bluuebunny/crossref_metadata_2025 huggingface dataset:
Measurements of total absolute cross sections for 0.2-100 eV electrons on H2
Authors: G Dalba  , P Fornasini  , I Lazzizzera  , G Ranieri  , A Zecca
Published: July 2002

Absolute total electron scattering sections for H2have been measured from exponential attenuation in the energy range 0.2-100 eV with 0.07 eV FWHM energy resolution. Resonant structures in the 11-15 eV energy range are presented for the first time with their absolute size. The overall absolute error on the cross section was estimated to be from +or-1.7 to +or-5% in different energy regions. Comparison with previous experimental and theoretical results are given. The authors' measurements are higher than the previous ones although not further than the combined error. A fair agreement can be found with the calculations of Hara (1968).
Observations of the collision-induced He I 23P-31D intercombination line in a laboratory plasma
Authors: D D Burgess  , C St Q Playford
Published: July 2002

The authors have observed the He I 23P-31D intercombination line by using laser absorption spectroscopy over a 140 m optical path in a laboratory plasma. Whilst electric-field-induced forbidden transitions are well known in laboratory plasmas, this is the first observation of a collision-induced intercombination line. Discussion is given of the likely underlying mechanisms.
Stark broadening of He I lines in a CO2laser-produced plasma
Authors: J -C Cauthier  , G - P Geindre  , C Goldbach  , N Grandjouan  , A Mazure
Published: July 2002

Comparisons of experimental and theoretical Stark broadening parameters of the 21P-31D, 23P-33D and 23S-33P helium lines at 6678, 5876 and 3889 AA are carried out in the high-density plasma produced by focusing a powerful pulsed CO2laser in helium at 1500 Torr. At low electron densities (ne<5*1017cm-3) the measured linewidths and lineshifts are in good agreement with published results of current theories. Model microfield method calculations were found to give accurate prediction of helium line-broadening parameters over an electron density range from 1017to 2*1018cm-3.
Observation of the 2S state of muonium in a vacuum
Authors: C J Oram  , C A Fry  , J B Warren  , R F Kiefl  , J H Brewer
Published: July 2002

The authors report the first detection of fast muonium atoms formed in the 2S state. Approximately 115 2S muonium atoms per second, with velocities from 0.006 to 0.02 times the speed of light, were produced in a vacuum from gold and aluminium foils bombarded with 5*104 positive muons per second having incident kinetic energy 2.1 MeV. The Lamb shift in the n=2 excited state has been measured to be 930(+570/-640) MHz.
A study of potential curve crossing in X-Ar complexes (X=Mg, Ca, Sr, Ba)
Authors: P Schwerdtfeger  , H Stoll  , H Preuss
Published: July 2002

Semiempirical pseudopotentials which are determined by fitting to experimental term values of Mg+, Ca+, Sr+, Ba+and Ar7+, are tested for ground and various excited states of the corresponding neutral atoms, using a spin-density functional approximation for valence correlation energies. Experimental triplet excitation energies are reproduced with an average error of 0.1 eV. The method is then applied to study adiabatic level crossing between ground and excited states in X-Ar complexes (X=Mg, Ca, Sr, Ba). Estimates of deactivation energies are given and are compared with results from deactivation cross section measurements.
Correlation effects in collisional depolarisation and redistribution of radiation
Authors: G Nienhuis
Published: July 2002

The author derives an expression for the spectrum of the radiation scattered by an atom that suffers binary collisions in the weak-field limit. The frequency of the incident radiation is in near-resonance with a multiplet transition. The authors accounts for strong atom-perturber correlations that are induced when the incident or the detected frequency is in the far wing of an absorption line. For an isolated transition between two degenerate atomic levels the model picture of quasistatic absorption followed by adiabatic reorientation of the excited state predicts a redistributed fluorescence line with a polarisation that is determined by geometrical factors.
Theoretical study of the lowest states of CsH and Cs2
Authors: G H Jeung  , F Spiegelmann  , J P Daudey  , J P Malrieu
Published: July 2002

The 19 lowest states of CsH and the 11 lowest states of Cs2have been calculated using a non-empirical relativistic effective core potential considering the caesium atom as a one-electron system. The core-valence polarisation and correlation energy has been included employing a recently developed perturbation treatment. The experimental atomic spectrum of caesium is well reproduced. The calculated energy curves show the remarkable precision of this procedure for the ground-state characteristics. For CsH, Re=4.47 Bohr (experimental 4.71 Bohr), De=1.80 eV (experimental 1.83 eV), omegae=871 cm-1(experimental 891 cm-1); for Cs2, Re=8.75 Bohr (experimental 8.78 Bohr), De=0.40 eV (experimental 0.45 eV), omegae=41 cm-1(experimental 42 cm-). The well-to-well transition energies are in error by less than 400 cm-1for experimentally known transitions.
Investigation of excitation transfer processes between hydrogen isotopes by means of laser fluorescence
Authors: K Follmer  , K Bergstedt
Published: July 2002

The hydrogen-deuterium excitation transfer process is investigated experimentally for the case of the n=3 levels. Furthermore, a numerical simulation is provided for testing the method of analysis. The corresponding cross section is evaluated and compared with theoretically calculated values. Results of the numerical simulation are discussed.
Bremsstrahlung in the field of bound electrons
Authors: E Haug  , M Keppler
Published: July 2002

