bluuebunny/crossref_metadata_2025 huggingface dataset:
NIGHT-SKY HIGH-RESOLUTION SPECTRAL ATLAS OF OH EMISSION LINES FOR ECHELLE SPECTROGRAPH WAVELENGTH CALIBRATION. II.
Authors: Osterbrock D. E. , Fulbright J. P. , Bida T. A.
Published: July 2002

The potential of night-sky emission lines recorded on every long-exposure astronomical spectrum, for wavelength calibration, is reemphasized. The previously published high-resolution atlas, based on spectra obtained with the Keck 10-meter telescope on Mauna Kea and the HIRES high-resolution echelle spectrograph, is extended from 9000 Å to 10600 Å, the present effective long-wavelength limit for reasonable exposure times with current CCD's. The extension of the atlas shows many OH night-sky lines, and makes it possible to identify them easily on high-resolution spectra. Accurate wavelengths and references to their sources are given. Measured intensity ratios for the resolved, well measured lambda-type doublets are presented, and the probable errors in the listed wavelengths of the unresolved doublets, based on them, are discussed. Observations and identifications of a number of lines of weak satellite or intecombination bands of OH in the night-sky spectrum are discussed, and the "not proven" result of a search for OH lines in the (10 - 5) and (10 - 4) bands is mentione.
Simultaneous saturation of two atomic transitions sharing a common level
Authors: C Cohen-Tannoudji  , S Reynaud
Published: July 2002

A dressed-atom approach is generalised for studying several effects which could be observed on an atomic beam interacting with two monochromatic resonant laser beams saturating two atomic transitions sharing a common level, a situation which occurs frequently in stepwise excitation experiments. The problem of the optimisation of the population of the upper state is investigated. Analytical expressions are derived for the positions, the widths and the weights of the various components of the fluorescence and absorption spectra. The results obtained at the limit where one of the two lasers has a much weaker intensity than the other one are interpreted perturbatively by treating to lowest orders the scattering of the weak laser beam by the atom dressed by the intense one.
Quenching of excited atoms by collisions with stable molecules
Authors: C Bottcher  , C V Sukumar
Published: July 2002

The authors are concerned with relatively simple semi-empirical models for quenching of excited atoms by stable molecules, i.e. the transfer of electronic energy into vibrational and translational degrees of freedom. The most notable example is the quenching for Na(33P) to the ground state by N2. The cross sections are first related to those for free-electron-molecule scattering in a 'first-order' theory and it is shown that this simple theory leads to numerical values which are far too small. Thus the authors invoke an ionic collision complex formed by partial charge transfer and succeed in obtaining reasonable total cross sections and final-state branching ratios from a hybrid theory. Calculations are carried out for excited Li, Na and K colliding with H2, N2and O2.
Diffusion of positrons in helium gas
Authors: R I Campeanu  , J W Humberston
Published: July 2002

The caution describing diffusion of positrons in helium gas is solved with the scattering and annihilation data previously obtained by Humberston (1973), and Campeanu and Humberston (1975) to obtain the positron annihilation rate under various conditions of temperature and electric field strength and for various initial velocity distributions. The results at zero electric field are in reasonably good agreement with the recent experimental results of Coleman et al., with the differences being attributed to inaccuracies in the theoretical annihilation data. There appears to be good agreement between the present equilibrium results for non-zero electric fields and the experimental results of Leung and Paul, and Lee et al., but the agreement is fortuitous since the experimental and theoretical results are almost certainly too low. With the present level of accuracy of the theoretical results it has not been possible to determine the detailed form of the initial velocity distribution of the positrons.
Optical interaction potentials from high-resolution differential cross section measurements at thermal energies. II. He(21S)+Ar
Authors: H Haberland  , K Schmidt
Published: July 2002

