are also reported and compared with experimental and other theoretical results.

Ab initio calculation of the deuterium quadrupole coupling in liquid water

Abstract: The quadrupole coupling constant and asymmetry parameter for the deuteron in liquid heavy water was determined using purely theoretical methods. Molecular‐dynamics simulations with the ab initio potential‐energy surface of Lie and Clementi were used to generate snapshots of the liquid. The electric‐field gradient at the deuteron was then calculated for these configurations and averaged to obtain the liquid quadrupole coupling constant. At 300 K a quadrupole coupling constant of 256±5 kHz and an asymmetry parameter of 0.164±0.003 were obtained. The temperature dependence of the quadrupole coupling constant was investigated.

Electric dipole polarizabilities of atomic valence states

Abstract: The second-order perturbation method for single-state multireference wave functions generated in complete-active-space self-consistent-field (CASSCF) calculations has been employed for computing electric dipole polarizabilities of different valence-electronic states of the first- (C,N,O,F) and second-row (Si,P,S,Cl) atoms. The results of calculations with polarized basis functions provide a uniform set of accurate polarizability data for those systems. Also, atomic quadrupole moment values obtained by using the CASSCF method are presented.

NMR study of kaolinite. 1. Silicon-29, aluminum-27, and proton spectra

Abstract: We have measured 29 Si, 27 Al, and 1 H NMR spectra of kaolinites with natural and synthetic origins, using high-resolution solid-state techniques such as the magic angle spinning and the multipulse sequences. The 29 Si spectra demonstrate clearly that two inequivalent Si sites are present, and the origin of the line width has been discussed quantitatively. The line shape of the 27 Al spectra is governed by the second-order quadrupole interaction and suggests the presence of two inequivalent Al sites with different quadrupole coupling constants and asymmetry factors

Study of quadrupole-perturbed quartets in the solid-state magic-angle spinning phosphorus-31 NMR spectra of phosphine-copper(I) complexes. 63Cu electric field gradients and anisotropy in the 31P,63Cu scalar coupling

Abstract: High-resolution solid-state 31 P NMR spectra of phosphine-containing Cu(I) complexes show field-dependent, distorted quartets in which the line separations are not constant. This phenomenon is due to the combination of scalar J( 31 P, 63 Cu) coupling with incompletely averaged dipolar an anisotropic J interactions. A simple analysis is presented on literature data involving a series of compounds of the P n CuX m type (P=triphenylphosphine and derivatives, X=halide) having different Cu coordination numbers and core geometries

Nonperturbative study of the damping of giant resonances in hot nuclei.

Abstract: The damping of dipole and quadrupole motion in {sup 16}O and {sup 40}Ca at zero and finite temperature is studied including particle-particle and particle-hole interactions to all orders of perturbation. We find that the dipole dynamics in these light nuclei is well described in terms of mean-field theory (time-dependent Hartree-Fock), while the quadrupole motion is strongly damped through the coupling to more complicated configurations. Both the centroid and the damping width of the quadrupole and dipole giant resonances show a clear stability with temperature as a consequence of the weakening of the interaction, which contrasts with the increase of the phase space.

Computational exploration of the six-dimensional vibration-rotation-tunneling dynamics of (NH3)2

Abstract: In order to address the well‐known problem that the nearly cyclic structure of (NH3)2 deduced from microwave spectra differs greatly from the hydrogen‐bonded equilibrium structure obtained from ab initio calculations, we have calculated the vibration–rotation–tunneling (VRT) states of this complex, and explicitly studied the effects of vibrational averaging. The potential used is a spherical expansion of a site–site potential which was extracted from ab initio data. The six‐dimensional VRT wave functions for all the lowest states with J=0 and J=1 were expanded in products of radial (van der Waals stretch) functions and free‐rotor states for the internal and overall rotations, which were first adapted to the complete nuclear permutation inversion group G36. Although the (expanded) potential is too approximate to expect quantitative agreement with the observed microwave and far‐infrared spectra, we do find several interesting features: The 14N quadrupole splittings and the dipole moment of the complex, whic...

Ground-state properties of neutron-deficient platinum isotopes

Abstract: The hyperfine structure splitting and the isotope shift in the λ=266 nm transition of Pt isotopes within the mass range 183 ≦A≦ 198 have been determined by Resonance Ionization Mass Spectroscopy (RIMS) in combination with Pulsed-Laser Induced Desorption (PLID). The Pt isotopes were obtained at the on-line isotope separator ISOLDE-3/CERN as daugthers of the primarily produced Hg isotopes. Magnetic moments, quadrupole moments, and changes in the mean-square charge radii are deduced and compared with results of a particle-triaxial rotor model and mean field calculations. Good agreement with experimental data (including nuclear level schemes and transition probabilities) can only be obtained if triaxial shape is admitted. The calculations yield a smooth transition in the shape of odd-A Pt nuclei from a slightly deformed, nearly oblate195Pt via triaxial197-187Pt to a strongly deformed nearly prolate177Pt.