The fourfold differential cross section of electron-electron bremsstrahlung from bound electrons is calculated by means of the impulse approximation. The atomic electrons are described by Hartree-Fock wavefunctions. Numerical results are given for parameters used in coincidence experiments.
Low-energy shape resonances in Lennard-Jones potentials
Authors: J Picart  , S Avrillier  , N T Minh
Published: July 2002

The range of very low collision energies where shape resonances may occur has been systematically explored by pure quantum mechanical calculations in the case of Lennard-Jones (12-6) systems. Semi-empirical formulae have been established which lead, from the characteristic parameter of the system: bz=2 mu epsilon rm
2/h(cross)2, to the prediction of the number of quasi-bound rovibrational states of the molecule temporarily created during the collision. It is shown that the number and the nature of the shape resonances are periodical functions of Bz
12/. These results can be used either to predict experimental results or to determine the potential parameters from experimental data.
Photoionisation of mercury
Authors: K Bartschat  , P Scott
Published: July 2002

The relativistic R-matrix method is used to calculate the total cross section for the photoionisation of mercury for photon energies in the range 10.43 eV (the ionisation potential of mercury) to 14 eV. These cross sections reveal a wealth of structure in this energy region. The asymmetry parameter, beta , for the angular distribution of the photoelectrons is also calculated.
The ion and velocity structure in a laser produced plasma
Authors: F E Irons  , R W P McWhirter  , N J Peacock
Published: July 2002

The early expansion of the plasma produced by the laser irradiation of a polyethylene foil in vacuum has been studied by making time resolved observations at right angles to the target normal. Spectral line emission has been scanned to give the spatial distribution of the ion species carbon I-VI. The lines observed are Doppler broadened by the ion streaming motion, and this broadening has been interpreted to give velocity as a function of time and of distance from the target normal. For one ion species (carbon V) a reconstruction has been made of the ion trajectories. Velocities of 2-3*107cm s-1have been deduced from Doppler shifts observed parallel to the target normal and are in agreement with previous time of flight measurements.
Quantum theory of Stark broadening by electrons
Authors: N Tran-Minh  , H Van Regemorter
Published: July 2002

The one electron approximation is used to determine the exact form of the wings of a line broadened by electron-atom collisions. The validity of this model is examined. The profile being proportional to the square of the scalar product of scattered waves, the impact approximation is obtained by replacing the exact form of the scattered waves by their asymptotic form. In this case of hydrogen the quasistatic approximation is obtained by using the JWKB method together with the usual assumptions.
Resonant rotational excitation of H2by electron impact
Authors: G Joyez  , J Comer  , F H Read
Published: July 2002

By using an electron impact spectrometer with a greatly improved energy resolution it has been possible to study the appearance of H2-resonances in the energy range 11.0 to 12.25 eV in reactions which lead to specific rotational-vibrational states of the final H2molecule. Rotational excitations having Delta J=-2 (i.e. 2 to 0, 3 to 1), Delta J=0 (i.e. 0 to 0, 1 to 1,2 to 2, etc), Delta J=+2 (i.e. 1 to 3,3 to 5), and Delta J=+4 (i.e. 1 to 5), accompanied by vibrational excitations Delta v=0, 1, 2, 4, 6 and 8 have been studied. Two series of resonances (series a and c) have been seen, and their symmetry classifications have been established as2Sigmag+and2Piurespectively.
A new electronic transition F2Σ +-A2 Π i of AlO
Authors: M Singh
Published: July 2002

Red degraded and double headed bands of AlO at 2381.64 AA and 2388.99 AA have been excited in a low pressure dc arc and photographed at high resolution. From a study of their fine structure they have been assigned to an electronic transition F2 Sigma +-A2 Pi i. The final state has been identified with the state A2 Pi i recently reported at 5282 cm-1 (T00). The initial state F2 Sigma + is a new electronic state located at 47190 cm-1 (T00). The rotational constants determined for these electronic states are given.
A derivation of general open-shell SCF equations for atoms and molecules with a Green function method
Authors: R Albat  , N Gruen
Published: July 2002

A new general method is described for obtaining SCF orbitals for electronic systems without restrictions of the symmetry of the open shells. With a Green's function method an optimal SCF operator is constructed for each state of a given configuration. The unperturbed one-particle Green's function is optimal with this operator and permits a quasiparticle interpretation. The SCF equations are derived in matrix formulation convenient for numerical applications within a limited basis set. It is shown that the SCF orbitals thus obtained satisfy the necessary conditions for the energy to be stationary. An application of the SCF method is given for the three lowest S states of the helium atom.
Metastable densities and excitation processes in the He-Cd laser discharge
Authors: P G Browne  , M H Dunn
Published: July 2002

Helium singlet and triplet metastable densities have been measured by a line absorption technique in a 3 mm bore capillary tube over the pressure range 0.5-15 Torr and current range 10-200 mA. With constant discharge current the singlet and triplet densities show pronounced maxima of 2*1012cm-3and 9*1012cm-3respectively at about 2 Torr. At constant pressure the metastable densities saturate for currents above approximately 20 mA. Experimental values are in fair agreement with those calculated using known cross sections for production and loss mechanisms. The addition of cadmium vapour to simulate the conditions of laser oscillation at 4416 AA and 3250 AA in Cd II has two principal effects; cadmium vapour reduces the metastable populations-almost halving them when the optimum cadmium pressure for lasing is reached, and further the metastable densities now saturate at much higher currents (60-120 mAA). The observed optimum performance of the He-Cd laser with respect to discharge current, helium filling pressure and cadmium partial pressure is related directly to the behaviour of the helium metastable densities as these parameters are varied.
Secondary ionization by metastable particles in nitrogen
Authors: S C Haydon  , O M Williams
Published: July 2002