For pt.I see ibid., p.2693 (1976). Differential cross sections for He(21S)+Ar have been measured for six relative kinetic energies between 21 and 180 MeV. At low kinetic energies the small-angle oscillations are clearly resolved. Except for the lowest energy (21 MeV) a well resolved rainbow peak is observed in the differential cross sections. The real and imaginary parts of the interaction potential are obtained from the data. The real part has an intermediate maximum in the potential ( Delta E=25 MeV, rmax=7 au) which causes the rainbow peaks. The imaginary part can be expressed by an exponential plus a Gaussian. The velocity dependence of the total and ionization cross sections is calculated and the possibility of an energy-dependent width of the potential is indicated.
Collisions of charged particles with highly excited non-hydrogenic atoms
Authors: I C Percival  , D Richards
Published: July 2002

A theory is presented and cross sections estimated for l to l' transitions produced by charged particles, particularly electrons, incident upon Rydberg atoms. Explicit formulae and ranges of validity are presented. The case of sodium is considered in detail.
Experimental determination of electron energy distribution functions in Townsend discharges in argon
Authors: T Makabe  , T Goto  , T Mori
Published: July 2002

A parallel-plate retarding-potential-difference technique has been used to measure electron energy distribution functions in non-self-sustaining Townsend discharges in argon. The experiments were performed for values of E/p0equal to 150 and 200 V cm-1Torr-1(where E is the electric field and p0the gas pressure reduced to 0 degrees C). The energy distribution functions were evaluated from the measured retarding characteristics using the analysis derived here for a parallel-plate energy analyser.
Determination of H++Cs(6s) potential from differential cross section measurements at energies 13.4-24.2 eV
Authors: H Scheidt  , G Spiess  , A Valance  , P Pradel
Published: July 2002

The relative differential cross sections for elastic scattering of H+by a caesium vapour has been measured. The reduced cross sections exhibit a characteristic rainbow oscillatory structure. The rainbow reduced-scattering angle is 63.5+or-1.4 eV deg. A best Morse potential has been determined by fitting the experimental data with semiclassical differential cross sections. The well depth of the Cs(6s)H+Morse potential is 0.77+or-0.05 eV at 10+or-1 a0, the exponential parameter being 2.5+or-0.1. For comparison the Cs(6s)H+molecular potential has been calculated using a pseudopotential approach with a large LCAO-MO basis. The interaction of the valence electron with the Cs+ion is represented by a Hellmann pseudopotential. When the non-linear coefficients of the trial wavefunction are optimised, satisfactory agreement is obtained with the experimentally determined potential.
Electron scattering from noble gases
Authors: A W Yau
Published: July 2002

Polarised-orbital calculations for the low-energy elastic scattering of electrons from helium, neon and argon are presented in the adiabatic-local-exchange approximation. The polarised orbitals are calculated using a perturbed Hartree-Fock scheme and the exchange kernels in the scattering ansatz are approximated by an equivalent local potential which, for neon and argon, is constructed by augmenting Hara's free-electron-gas potential (1967) with the semiclassical approximation near the origin. In principle, all multipole moments of the electron-atom interaction are included. Partial-wave phaseshifts and scattering cross sections are presented.
Distorted-wave approximations for the triple-differential cross section for ionisation of helium by electron impact
Authors: B H Bransden  , J J Smith  , K H Winters
Published: July 2002

A distorted-wave model is developed, for the ionisation of helium by electron impact at intermediate energies, which takes into account polarisation and absorption in the initial state. The triple-differential cross sections are calculated in the energy range 80-250 eV and the results are compared with experimental data.
Electron impact dissociation cross sections in hydrogen and deuterium, leading to Balmer alpha and beta emission
Authors: C Karolis  , E Harting
Published: July 2002

Dissociation of H2and D2by electron impact, leading to the emission of Balmer alpha and beta radiation, has been studied for electron energies up to 100 eV. Normalised excitation functions, their isotope ratio, and polarisation are reported. Care was taken to eliminate magnetic and electric fields, and to allow for molecular radiation near the Balmer lines. Compared with previous reported work the shape of the cross sections found is substantially the same, while showing an additional threshold near 43 eV, but the isotope ratios are markedly different at low energies while agreeing at higher energies. Complete polarisation measurements are reported for the first time.
Photoabsorption and photoionisation measurements on some atmospheric gases in the wavelength region 50-340 AA
Authors: B E Cole  , R N Dexter
Published: July 2002