Multipole moments and Maxwell's equations

Abstract: Maxwell's equations provide the basis for explaining many electromagnetic phenomena, both in vacuum and in matter. While some of these effects may be described within the electric dipole approximation D= in 0E+P and H= mu -10 B, there are others which require the inclusion of higher multipole terms in the fields D and H. Various practices for doing this have been adopted in the past, such as the use of traceless quadrupole and higher moments, and the neglect of certain moments. Some of these practices are shown to lead to Maxwell equations which are not translationally invariant, in that they depend on the choice of origin to which the multiple moments are referred, even though they may also yield expressions for observables which do not possess this defect. It is also shown how to obtain general multipolar forms for D and H which ensure origin-independent Maxwell equations.

Electronic pseudocharge model for the Cu(111) longitudinal-surface-phonon anomaly observed by helium-atom scattering.

Abstract: The phonon-dispersion curves of the Cu(111) surface have been calculated in the framework of an electronic multipole model. The electronic pseudocharges are located at the midpoints between nearest-neighbor ions of the lattice and are expanded in terms of their dipole and quadrupole deformabilities. The anomalous longitudinal resonance observed by helium-atom scattering (HAS) is accounted for by increasing the dipolar and quadrupolar bulk deformabilites at the surface. Moreover the model explains in a straightforward way the apparent contradiction between force constants obtained from a fit of the HAS data and of similar dispersion curves measured by electron scattering. Thus the enhanced inelastic HAS intensity of the longitudinal resonance, which previously required extreme force-constant adjustments, is instead attributed to an additional contribution from the inelastic interaction of the He atoms with the dipolar and quadrupolar modulations of the surface electron density.

Finite element multiconfiguration Hartree-Fock calculations on carbon, oxygen, and neon: the nuclear quadrupole moments of carbon-11, oxygen-17, and neon-21

Abstract: The authors report new calculations for the nuclear quadrupole moments of {sup 11}C, {sup 17}O, and {sup 21}Ne, making use of a new finite-element multiconfiguration Hartree-Fock method. They calculate electric field gradients at the nuclei and combine these with measurements of nuclear quadrupole coupling constants to arrive at the values.

Structural, Mössbauer, and EPR investigations on two oxidation states of a five-coordinate, high-spin synthetic heme. Quantitative interpretation of zero-field parameters and large quadrupole splitting

Abstract: P2,/n. The five-coordinate iron atom is bonded to four porphyrinato nitrogens (Fe-N,) = 2.107 (14) A and to an oxygen atom of the acetate ion (FA,, = 2.034 (3) A), placed inside the molecular cavity of the picket-fence porphyrin. Mhsbauer spectra were recorded in the two oxidation states of the complex at temperatures varying from 1.5 to 200 K in fields of 0-6.21 T. The ferrous complex has a large quadrupole splitting, AEQ = 4.25 mm s-I, nearly independent of temperature. In the ferric species, the quadrupole splitting, AEQ = 1.1 mm s-', is as normally found in ferric high-spin iron porphyrins. The spin-Hamiltonian analysis of the spectra yields the zero-field parameters D = -0.9 cm-' and E/D = 0.33 and the magnetic hyperfine parameters A,, = -17 T and A, = -13.3 T in the ferrous complex (spin S = 2) and D = 7.5 cm-I, E/D = 0 and Ax,,= = -20 T in the ferric species (S = 5/2). The values of the zero-field parameters of the ferric species are confirmed by EPR analysis; the g values are g, = 1.960, g, = 2.017, and g, = 2.00. The zero-field splittings and effective g values in the ferric complex are interpreted in terms of a crystal-field model. Theoretical estimates of the quadrupole splitting and zero-field parameters in the ferrous complex are given on the basis of molecular-orbital calculations. The relation between the zero-field tensor (D) and electronic and X-ray structure in the ferrous species is discussed. Parallel to the discovery of the unusually large quadrupole splitting, PE - 4 mm s-', in the ferrous state of the prosthetic group term4 p460 of the multiheme enzyme hydroxylamine oxidoreductase from the bacterium Nitrosomonas europeae,'V2 comparably large splittings were observed in a number of synthetic five-coordinate porphyrins of the formula [Fe(X)(Porph)]-, where X is an anionic oxygen- or nitrogen-donor ligand or a halide ion.38 In order to gain more insight into the electronic structure of the iron ions in these complexes, field- and temperature-dependent MBssbauer measurements have been performed in the synthetic phenolato porphyrinato complex [ Fe"(OC6H,)(TPP)]- by Lang et a1.* For the derivation of a set of zero-field parameters consistent with both the susceptibility and MBssbauer data in the phenolate complex, these authors introduced weak antiferromagnetic exchange coupling between the paramagnetic iron centers.8 Strong correlations between the sign and magnitude of the zero-field parameter D and the value of the exchange-coupling constant were found in the simulations of the magnetic susceptibility data. However, the spin-Hamiltonian parameters inferred from the Mbsbauer spectra are rather insensitive to the weak intermolecular couplings. The value of the zero-field parameter D obtained by Lang et al. is small and negative (see Table I). The principal components of the magnetic hyperfine tensor (A) in the complex are negative, which is characteristic of a dominant Fermi contact interaction. Furthermore, the A tensor shows anisotropy, lAll < IA, 1, originating from spin-dipolar interaction. The anisotropy ob the A tensor and the positive sign of the main component of the electric-field-gradient (EFG) tensor indicate that the iron(I1) ion in the phenolate complex possesses a half-filled subshell of spin-parallel electrons plus an "excessn electron of opposite spin in an oblate orbital of d, ty~,~*,' The weak temperature dependence of the quadrupole sphttmg in the range 1-300 K shows that the ground orbital state is energetically well-separated from excited orbital states by energy gaps typically 1 order