The spatial dependence of the coefficient for secondary electron emission by N2(A3Sigmau+) metastable molecule impact on the cathode in an ionization growth experiment in nitrogen at E/p=100 V cm-1Torr-1is examined over a wide range of electron separations. The measurements provide the first detailed experimental confirmation of the fractional diffusion theory in which the spatial dependence is predicted. In particular, the effects of trace concentrations of impurity of the metastable particle population have been evaluated quantitatively, enabling the extrapolation of the data to the case of ionization growth in nitrogen which is free of impurity contamination. The effects of impurities on the cathode surface are examined and the efficiency of secondary electron production following metastable particle impact is estimated.
A new method for measuring relative oscillator strengths using a CW dye laser
Authors: M Crance  , P Juncar  , J Pinard
Published: July 2002

A new method for measuring relative oscillator strengths is described. The basic idea is the same as in the classical hook method, i.e. the study of anomalous dispersion of a vapour in the neighbourhood of an absorption line by using a two-beam interferometer. The improvement consists in measuring directly the optical path difference as a phase. Therefore in principle very precise values of relative oscillator strengths can be obtained, provided that the analysis of the transmission is performed with a sufficiently high resolving power. In order to solve this problem, the two transitions 1s5-2p6and 1s5-2p4of Ne I have been studied. The ratio of oscillator strengths (f1s5-2p6)/(f1s5-2p4)=2.60+or-0.15 was obtained.
Positron scattering by molecular nitrogen
Authors: E S Gillespie  , D G Thompson
Published: July 2002

Elastic momentum transfer and rotational excitation cross sections have been calculated for positron scattering by nitrogen molecules. The potential describing the interaction between the molecule and the positron consists of static and distortion contributions. The latter is of the Temkin type and results are presented for positron scattering by neon and argon, as well as molecular nitrogen, to show that this approximation may be qualitatively correct for positron energies greater than 2 eV. Wavefunctions and potentials are of single-centre form and the associated convergence problems are fully discussed.
Regge poles and the Watson-Sommerfeld transformation in low-energy atomic collisions
Authors: C V Sukumar  , S L Lin  , J N Bardsley
Published: July 2002

The usefulness of complex angular momentum techniques is examined in the context of low-energy atomic collisions. For elastic e-Ar collisions at energies between 50 eV and 200 eV the observed structure in the differential cross sections is reproduced by the contribution from three Regge poles. For a Lennard-Jones potential representing p-Ar collisions the contributions from many poles are significant and the interference between these contributions leads to serious numerical difficulties in the application of the Watson-Sommerfeld transformation. The Remler model is shown to give a reliable phenomenological procedure for fitting measured differential cross sections for ion-atom collisions, even though the physical basis of the model is questionable.
57Fe Mossbauer effect and magnetism down to 0.98K in the high-spin (5T2) to or from low-spin (1A1) system dithiocyanato bis(4-methyl-1, 10-phenanthroline) iron (II)
Authors: E Konig  , G Ritter  , W Irler  , B Kanellakopulos
Published: July 2002

The57Fe Mossbauer effect and the magnetism of the ferrous compound Fe(4-CH3-phen)2(NCS)2have been studied in detail between 4.2 and 309K and between 0.98 and 303.7K, respectively (phen=1,10-phenanthroline). Between 245 and 120K the compound shows a high-spin (5T2) to or from low-spin (1A1) transition centred at 215K. A study of magnetic hyperfine interactions at 4.2K shows that Vzz(1A1)<0. The reversal of the texture based line asymmetry in the Mossbauer spectrum associated with the5T2to or from1A1transition is interpreted by a sign reversal of Vzzresulting in Vzz(5T2)>0. Debye-Waller factors were determined from the saturation corrected areas of the Mossbauer spectra for the5T2and1A1ground states, individually. The values of -lnf5T2and -lnf1A1are well reproduced by the Debye model between 175 and 250K ( Theta5T2=126.2K) and between 105 and 222K ( Theta1A1=150.2K) respectively.
An electron spin resonance study of copper (II) tetraphenyl porphyrins
Authors: C O Clark  , C P Poole Jr  , H A Farach
Published: July 2002

Copper tetraphenyl porphyrins (TPP) magnetically diluted with free base and zinc porphyrins in the concentration range 5% to 50% were studied by electron spin resonance at X-band. A powder pattern computer program gave a good match to the experimental spectra and produced the Hamiltonian parameters gperpendicular to =2.057, g/sub ///=2.208, Aperpendicular to Cu=35 G, A/sub //Cu/=230 G and AN=19 G. The four equally coupled nitrogens produce superhyperfine structure with the 1:4:10:16:19:16:10:4:1 intensity ratio. The individual component linewidth increased with concentration from 4G at 5% to 10 G at 50% Cu TPP.
On the critical temperature of the two-dimensional Ising spin glass
Authors: P Reed
Published: July 2002