Photoabsorption cross sections were obtained for the gases N2, NO, O2, and N2O between 50 and 340 AA and for CO between 180 and 340 AA. Partial sum rule contributions to the total oscillator strength are calculated from the data and are compared with theoretical calculations. Secondly, using a double-ionisation chamber, multiple-ionisation yields are obtained for N2and some rare gases at selected wavelengths between 180 and 300 AA. The results are related to those of other authors who have measured oscillator strengths for partial double-ionisation channels.
Differential study of H--inert-gas collisions
Authors: V Esaulov  , D Dhuicq  , J P Gauyacq
Published: July 2002

H-scattering by He, Ne and Ar in the 0.08-2.0 keV energy range has been studied. The energy-loss spectra of scattered particles were obtained using the time-of-flight method and electrostatic analysis. Reduced differential cross sections for elastic and inelastic processes are reported. The dominant inelastic process is single-electron detachment which has a very low angular threshold. Strong inelastic processes leading to H-atom excitation are observed. The weakly bound electron of H-can be treated independently of the molecular (H-inert-gas atom) core and the theoretical differential cross sections of H--inert-gas collisions can then be obtained by multiplying the appropriate H-inert-gas cross section by the detachment or survival probability for the outer electron calculated by Devdariani, (1972), in the 'zero-radius potential' approximation.
Electron-atom momentum-transfer cross sections for caesium from the measured electron thermal conductivity
Authors: B Stefanov
Published: July 2002

A method is proposed to determine the dependence of the electron-atom cross section on the energy from measured values of the thermal-conductivity coefficient in a plasma. Preliminary results on Cs are reported.
Carbon K-shell excitation in small molecules by high-resolution electron impact
Authors: M Tronc  , G C King  , F H Read
Published: July 2002

The excitation of 1s carbon electrons has been observed in CO, CH4, CF4, CO2, COS, C2H2and C2H4by means of the electron energy-loss technique with high resolution (70 meV in the 300 eV excitation energy range) and at an incident electron energy of 1.5 keV. The energies, widths and vibrational structures of excited states corresponding to the promotion of 1s carbon electrons to unoccupied valence and Rydberg orbitals have been obtained. The validity of the equivalent-core model, and the role of resonances caused by potential barriers, are discussed.
Positronium formation in positron scattering from hydrogen and helium atoms: the distorted-wave approximation
Authors: P Mandal  , S Guha  , N C Sil
Published: July 2002

The distorted-wave approximation is applied to investigate the rearrangement collision problem of positronium formation in the ground state with positron scattering from hydrogen and helium atoms for the positron energy ranges 8.704-200 eV and 20-100 eV respectively.
Ion pair formation in H, D+Ar, Kr, Xe collisions from threshold to 100 eV
Authors: W Aberle  , J Grosser  , W Kruger
Published: July 2002

Integral cross sections are reported for ion pair formation in collisions between hydrogen (H,D) and noble-gas (Ar,Kr,Xe) atoms from threshold to 100 eV. The energy dependence of the cross sections is strongly oscillatory. A curve crossing model provides a good semi-quantitative explanation of the results.
Photoelectron studies of the xenon 5p branching ratio using synchrotron radiation
Authors: H J Levinson  , I T McGovern  , T Gustafsson
Published: July 2002

Measurements of the photon energy dependence of the branching ratio of the spin-orbit split 5p levels in xenon, in the range 16 to 33 eV are reported. The authors' data are compared with three recent calculations. They find best agreement with one based on the relativistic random-phase approximation which extends down to 27 eV. In the near threshold region, the other available calculations are not in agreement with their data. They also describe a simple modification of a commercial cylindrical mirror energy analyser for angular distribution studies.
Light statistical dependence of saturated two-photon transitions
Authors: G H Alber  , P Zoller
Published: July 2002

