Structures and energetics of carbon bridged C 60 clusters

The structures and energetics of carbon bridged C₆₀ clusters (C ₆₀ ) _n C_m have been studied by simulated annealing technique within the tight-binding molecular-dynamics. The “sp² addition” ball-and-chain dimers exhibit odd-even alternations over the number of chain atoms, with the dimers containing even chain atoms more stable against dissociation than their immediate neighbors containing odd chain atoms. In addition to the usual “sp² addition” dimers, a pentagon-linked C₁₂₁ isomer and a hexagon-linked C₁₂₂ isomer are also found to be stable. Based on our tight-binding calculations, trimers and larger clusters can be simply regarded as being made up of independent or weakly interacting dimers, if the C-C₆₀ joints on a single cage are not too close to each other. Large C₆₀ clusters connected by chains each containing only one or two carbon atoms have similar stability to that of constituent dimers, indicating the possibility to form stable C₆₀-carbon polymers. Received 17 January 2001 and Received in final form 26 February 2001     

SiC bonding in ( C 60 ) n Si m clusters

This paper deals with a new type of SiC bonding where silicon atom seems to bridge C₆₀ molecules. We have studied films obtained by deposition of (C₆₀)_nSi_m clusters prepared in a laser vaporization source. Prior deposition, free ionized clusters were studied in a time-of-flight mass spectrometer. Mixed clusters (C₆₀)_nSi_m were clearly observed. Abundance and photofragmentation mass spectroscopies revealed the relatively high stability of the (C₆₀)_nSi _n ⁺ , (C₆₀)_nSi _{n - 1} ⁺ and (C₆₀)_nSi _{n - 2} ⁺ species. This observation is in favor of the arrangement of these complexes as polymers where the C₆₀ cages may be bridged by a silicon atom. Free neutral clusters are then deposited onto substrate making up a nanogranular thin film (≃ 100 nm). The film is probed by Auger and X-ray photoemission spectroscopies, but above all by surface enhanced Raman scattering. The results suggest an unusual chemical bonding between silicon and carbon and the environment of the silicon atom is expected to be totally different from the sp³ lattice: ten or twelve carbon neighbors might surround silicon atom. The bonding is discussed to the light of the so-called fullerene polymerization as observed for pure fullerite upon laser irradiation. This opens a new route for bridging C₆₀ molecules together with an appreciable energy bonding, since the usual van der Waals bonding in fullerite could be replaced by an ionocovalent bond. Such an assumption must be checked in the future by XAS and EXAFS experiments. Received 15 November 2000     

From direct to absolute mass measurements: A study of the accuracy of ISOLTRAP

For a detailed study of the accuracy of the Penning trap mass spectrometer ISOLTRAP all expected sources of uncertainty were investigated with respect to their contributions to the uncertainty of the final result. In the course of these investigations, cross-reference measurements with singly charged carbon clusters ¹²C⁺ _n were carried out. The carbon cluster ions were produced by use of laser-induced desorption, fragmentation, and ionization of C₆₀ fullerenes and injected into and stored in the Penning trap system. The comparison of the cyclotron frequencies of different carbon clusters has provided detailed insight into the residual systematic uncertainty of ISOLTRAP and yielded a value of 8×10^-9. This also represents the current limit of mass accuracy of the apparatus. Since the unified atomic mass unit is defined as 1/12 of the mass of the ¹²C atom, it will be possible to carry out absolute mass measurements with ISOLTRAP in the future. Received 7 June 2002 Published online 6 November 2002 RID="a" ID="a"e-mail: a.kellerbauer@cern.ch RID="b" ID="b"Current address: Centre de Physique des Particules de Marseille, 13288 Marseille Cedex 9, France.     

Stern Gerlach interferometry with metastable argon atoms: an immaterial mask modulating the profile of a supersonic beam

A new Stern Gerlach interferometer operating with a nozzle beam of metastable argon atoms Ar^* (3p⁵ 4s, ³ P ₂) is described. The selection of incoming (polarisation) and outgoing (analysis) Zeeman sublevels is achieved by use of laser induced transitions at two wavelengths, 811.5 nm (closed J = 2 → J = 3 transition) and 801.5 nm (open J = 2 → J = 2 transition). Linear superpositions of Zeeman sublevels, just beyond the polariser and just before the analyser, are prepared by means of two zones where Majorana transitions take place. In between, a controlled magnetic field configuration (the phase object) is produced within a triple μ-metal shielding. Standard interference patterns are obtained by scanning the field and detecting the atoms by secondary electron emission from a Faraday cup. When a static radial magnetic gradient is used, the beam profile is modulated by interference. The transverse pattern, which can be translated at will by adding a homogeneous field, is observed for the first time using a multi-channel electron multiplier followed by a phosphor screen and a CCD camera. The results satisfactorily agree with all theoretical predictions. Received 27 June 2002 / Received in final form 20 September 2002 Published online 4 February 2003 RID="a" ID="a"e-mail: perales@lpl.univ-paris13.fr RID="b" ID="b"UMR 7538 du CNRS     