The critical temperature of any two-dimensional Ising model can in principle be determined exactly, providing there is a finite unit cell. This property is used to investigate the dependence of the critical temperature on the size of the unit cell where the exchange coupling between spins in the unit cell are assigned from a Gaussian distribution. It is found that as the number of spins in the unit cell is increased the critical temperature falls. Extrapolation to infinite unit cell, which corresponds to a spin glass, indicates that the critical temperature vanishes.
Scattering theory of low-energy photon diffraction
Authors: K Ohtaka
Published: July 2002

To apply the layer iteration method of low-energy electron diffraction to the diffraction of UV and visible light, the transmission and reflection coefficients of the electromagnetic waves are derived for a two-dimensional periodic array of dielectric spheres with an isotropic dielectric constant. The concepts of phase shifts and t matrix are introduced for light scattering. The theory is a direct extension of Kambe's LEED theory (1968). Also, the secular equation obtained for the bandstructures of photons is a generalisation of the Korringa-Kohn-Rostoker formalism of electronic band calculation.
Spin-polarised low-energy electron diffraction
Authors: R Feder
Published: July 2002

A review is given of elastic spin-polarised low-energy electron diffraction (SPLEED) from crystalline surfaces. For the two relevant spin-dependent interaction mechanisms, spin-orbit coupling, and exchange, dynamical theories are presented with emphasis on symmetry principles, potential models and multiple scattering formalisms. Brief accounts of computational aspects and experimental apparatus (polarised-electron source, spin detectors) are followed by a survey and discussion of experimental and theoretical results for non-magnetic and magnetic surfaces. Finally, the impact of SPLEED on electron emission methods is pointed out, and the prospects of SPLEED as a powerful tool for surface analysis are assessed.
Topological order in an n=4 vector model
Authors: T Nattermann
Published: July 2002

A two-dimensional weakly anisotropic n=4 vector model with planar rotator symmetry in the two complementary n=2 subspaces is considered. It is shown that the low-temperature phase is characterised by quasi-long-range order. The transition to the disordered phase is of 'Kosterlitz-Thouless type and restores the full symmetry of the Hamiltonian. The influence of the anisotropy strength on the transition temperature, and the temperature dependence of the correlation length and the exponent eta is calculated. Contrary to a recent investigation by Bak (1979) argues that there is no additional intermediate phase with orientational order.
The temperature dependence of methyl tunnelling motion in three acetates
Authors: S Clough  , A Heidemann  , M Paley
Published: July 2002

High-resolution inelastic neutron scattering is used to study the excitations of the three proton spin symmetry species of the tunnelling CH3group in sodium, zinc and ammonium acetates at temperatures below 30K. The inelastic and quasi-elastic peaks are found to have similar widths. This shows that the thermal excitations of the three species due to fluctuations of the hindering barrier are statistically independent. The methyl tunnelling splittings are obtained from the positions of the inelastic peaks at low temperature. The tunnel splitting decreases in each case with increasing temperature. The observed splitting is in approximate agreement with a thermally weighted average of the splittings of the ground and first excited torsional states.
Evidence for a common origin of the Urbach tails in amorphous and crystalline semiconductors
Authors: D J Dunstan
Published: July 2002

The absolute strengths of the exponential absorption tails of amorphous semiconductors can be correctly predicted from the band-to-band absorption spectra and the exponents of the tails only. This result shows that the tails are due to transitions between band-derived states, and that the mechanism giving rise to these states is common to all these materials. The same mechanism appears to operate in crystals, where it may be associated with band gap fluctuations.
Intermolecular modes of solid carbon disulphide
Authors: B M Powell  , G Dolling  , B H Torrie  , G S Pawley
Published: July 2002

The frequencies of particular intermolecular modes of vibration and libration in single crystals of solid carbon disulphide have been measured by coherent inelastic neutron and Raman scattering techniques 148K. Measurements were made at the special points in the Brillouin zone Gamma , S, R, Z, A, Q and Y and, including earlier infrared results, 37 of the 42 distinct frequencies at these points have been identified. Six of the ten dispersion curves along Lambda (001) have been measured and three dispersion curves have been outlined along Delta (100). Values of the elastic constants C11, C33, C44and C55have been determined from the acoustic branches. The present results are compared with calculations based on two different models for the interatomic potentials. Both models calculate the general level of the intermolecular frequencies to be too high and the bandwidth of the frequencies to be too large. Neither model can be considered an adequate description of the present results.
A new multicritical point in anisotropic magnets. III. Ferromagnet in both a random and a uniform field
Authors: S Galam
Published: July 2002

For pt.II see ibid., vol.14, p.3603 (1981). Uniaxially anisotropic magnetic systems with both a random and a uniform magnetic field along the easy axis exhibit rather exotic phase diagrams as function of the anisotropy (a), temperature(T), random field (H0) and uniform field (H). For fixed strong anisotropies, below the tricritical temperature. the coexistence surface (H=0) bifurcates for H not=0 into a pair of symmetrical 'wings' of first-order transitions, bounded by two critical loci which meet at the tricritical point at H=0. On the other side, for fixed weak anisotropies the spin-flop phase spreads out into two symmetrical 'horns' (for H>0 and H<0) containing two tricritical lines which meet at the bicritical point at H=0. At intermediate fixed anisotropies, for large uniform fields, the upper parts of the 'horns' overlap the 'wings'. The global four-dimensional phase diagram has been explicitly constructed using mean-field theory. In the vicinity of the new multicritical point, there is a large variety of critical (random Ising-like, pure xy-like), normal and special bicritical (random Ising-like), tricritical (random Ising-like, pure xy-like), fourth-order (random Ising-like) behaviours, as well as crossovers between these behaviours. The results are applicable to the description of dilute antiferromagnets in a uniform field.
Electronic structure of a model stage-1 graphite acceptor intercalate
Authors: G Campagnoli  , E Tosatti
Published: July 2002