The light statistical dependence of stationary saturated two-photon transitions is discussed. While at low intensities, in comparison with light having a stable amplitude, Gaussian amplitude fluctuations of the exciting radiation are more effective in populating the excited resonant state, light with amplitude fluctuations is less effective in saturating a two-photon transition. The amplitude fluctuations are shown to wash out the resonance curves. While at low intensities the effective Stark shift (i.e. the maximum of the dispersion curve) is larger by a factor of three in light with Gaussian amplitude fluctuations compared with radiation of constant amplitude, this enhancement factor becomes intensity dependent with increasing intensity.
Collisions of slow electrons with methane: ionisation, fragmentation and resonances
Authors: D Mathur
Published: July 2002

Ionisation efficiency and appearance potential functions for CH4+, CH3+, CH2+and CH+have been measured using a monoenergetic electron beam colliding with methane. The upper limit for the first ionisation potential of CH4is determined to be 12.71 eV. Appearance energies for the ionic fragments have yielded values for heats of formation which indicate a large reverse activation energy in the case of CH+. Structure in the ion yield curves for CH4+, CH3+and CH2+in the energy region 19-22 eV is in qualitative agreement with the results of Marmet and Binette (1978) and may be due to highly excited states of methane. The first excited state of CH+as well as four excited states of CH2+are observed. Other structure reported in various RPD measurements is not reproduced. The electron transmission function of CH4, measured on two spectrometers over 1-20 eV, reveals a broad minimum at 7.8 eV and a resonance at 18.5 eV.
Electronic states of the CuF molecule. I. Analysis of rotational structure
Authors: F Ahmed  , R F Barrow  , A H Chojnicki  , C Dufour  , J Schamps
Published: July 2002

The electronic spectrum of the CuF molecule has been photographed at moderate resolution in absorption and some regions have been studied in emission, using spectra excited at reduced rotational temperature in a composite wall hollow cathode discharge. Three new systems, all with the ground state as lower state, have been discovered: their upper states are a3Sigma+, A(0+) and D( Omega =1). Constants for63Cu19F are given.
Theoretical study on low-lying electronic states of Na2
Authors: G Jeung
Published: July 2002

Nineteen ground and excited states have been calculated using a HF pseudopotential, a valence CI, and a core-valence perturbation. This work provides extremely precise adiabatic potential curves that will facilitate the interpretation of the experimental data. Three currently employed calculational methods (model potential, frozen-core all-electron, HF pseudopotential) are also discussed.
Multiply charged ions induced by multiphoton absorption processes in rare-gas atoms at 1.064μm
Authors: A L'Huillier  , L A Lompre  , G Mainfray  , C Manus
Published: July 2002

Multiply charged ions are easily formed in rare gases by multiphoton absorption processes. For Kr and Xe up to quadruply charged ions are formed. They are induced by a bandwidth-limited 50 ps laser pulse at 1.064 W cm-2intensity range. Doubly charged ions are formed through the absorption of a very large number of photons (29 for Xe) in a direct transition from the ground state of the atom to the second ionisation limit. The percentage of Xe2+ions relative to Xe+ions is 1% at 1013W cm-2, and reaches 20% at 1014W cm-2. At 1.06 mu m, the removal of two electrons from the ground state of rare-gas atoms in a single step is responsible for the creation of doubly charged ions. This could be a general rule for atoms which have two or more electrons in the outer shell.
Measurements of the dissociative recombination coefficients of O2+, NO+and NH4+in the temperature range 200-600K
Authors: E Alge  , N G Adams  , D Smith
Published: July 2002

Measurements are presented for alphat, the dissociative recombination coefficients with electrons of O2+, NH4+and NO+under truly thermalised conditions within the temperature range 200-600K using a flowing afterglow/Langmuir probe apparatus, alphat(O2+) is found to vary as approximately T-0.7in close accord with previous pulsed afterglow data for alphat(O2+) and alphae(O2+) and with values for alphae(O2+) inferred from ion trap data. alphat(NH4+) is found to vary as approximately T-.06. alphat(NO+) is found to vary as approximately T-0.9which is reasonably consistent with previous pulsed afterglow data for alphat(NO+). These data are also compared with values of alpha3(NO+) measured in a pulsed afterglow experiment and those derived from ion trap and merged beam cross section data and from atmospheric observations.
High Rydberg transitions in the principal and intercombination series of mercury
Authors: M A Baig
Published: July 2002