Radiative cooling of fullerene anions in a storage ring

Thermionic emission from hot fullerene anions, C_N ^-, has been measured in an electrostatic storage ring for even N values from 36 to 96. The decay is quenched by radiative cooling and hence the observations give information on the intensity of thermal radiation from fullerenes. The experiments are analysed by comparison with a simulation which includes the quantisation of photon energy and the statistics of emission. Experiments with heating of the molecules with a laser beam confirm the interpretation of the observations in terms of radiative cooling and give an independent estimate of the cooling rate for C₆₀ ^-. The measured cooling rates agree in general within a factor of two with the prediction from a classical dielectric model of a thermal radiation intensity of ∼ 300 eV/s for C₆₀ at 1 400 K, scaling approximately with the 6th power of the temperature and with the number of atoms in the molecule. Received 12 March 2001 and Received in final form 12 June 2001     

The measurement of the electronic g-factor in hydrogen-like ions --A promising tool for determining fundamental and nuclear constants

We describe a double-Penning-trap experiment suitable for testing QED in strong fields by determining the electronic g-factor of a single hydrogen-like ion in its ground state. Our measurements on ¹²C^{5 +} reach a relative accuracy of 2×10^-9, where the largest uncertainty results from the mass of the electron. Together with equally precise theoretical predictions therefore, it is possible to evaluate a new value for the electron's mass. The possibilities to obtain other fundamental constants and nuclear parameters are lined out. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"e-mail: t.beier@gsi.de     

Electron-phonon interaction in carbon schwarzites

The recent synthesis of random schwarzites has stimulated the present ab initio calculation of the electronic structure and electron-phonon interaction in two different periodic D-type schwarzites, fcc-(C₂₈)₂ (made of 24 seven-membered rings per unit cell) and fcc-(C₆₄)₂ (made of 12 eight membered and 48 six-membered rings per unit cell). Like in fullerenes, also in schwarzites the electron-phonon interaction potential is found to increase with the absolute Gauss curvature, though it remains smaller than for doped fullerenes. Received 19 December 2002 Published online 1st April 2003 RID="a" ID="a"e-mail: marco.bernasconi@unimib.it     

A novel electron scattering apparatus combining a laser photoelectron source and a triply differentially pumped supersonic beam target: characterization and results for the resonance

A novel electron scattering apparatus for high resolution studies of angle-differential elastic and inelastic electron scattering from atoms and molecules in the gas phase is described and its performance characterized. It combines a laser photoelectron source, a triply differentially pumped collimated supersonic beam target (half angle 0.015 rad, background to beam density ratio < 0.01), and several electron multipliers for simultaneous detection of elastically scattered electrons and metastable atoms (or molecules) due to inelastic scattering. In detailed test measurements of the yield for the production of metastable He^*(2³S₁) atoms around its threshold, the dependence of the overall energy width on various experimental parameters has been investigated. So far a resolution down to 7 meV (FWHM) has been obtained. Under such conditions we have investigated the profile of the He^- (1 s 2 s ^{2 2} S _1/2 ) resonance at the scattering angles 22 ^° , 45 ^° , and 90 ^° . From a consistent fit of the measured profiles by resonant scattering theory we determine a new value for the resonance energy ( E _r = 19.365(1) eV) and an accurate resonance width ( Γ = 11.2(5) meV). These results are consistent with the previously recommended values. Received 23 July 2002 Published online 29 October 2002 RID="a" ID="a"e-mail: hotop@physik.uni-kl.de RID="b" ID="b"Permanent address: Department of Physics and Astronomy, Drake University, Des Moines, IA 50311, USA.     