A simple but self-consistent LCAO scheme is used to study the pi -electronic structure of an idealised stage-1 ordered graphite acceptor intercalate, modelled approximately on C8AsF5. The resulting non-uniform charge population within the carbon plane, band structure, optical and energy loss properties are discussed and compared with available spectroscopic evidence. The calculated total energy is used to estimate migration energy barriers, and the intercalate vibration mode frequency.
The sodium tungsten bronzes: a study of the changes in electronic structure with composition using high-resolution electron spectroscopy
Authors: M D Hill  , R G Egdell
Published: July 2002

The variation with composition of the electronic structure of polycrystalline sodium tungsten bronzes NaxWO3(0<x<1) has been studied using a combination of high-resolution electron spectroscopic techniques. Samples on either side of the metal-nonmetal transition (x approximately 0.25 from conductivity data) were used. Low-energy electron energy-loss spectra (LEELS) agree well with those predicted in previous optical studies. In particular the effective electron mass (m*) shows the expected variation with bulk composition. Ultraviolet photoelectron spectroscopy (UPS) has been used to study the valence and conduction bands. Measurements of work function ( phi ); density of states at the Fermi level (g(EF)); and effective electron mass at the Fermi level (mEF*) all show linear variations with bulk composition, in agreement with the findings of magnetic susceptibility and low-temperature specific heat capacity studies. A linear variation of the ratio of the intensities of the conduction and valence bands with bulk composition is also observed across the whole composition range. The variation in shape of the conduction band with composition is consistent with the formation of an impurity band which overlaps a nearly-free-electron-like conduction band that narrows with increasing sodium content. The data obtained in this study are consistent with a mechanism for the metal-nonmetal transition involving the formation of localised small polarons when the electron concentration falls below a critical value.
Superposition model for sixfold-coordinated Cr3+in oxide crystals (EPR study)
Authors: K A Muller  , W Berlinger
Published: July 2002

The effect of uniaxial (100), (111) and (110) stresses on the EPR fine-structure lines of Cr3+on octahedral Ti4+sites in SrTiO3and Mg2+sites in MgO have been measured at 300K. The spin-lattice strain coefficients G11and G44were determined. From them, the authors deduced the intrinsic superposition-model parameters b2=2.37+or-0.04 cm-1and t2=-0.36+or-0.01 in SrTiO3, and b2=2.34+or-0.01 cm-1and t2=-0.12+or-0.11 in MgO, respectively. A substantial dependence of G11strain coefficients on charge misfit between Cr3+and the substituted ion was found as has been calculated theoretically by Sangster (1981). The small negative t2exponents and positive b2reported here are interpreted as resulting from the three occupied t2gorbitals of the4A2gground state of Cr3+as compared with the6A1gof Fe3+. A consistent analysis yields a maximum of the positive b2(R) of Cr3+at Rmax=2.102+or-0.005 AA in MgO, whereas the negative b2(R) of Fe3+has a minimum at Rmin=1.7+or-0.2 AA. The deduced b2(R) dependence is confirmed for trigonal LaAlO3and Al2O3, where the known b20(Fe3+)/b20(Cr3+) ratio and sign are quantitatively accounted for.
Charge-density waves in the quasi-one-dimensional compounds NbTe4and TaTe4
Authors: F W Boswell  , A Prodan  , J K Brandon
Published: July 2002

Superlattice spots observed by X-ray and electron diffraction in the tetragonal, metallic, quasi-one-dimensional compounds NbTe4and TaTe4are interpreted in terms of charge-density waves. It is shown that three slightly different types of columns occur in the crystals. In NbTe4at room temperature two types of chains have incommensurate distortions with wavevectors q1=(0, 0, 0.311c*) and q2=(0.5a*, 0.5b*, 0.344c*): the third type has a very weak distortion with q3=(0.5a*, 0,1/3c*). The wavevectors for TaTe4are as above except that the component along the metal chain is exactly1/3c* in each case. The vectors q1and q2are independent of temperature but can be varied by substitution of Ta for Nb. These are structurally the simplest inorganic crystal structures in which CDW have been observed and are the only reported crystals in which three CDW coexist.
The anomalous electric field effect in the EPR of CdF2:Gd3+
Authors: N Guskos  , J Kuriata  , I H Salikhov
Published: July 2002

It is shown that at high electric fields the EPR of CdF2:Gd3+exhibits features not found earlier for almost ionic gadolinium-doped CaF2and BaF2single crystals. These can be ascribed to the semiconducting properties of CdF2:Gd3+. Two possible explanations of the observed phenomena are suggested. Both of these take into account the effects accompanying the current carried through the sample rather than the effect of the applied electric field.
Magnetic field and electron states in two-dimensional disordered systems
Authors: L Schweitzer  , B Kramer  , A MacKinnon
Published: July 2002