The absorption spectrum of mercury vapour has been recorded at high resolution (+or-0.003 AA) in the 1150-1350 AA region with synchrotron radiation as the background source of the continuum. Observations comprise the principal series: 6s21S0to 6snp1P10(8<n<or=37) and the intercombination series: 6s21S0to 6snp3P10(8<n<or=48). An intensity anomaly occurs at high n values with3P10gaining most of the oscillator strength at the expense of the1P10transitions. Simple intermediate coupling calculations show that this is due to a change from LS to jj coupling with increasing n values. A two-dimensional Lu-Fano plot it is also presented and this exhibits interchannel interactions between the1P10channels. As a result of the new measurements, the previously somewhat uncertain determination of the first ionisation potential is reliably confirmed as 84184.10+or-0.20 cm-1.
Semiclassical many-mode Floquet theory. II. Non-linear multiphoton dynamics of a two-level system in a strong bichromatic field
Authors: Tak-San Ho  , Shih-I Chu
Published: July 2002

For pt.I see Chem. Phys. Lett. vol.96, p.464 (1983). The non-linear multiphoton excitation dynamics of two-level systems under the influence of two strong linearly polarised monochromatic fields is studied exactly for the first time along the semiclassical many-mode Floquet theory. The present approach relaxes the restrictions of the conventional generalised rotating-wave approximation (GRWA) and allows correct treatment of various non-linear features such as power broadening, resonance shift, asymmetry in absorption lineshape, etc. By extending the nearly degenerate perturbative method of Salwen (1955), approximate analytical expressions for transition probabilities are obtained for multiphoton resonance processes for which (n+1) photons of one field are absorbed, whereas n photons of the other field are emitted, where n is an arbitrary integer. Detailed comparison of the analytical, the GRWA and the numerically exact results is given. Also presented is a case study of the time evolution of spin 1/2 systems in multi-quantum NMR transitions driven by pulsed bichromatic radiofrequency fields.
Spectral shapes for atomic-field bremsstrahlung produced by beta particles of yttrium-90 in thick targets of Cu. Cd, Ta and Pb
Authors: Shivaramu
Published: July 2002

Experimental results are presented for the spectral shapes for atomic-field bremsstrahlung produced by complete absorption of beta electrons from the decay of90Y in targets of Cu, Cd, Ta and Pb over the photon energy range from 600 keV to 2 MeV. The experimental data are compared with numerical calculations based on the formulation of Pratt et al., the Born approximation theory of Bethe-Heitler and with the classical theory of Kramers. Although small discrepancies exist at some spectral points scattered throughout the energy region studied, the overall agreement between the theory of Pratt et al. and experiment is good. The experimental data agree with the Elwert-corrected Bethe-Heitler theory within experimental uncertainties for Cu, and a systematic positive deviation is found for Cd, Ta and Pb. The overall agreement between the classical theory of Kramers and experiment is very poor. Agreement between Kramers' theory and experiment is only seen within experimental uncertainties in certain restricted energy regions for Cd, Ta and Pb.
Photoionisation of atomic sulphur
Authors: S T Gibson  , J P Greene  , B Ruscic  , J Berkowitz
Published: July 2002

The photoionisation spectrum of atomic sulphur prepared by the reaction of H atoms with H2S, is presented from its ionisation threshold to 925 AA. Autoionisation peaks assigned to quasidiscrete levels . . . (2D degrees )nd3S degrees ,3P degrees are sharp, whereas those designated . . . (2D degrees )nd3D degrees are broad. This latter feature parallels the discontinuity observed between first-row and heavier elements in the noble gases and halogens. There is evidence for severe perturbation in the . . , (2D degrees )nd3P degrees series, which could conceivably be attributed to a proximate 3s3p53P degrees state.
Monte Carlo simulation of electrical discharges in gases
Authors: R W L Thomas  , W R L Thomas
Published: July 2002