Rotational spectra, structure, Kr‐83 nuclear quadrupole coupling constant, and the dipole moment of the Kr‐benzene dimer

Abstract: Rotational spectra were observed for the Kr‐82, Kr‐83, Kr‐84, and Kr‐86 isotopic species of the Kr‐benzene dimer with the Mark II Balle‐Flygare Fourier transform microwave spectrometer. The spectra are those of a symmetric top, with the Kr on the symmetry axis of the benzene. In the most abundant Kr‐84 benzene dimer the rotational constants B0, DJ, and DJK were found to be 795.6821(1) MHz, 1.315(1) kHz, and 7.895(4) kHz, respectively. R, the Kr to benzene center‐of‐mass distance, is 3.663 A and the zero‐point vibrational bending of the benzene with respect to R gives an average angular displacement of 16.6°. The Kr‐83 quadrupole coupling constant was determined experimentally to be −5.201(5) MHz and the dipole moment, 0.136(2) D. A theoretical prediction of these electrically induced properties gives results for a distributed multipole model which are superior to those using central multipoles.

Ramsey technique applied in a Penning trap mass spectrometer

Abstract: The Ramsey method has been applied in an experiment aiming for accurate mass determination of unstable isotopes. The ion motion in a Penning trap has been excited with time-separated oscillatory fields and Ramsey fringes were observed in the case of dipole and quadrupole excitation. The experimental resonances are in good agreement with theory. Further applications of the technique are discussed.

An electron spin-echo envelope modulation study of copper(II)-doped single crystals of L-histidine hydrochloride monohydrate

Abstract: Single-crystal electron spin-echo envelope modulation (ESEEM) spectroscopy was utilized to accurately measure the 14 N hyperfine and quadrupole coupling for the remote or noncoordinating nitrogen of histidine imidazole ligated to copper in doped L-histidine HCl.H 2 O. The anisotropic part of the 14 N hyperfine tensor is of the form (A, 0,-A). Nevertheless, the hyperfine tensor could be simply modeled by a multipoint representation of delocalized spin density both in the postulated copper unpaired orbital and on the imidazole moiety itself. Hence, the anisotropic part of the remote nitrogen hyperfine tensor may be a better probe of the ligand geometry and copper unpaired orbital than the isotropic hyperfine component. An analysis invoking the Townes-Dailey approximation was applied to the derived quadrupole tensor of the remote nitrogen of imidazole in Cu(II)-doped L-histidine HCl.H 2 O

Hydrogen bonded complexes of HCl with CO, C2H2, C2H4, PH3, H2S, HCN, H2O and NH3

Abstract: The results of ab initio quantum chemical calculations carried out using the MP2 perturbation and averaged coupled pair functional (ACPF) methods are reported on hydrogen bonded binary complexes of HCl with CO, C2H2, C2H4, PH3, H2S, HCN, H2O and NH3. The geometries, vibrational frequencies and related quantities such as zero point vibrational energies, librational amplitudes, infrared intensities, etc., have been computed at the MP2 level, while the ACPF method has been used to obtain electric field gradients, dipole moments and also dissociation energies. The theoretical predictions of geometries, intermolecular stretching frequencies, librational amplitudes and 35Cl nuclear quadrupole coupling constants are in good agreement with the corresponding experimental data. The effects of basis set superposition have been found to be significant in the case of the dissociation energies. Exploratory calculations on the NH3 … HCl complex, using atomic natural orbital (ANO) bases, demonstrate, however, that a sign...