The density of states and the exponential decay of a two-dimensional disordered system subject to a magnetic field are calculated using a new iterative technique. The influence of the boundary conditions is investigated. For cylinder geometry states in the centres of the magnetic sub-bands are found that extend throughout the whole system. For strip geometry the existence of disorder-independent edge states is shown. These extend along the whole length of the boundary of the sample. The implications of the results for the explanation of the quantum Hall effect are discussed.
Boron induced changes in the electronic structure of CePd3
Authors: E Wuilloud  , W -D Schneider  , B Delley  , Y Baer  , F Hulliger
Published: July 2002

The intermetallic compound system CePd3Bx, with x=0, 0.12, 0.25 and 0.5, has been studied by X-ray photoemission, Bremsstrahlung isochromat and electron energy loss spectroscopy. As a function of the boron concentration strong variations of the occupied and unoccupied parts of the valence bands as well as of the 3d core levels are observed. An analysis of the data within the Gunnarsson-Schonhammer (GS) model (1983) yields an f occupation varying from nfapproximately 1 in CePd3B0.5to nfapproximately 0.9 in CePd3(90K), while the f conduction electron hybridisation remains about constant over the entire composition range with Delta =150+or-20 meV. Moreover a nonlinear dependence of the lattice contraction on the f occupancy is found which is well accounted for by a calculation performed within the GS model.
A deep centre with excited states in MOVPE GaP grown under high phosphorus pressure
Authors: X -Z Yang  , L Samuelson  , H G Grimmeiss
Published: July 2002

Using junction-space-charge techniques, a deep centre with excited states has been studied in undoped MOVPE n-type GaP grown under high phosphorus pressure. The ground-state and excited-state energy positions have been determined to be 0.82 and 0.16 eV below the conduction-band edge respectively. The photoionisation cross section for electrons shows a pronounced photo-thermal excitation behaviour. The kinetic properties of electron excitation and recombination are analysed using a model for two-step excitation. A method for determining the thermal activation energy of the dominant excited state is described. Comparing the results with those published previously, a possible relation of the centre to Ni(d9) is discussed.
Clustering of atoms and the metal-non-metal transition
Authors: W Geertsma
Published: July 2002

Several liquid alloys of alkali metals (A=Li, Na, K, Rb, Cs) with group-IV (B=Si, Ge, Sn, Pb) alloys show strongly anomalous electrical transport properties for compositions AB and/or A4B. In some cases a nearly complete charge transfer leads to a nearly non-metallic behaviour (for example in Li4Sn and KPb). These anomalies are explained in terms of calculations of the electronic structure, using a tight-binding model and taking into account on-site and nearest-neighbour Coulomb interactions, clustering of B atoms and short-range order. In particular near the composition AB the presence of tetrahedral clusters B44-has a large influence on the properties on the liquid alloy.
Landau theory of incommensurate phases in thiourea
Authors: K Parlinski
Published: July 2002

The phenomenological Landau-type free-energy expansion which contains only second-, third- and fourth-order invariants of the order parameter is proposed for thiourea. The finite set of harmonics and subharmonics which completely describe any commensurate phase is specified. The observed phases 1/3, 1/7, 1/8 and 1/9 arise owing to the couplings of the Umklapp type between higher harmonics. The reasons for the disappearance of the phase 1/7 at high pressure are discussed. The commensurate phases n/m where n and m are any integers, also follow from the free-energy expansion. The results of numerical calculations show the existence of harmonics and subharmonics for high commensurate phases. The subharmonics are related to discommensurations.
Magneto-optical studies of strained PbTe
Authors: J Singleton  , E Kress-Rogers  , A V Lewis  , R J Nicholas  , E J Fantner  , G Bauer  , A Otero
Published: July 2002

Interband magneto-optical studies are reported on strained and unstrained films of PbTe grown on BaF2, NaCl and PbTe substrates. By a careful orientational study of the magneto-absorption of these films it has been possible to deduce an accurate set of band parameters for PbTe, using the six-band Mitchell-Wallis k.p model. It is demonstrated that the inbuilt strain in the layers is an important parameter, which can cause changes in both the bandgap, and the momentum matrix element coupling the L3-6-and L1+6+bands. Both uniaxial and hydrostatic components of strain are shown to be present, and these are used to deduce the deformation potentials for PbTe. The anisotropy of the conduction and valence bands is shown to be independent of the strain.
Electron spin resonance of vanadium in silver chloride
Authors: S U Cheema  , M J A Smith
Published: July 2002

Vanadium-doped silver chloride is prepared by dipping vanadium metal into molten silver chloride. The vanadium enters the lattice substitutionally in the doubly ionized state. Silver-ion vacancies are associated with the vanadium ions in next-nearest cation positions at 93 °K. The symmetry of the crystal field at the vanadium ions is tetragonal and the values of the constants in the spin Hamiltonian areD= (557·2 ± 0·9) × 10-4cm-1,A= (77·7 ± 0·5) × 10-4cm-1,B= (76·3 ± 0·5) × 10-4cm-1,gparallel= 1·9657 ± 0·0007 andgperpendicular= 1·9666 ± 0·0007. At 415 °K the vacancies are no longer associated. The field symmetry is octahedral and the values of the constants in the spin Hamiltonian areA= (77·7 ± 0·5) × 10-4cm-1andg= 1·9657 ± 0·0007. The situation is similar to that reported by Grossberg and by Daehler for manganese in silver chloride. The temperature variation in the width of lines which constitute the outer fine-structure components of the spectrum gives an activation energy of 0·30 ± 0·05 ev for vacancy motion. Vanadium-doped silver chloride is not found to be noticeably photo-active.
The application of pseudopotentials to low-energy electron diffraction II: Calculation of the reflected intensities
Authors: J B Pendry
Published: July 2002