Magnetic properties of the cyclic spincluster Fe6:bicine

The magnetic properties of the cyclic compound [Fe₆(bicine)₆] LiClO₄ ^. 2MeOH are reported. The cluster Fe₆(bicine)₆ forms an antiferromagnetically coupled ring structure of Fe ^III ions. The magnetic susceptibility is measured between 2 and 300 K and yields the exchange coupling of J/k _B = - 27.5±0.5 K. The field dependence of the magnetic moment is studied at 3 and 6 K in magnetic fields up to 5 T. The zero-field splitting of the first excited spin states with S = 2 and 3 are determined by ESR at 94 GHz. The intra-molecular interactions of the Fe ^III ions are analyzed and the on-site anisotropy of the Fe ^III due to the ligand-configuration is determined to d /k _B = - 0.633±0.008K. Received 28 October 2002 / Received in final form 22 February 2003 Published online 20 June 2003 RID="a" ID="a"e-mail: bernd@piobelix.physik.uni-karlsruhe.de     

Magnetic properties of dc magnetron sputtered and evaporated amorphous films

Compositions of Fe₈₁B_13.5Si_3.5C₂, Fe₆₇Co₁₈Si₁B₁₄ and Fe_5.85Co_72.15Mo₂B₁₅Si₅ were deposited on to rigid and flexible substrates for the first time using a dc sputtering source as part of a novel Rotating Cryostat (RC). The films sputtered on silicon and glass show only isotropic magnetisation, whereas those sputtered on a polyimide (Kapton^TM) substrate exhibited either isotropic or anisotropic magnetisation depending upon composition. Similar findings were obtained for equivalent evaporated films. Received 23 October 2001 and Received in final form 29 January 2002     

Diode laser slit-jet spectra and analysis of the fundamental of 1-chloro-1,1-difluoroethane (HCFC-142b)

High resolution infrared spectra (0.001 cm^-1) have been measured for mixtures of 1-chloro-1,1-difluoroethane in Ne, expanded in a supersonic planar jet. The ν ₇ fundamental has been analyzed for both isotopic species, CH₃CF₂ ³⁵Cl and CH₃CF₂ ³⁷Cl. A weak b-type component has been observed for the first time. Received 20 May 2002 / Received in final form 10 July 2002 Published online 24 September 2002 RID="a" ID="a"Also: Dipartimento di Scienze Fisiche Universitá di Napoli “Federico II” Complesso Universitario di M.S. Angelo, 80126 Napoli, Italy. e-mail: m.snels@isac.cnr.it     

Extraction of the neutron charge form factor G<Superscript>n</Superscript><Subscript>E</Subscript>(Q<Superscript>2</Superscript>) from the charge form factor of deuteron G<Superscript>d</Superscript><Subscript>C</Subscript>(Q<Superscript>2</Superscript>)

We extract the neutron charge form factor G ⁿ _E(Q ²) from the charge form factor of deuteron G ^d _C(Q ²) obtained from T ₂₀(Q ²) data at 0≤Q ²≤ 1.717 (GeV/c)². The extraction is based on the relativistic impulse approximation in the instant form of the relativistic Hamiltonian dynamics. Our results (12 new points) are compatible with existing values of the neutron charge form factor of other authors. We propose a fit for the whole set (36 points) taking into account the data for the slope of the form factor at Q ² = 0. Received: 26 July 2002 / Accepted: 18 September 2002 / Published online: 4 February 2003 RID="a" ID="a"e-mail: krutov@ssu.samara.ru RID="b" ID="b"e-mail: troitsky@theory.sinp.msu.ru Communicated by V.V. Anisovich     

Exchange coupling in ultrathin epitaxial yttrium iron garnet films

Magnetic exchange coupling has been observed for ultrathin films of yttrium iron garnet (Y₃Fe₅O₁₂ or YIG). Single-crystalline YIG films were prepared on yttrium aluminium garnet (Y₃Al₅O₁₂ or YAG) substrates by pulsed laser deposition. (111) and (110) oriented substrates were used. Film thicknesses were varied from 180 ? to 4600 ?. Epitaxial growth of YIG on YAG was obtained in spite of the lattice mismatch of 3%. Magnetic hysteresis loops recorded for ultrathin YIG films have a “bee-waist” shape and show a coupling between two different magnetic phases. The first phase is magnetically soft YIG. A composition study by secondary ion mass spectroscopy shows the second phase to be Y₃Fe_5-xAl_xO₁₂ due to the interdiffusion of Fe and Al at the film/substrate interface. This compound is known to be magnetically harder and to have weaker magnetization than YIG. The coupling of the two phases leads to a hysteresis loop displacement at low temperatures. This displacement varies differently with film thickness for two substrate orientations. Assuming an interfacial coupling, the maximal interaction energy is estimated to be about 0.17 erg/cm² at 5 K for (111) oriented sample. Received 3 June 2002 / Received in final form 7 October 2002 Published online 27 January 2003 RID="a" ID="a"Presently at LPM, Université H. Poincaré, BP 239, 54506 Vandœuvre-lès-Nancy e-mail: popova@lpm.u-nancy.fr     