The second paper in this series casts the Boudreaux-Heine method of calculating low-energy electron diffraction (LEED) intensities into a matrix form suitable for extension to complicated surface structures. Calculations are made for the 00 and 01, macron beams reflected from a nickel 111 surface, for norrmal incidence, using the potential described in paper I. Predicted peak positions agree with experiment within ±2 ev and relative intensities agree qualitatively. We interpret our results in terms of band structure. A second calculation for the same face of a nickel crystal with the surface layer displaced by 5% of the bulk spacing between layers is presented, the results of which are interpreted in terms of the large ratio between forward and backward scattering cross section of atoms.
Collective motion in liquids
Authors: J Hubbard  , J L Beeby
Published: July 2002

A theory of collective motion in liquids is described. This theory is based upon a new approximation which may be regarded as either a generalization of the random phase approximation or, alternatively, as a generalization of the phonon theory of solids. The theory is applied to predict the form of the inelastic coherent neutron scattering from liquids making use of a particular model. Other applications, to the theory of amorphous solids, hot crystals, electron plasmas, etc., are indicated.
On the mechanism of Eu→Mn energy transfer in monocrystalline NaCl
Authors: J O Rubio  , A F Munoz  , G H Monoz  , M E Lopez-Morales
Published: July 2002

Integrated intensity and lifetime measurements of the Eu2+and Mn2+emissions were carried out in the temperature range 10-300 K in order to obtain information on the mechanism of the Eu to Mn energy transfer in monocrystalline NaCl. Calculations based on the Forster-Dexter model of energy transfer indicate that the fast rise time of the Mn fluorescence after pulse excitation of Eu2+can only be explained if a short-range interaction such as an electric dipole-quadrupole of exchange-type interaction is active. These calculations also substantiate the model proposed by Rubio and co-workers (1987) in the sense that energy transfer from Eu2+to Mn2+in the lattice of NaCl takes place in the Eu-Mn pairs which are preferentially formed in this material.
The partial structure factors of molten cuprous chloride from neutron diffraction measurements
Authors: D I Page  , K Mika
Published: July 2002

The partial structure factors and radial distribution functions for molten CuCl at 713 K have been determined by the scattering of neutrons from samples of varied isotopic composition. A marked difference is observed between the Cu-Cu and Cl-Cl distributions. The data are discussed in terms of both a hard sphere model and an associated (covalent type) molecular model.
Theory of averaged low energy electron diffraction data
Authors: J B Pendry
Published: July 2002

Averaged LEED spectra can be fitted with good accuracy by a curve of kinematic form. However, theoretical arguments show that a true kinematic average is impossible. The paradox is resolved by a new theory which shows that averages have a quasi-kinematic form with parameters different from those of the true kinematic theory. Some of the new parameters can be simply related to old ones, but others involve complicated corrections. Finally second order averaging schemes are proposed whereby more information can be extracted from experiments.
Localized surface impurity modes in magnetic insulators
Authors: J B Parkinson
Published: July 2002

The energies of the localized modes associated with a substitutional impurity in the surface layer of a magnetic insulator are calculated as a function of exchange coupling and spin. Results are presented for 2D square and 3D simple cubic crystal structures for both ferromagnets and antiferromagnets. For certain ranges of the parameters these modes may lie above the usual spin-wave band.
Novel design of a 'spherical' electron spectrometer
Authors: K Jost
Published: July 2002

The design procedure of a 'simulated' spherical electron spectrometer is described. This device has spherical equipotentials in the region of the beam trajectories, whereas its electrodes are non-spherical. The electron optical properties of the new device are very close to those of a spherical spectrometer. Its geometric width, however, is only about one-third that of a conventional hemispherical spectrometer. Because of its simple electrode structure, gridded or corrugated electrodes can be used easily to suppress background electrons. A prototype of the new instrument was employed as an electron monochromator. An energy spread of less than 25 meV was achieved.
A microjoule nanosecond dye laser photolysis system
Authors: D -J Lougnot  , G Dolan  , C R Goldschmidt
Published: July 2002

A flash photolysis system, in which a very low-energy pulse of actinic light is highly correlated with the analysing light beam, is described. The performances of the system are exemplified by measuring, with a 3 ns time resolution, the singlet-singlet transient absorption signal of rhodamine 6G obtained with a 5 mu J excitation dye laser pulse and a non-pulsed 75 W xenon analysing light source.
Experimental techniques in field ionisation and field desorption mass spectrometry
Authors: H D Beckey
Published: July 2002

A review is given of experimental techniques and instrumental design of field ionisation and desorption sources. Special emphasis is laid on a comprehensive account of the various types of ion emitter. Different sample-loading techniques are also discussed. Automatic FD emitter-heating devices and laser-assisted FD are also described.
Instrument reliability
Authors: R I Wright
Published: July 2002

The assessment of instrument reliability using mathematical models based on field data or laboratory test results is discussed. Techniques are described for the prediction of the reliability of an instrument for which no failure information is available, and a distinction is drawn between total failure and calibration faults. The factors influencing the reliability required of an instrument are discussed. By the application of some simple design principles and use of redundancy and diversity where necessary, it is shown how an instrument or instrument system can be designed to a specified reliability target.
Cellule à joints métalliques pour étude de l'adsorption par spectrographie infrarouge
Authors: R Foulatier  , D Lamarre  , J Fraissard
Published: July 2002

Nous decrivons une cellule metallique, a joints d'or presentant un volume mort negligeable et permettant d'etudier par spectroscopie infrarouge la structure des complexes adsorbes jusqu'a des temperatures de l'ordre de 250 C.
Minimizing aliasing errors of sensors with digital output
Authors: E -G Woschni
Published: July 2002

An outline is given of the nature of aliasing errors, which are caused if the signal is not band limited before sampling. In the case of sensors with digital output, bandpass filtering before sampling is often impossible because of the direct coupling with the process to be measured. The errors are treated, leading to exact as well as to approximate results. Several ways of minimizing the errors are considered which take into account the influence of signal processing after sampling and the consequences of non-ideal demodulation filtering (interpolation). The investigations show among other things the new and important result that, in the case of a mean operation, the sampling frequency may only be half the value due to the sampling theorem. Beyond this, the pulse shape of the samples should be chosen in such a way that the spectral density decreases with frequency as fast as possible. A numerical example demonstrates the efficiency of these methods.
A wind tunnel metabolism chamber with small volume and good air flow characteristics
Authors: G S Bakken  , D J Erskine  , M T Murphy
Published: July 2002

A wind tunnel metabolism chamber is described which has a low (4.2 to 1) ratio of total to working volume to facilitate respiratory gas exchange measurements on unrestrained animals. Fine mesh screens smooth air flow to obtain 2% RMS turbulence and +or-5% uniformity. Simultaneous combinations of air temperature from -10 to 50 degrees C and wind from 0 to 3.6 m s-1can be maintained indefinitely. A window admits simulated solar radiation to the working section.
The application of a PDP-8/1 computer to data acquisition and to the control of chemical instrumentation. I. A generalized interface and its use with a JEOL C60HL NMR spectrometer for multiscan signal averaging
Authors: R G Jones  , P Partington  , B W Ready  , T Trill
Published: July 2002

The philosophy and design of a generalized multipurpose quick change interface for a PDP-8/I computer is described. The application of this system to the problem of multiscan signal averaging of nuclear magnetic resonance spectra with a JEOL C60HL spectrometer is presented and a software description is given. A comparison of this type of system with the hard-wired dedicated computer of averaged transients is made.
Adhesives in instrumentation
Authors: J Shields
Published: July 2002

The use of adhesives for the assembly of instruments or associated parts is broadly reviewed with particular reference to applications in mechanical, optical, electronic and nucleonic instrumentation. The nature of adhesive materials and their advantages and limitations for instrument construction are discussed with emphasis on the importance of other properties for their integrity of instrument performance. Factors to be considered in selecting an adhesive for instrument assembly are outlined in a concluding summary. A recent bibliography of adhesion and adhesives is appended to provide designers with further information on bonding materials and techniques.
Wet chemical deposition of thin films on cylindrical substrates
Authors: M Oron
Published: July 2002

A small apparatus for internal thin-film coating of short glass tubes are designed and constructed. A predetermined amount of the suitable wet-chemical solution is introduced into the tube which is then spun and dried by infrared heating in a controlled manner until all the carrier-liquids evaporate, leaving a solid thin film coating on the inside of the spinning substrate. Accurate control of speed, heating, tube concentricity and internal surface finish is essential for obtaining uniform coatings.
Fast responding mark/space ratio decoder for use with 'slug' Josephson interferometers
Authors: J M A Wade
Published: July 2002

A circuit is described which produces a DC voltage that is related approximately linearly to the critical current of the Josephson junction. This is achieved without any form of filtering thus giving the rapid response that is desirable for the stable operation of a self-balancing potentiometer system. The response time of this circuit can be reduced to about 1 mu s.
Simple viscometer for samples less than 1 ml
Authors: C B Lindemann  , R Rikmenspoel
Published: July 2002

A falling ball viscometer is described requiring a sample volume less than 1 ml. Photoelectric read out allows for a fast and simple measurement procedure. The accuracy of the instrument is +or-1.5% in the viscosity range 1-70 cP.
A stabilized superheterodyne microwave interferometer with high resolution
Authors: J Mygind
Published: July 2002

A stabilized microwave interferometer similar in principle to the classical Rayleigh refractometer is described. A model based on superheterodyne technique and phase detection at the intermediate frequency indicates that the information contained in the amplitude and phase can be extracted separately. In the actual circuit the overall noise figure is 15 dB and the dynamic range exceeds 160 dB in a 3 dB bandwidth of 0.66 Hz. DC drift limits the useful integration time to 100 s. Relative phase angles are determined within 2.0 degrees in the most disadvantageous case. Methods for suppression of leakage signals and a number of modifications of the interferometer approach are briefly considered.
An improved 10 m length stabilized base
Authors: P H Sydenham
Published: July 2002

A 10 m, thermally stabilized steel measuring base has been constructed in a deep underground laboratory. Length is maintained constant to at least 2 parts in 109per hour by an on-off temperature control system. Results are compared with those obtained from a similar base built in 1968, which was situated in a surface laboratory.