Carbon clusters for absolute mass measurements at ISOLTRAP

The cyclotron frequencies of singly charged carbon clusters C_n ⁺ (n ≥ 2) were measured with the Penning-trap mass spectrometer ISOLTRAP at ISOLDE/CERN. The present limit of mass accuracy δm/m = 1.2 ^. 10^-8 and the extent of the mass-dependent systematic shift (δm/m)_sys = 1.7(0.6) ^. 10^-10/u ^. (m - m _ref) of the setup were investigated for the first time. In addition, absolute mass measurements by use of pure clusters of the most abundant carbon isotope ¹²C are now possible at ISOLTRAP. Received: 21 March 2002 / Accepted: 16 May 2002 / Published online: 31 October 2002 RID="a" ID="a"Present address: CERN, CH-1211 Geneva 23, Switzerland; e-mail: klaus.blaum@cern.ch     

Cross-section for collisions of ultracold Li with Na

In a magneto-optical trap (MOT) we are able to simultaneously trap and cool ⁷Li and Na. We investigated the loading behavior of the cloud of Li atoms in presence of the overlapped cloud of cold Na atoms, and, by blocking the weak repumping beam for Na, compared it with the loading curve for Li atoms only. Out of these loading curves we calculated the collision cross-section of Na on Li to be 10^-11 cm ³ /s. Received 11 January 2002 / Received in final form 5 April 2002 Published online 24 September 2002     

Rotational alignment of the h11/2 band in 157Dy

High-spin states of ¹⁵⁷Dy were investigated using in-beam γ-ray spectroscopy techniques with a ¹⁵⁰Nd(¹²C, 5n) reaction. Three rotational bands with the h _9/2, i _13/2 and h _11/2 configurations were observed up to (43/2^-), 53/2⁺ and 45/2^-, respectively. Interband M1 transitions in the h _11/2 (high-Ω) band were also measured up to the highest-spin state. The high-spin states were well reproduced by calculations using the tilted-axis-cranking model (TAC). The B(M1)/B(E2) ratio, Routhian and the tilted angle of the angular-momentum vector are found to be in good agreement with the result of the TAC calculation. Received: 22 August 2002 / Accepted: 26 September 2002 / Published online: 19 November 2002 RID="a" ID="a"e-mail: hayakawa@jball4.tokai.jaeri.go.jp Communicated by W. Henning     

Effect of interfacial alloying on the surface plasmon resonance of nanocrystalline Au-Ag multilayer thin films

Nanocrystalline Au and Ag in multilayer thin film form with Au/Ag/Au structure were prepared by high pressure (∼40 Pa) d.c. sputtering techniques. The Ag concentrations in Ag_xAu_1-x films were changed from x = 0 to 1. These multilayer films with varying Ag concentration showed significant changes in microstructures obtained from TEM and XRD analyses. The optical absorption spectra of these multilayer films showed a single plasmon band confirming the formation of Au-Ag alloy. We ascribe this alloying to the interfacial reactions in nanophase limited at the Au-Ag interface. The red-shift and broadening of the plasmon bands with the increase in silver concentration could be associated to the increase in size of the nanoparticles and its distribution. The observed red shift in the plasmon band may be associated with the change in electronic structure at the Au-Ag interface due to configuration mixing of the atomic energy levels of Au and Ag. Received 17 October 2002 / Received in final form 26 February 2003 Published online 23 May 2003 RID="a" ID="a"e-mail: msakp@mahendra.iacs.res.in     

Nonlinear dielectric response of antiferroelectric liquid crystals in the smectic Cα * phase

Nonlinear dielectric response of antiferroelectric liquid crystals has been studied in the smectic C_α* (SmC_α*) phase. The linear dielectric spectrum shows a single relaxation of Debye type and its relaxation frequency is as high as one hundred kHz. The profile of the third-order nonlinear dielectric spectrum varies in complex fashion with temperature. In the vicinity of the SmA-SmC_α* phase transition, experimentally obtained nonlinear spectra are well described by those calculated with phenomenological theory of Landau type. The soft mode of the SmC_α* phase shows critical slowing-down near the transition temperature. In the lower-temperature region of the SmC_α* phase, the contribution from the soft mode of the SmC_α* phase reduces and the other relaxation mode with Debye-type spectrum appears at several hundred Hz in the nonlinear spectrum. The appearance of this low-frequency mode suggests that the cooperative fluctuation of directors over long range exists in the SmC_α* phase. Received 19 April 2002 and Received in final form 23 July 2002 RID="a" ID="a"e-mail: kimura@exp.t.u-tokyo.ac.jp