New Monte Carlo methods of calculation for tracing the motion of electrons in gases are described. Two problems have been treated by these methods, and the techniques applied to electron swarms in neon. Firstly, the probability of back-scatter of electrons to the cathode has been estimated for values ofE/p0between 10 and 200 V cm-1torr-1(Eis the electric field andp0the pressure reduced to 0 °C) and for emission energies up to the lowest excitation potential. Secondly, Townsend primary ionization coefficients α/p0and drift velocities have been evaluated for values ofE/p0from 20 to 400 V cm-1torr-1. It is shown that the use of the ergodic hypothesis is not valid, yielding values of α/p0which are up to 40% too high in neon, and that entire avalanches must be simulated before meaningful results can be obtained. The present results agree well with those obtained recently at Swansea using the Boltzmann equation, indicating that the Lorentz approximation employed in the latter method leads to only small errors in estimates of the mean properties of the electron swarms.

The equilibrium of the swarm with the field is considered in detail. It is shown that, for uniform fields, deviations from exponential growth of current with increasing electrode separation occur if the interelectrode voltage is not at least three times the mean energy of electrons impinging on the anode.
Rotational structure in the A-X system of InBr
Authors: V B Singh  , A K Rai  , S B Rai  , D K Rai
Published: July 2002

Rotational structure in the A-X system of the indium monobromide molecule has been studied with moderately high resolution. Five bands, namely (2, 0), (1, 0), (0, 0) (0, 1) and (0, 2), were rotationally analysed and rotational constants were determined in the A and the X states for both the isotopic molecules In 79Br and In 81Br. Predissociation has been observed at J=134 in the upsilon =2 level of the A 3 Pi 0 state. This gives an upper limit to the dissociation energy of the InBr molecule of 3.5 eV.
High vibrational level predissociation in the A2Σ+state of OD
Authors: J Czarny  , P Felenbok  , H Lefebvre-Brion
Published: July 2002

Using high resolution spectrograms, the broadened linewidths for several bands of the B2Sigma+-A2Sigma+system of OD have been reinvestigated. Comparison between observed and calculated halfwidths shows that the high-vibrational level predissociation is a case c+one, due to a first order spin orbit interaction, with a repulsive4Pi state which crosses the A2Sigma+state of OD at r=1.65 AA between v=7 and v=8, and between v=5 and v=6 for OH. The vibration- rotation interaction is shown to be responsible for the strong halfwidth dependence on the rotational quantum number.
The elastic scattering of positrons by atomic hydrogen
Authors: J W Humberston  , J B G Wallace
Published: July 2002

Kohn's variational method is used to calculate the positron-hydrogen scattering length and the S wave phase shifts at several energies below the positronium formation threshold, and good agreement is obtained with the results of other recent variational calculations. A detailed analysis of the structure of the zero energy wavefunction is made, and the wavefunction is used to calculate the electron-positron annihilation rate parameter Zeff, with the result 8.9, and the angular correlation of the two annihilation gamma rays. The energy dependence of Zeff is also investigated using the positive energy wavefunctions.
Harmonic oscillator integrals
Authors: W Witschel
Published: July 2002

Using second quantization and operator techniques the square of the Franck-Condon overlap integral for different frequencies is derived algebraically. In contrast to similar work no integrations, series expansions of oscillator states or theorems from the theory of special functions are applied.
A unified calculation on the Stark broadened profile of Lyman-α
Authors: C Bottcher
Published: July 2002

The generalized phase shift theory previously discussed by the author (1971) has applied to the Stark broadening of the 2P-1S line in hydrogen. Complete allowance is made for both electron and proton dynamics. The results differ somewhat from those obtained by Smith et al. (1969) using another unified theory. The scanty experimental data does not make it possible to choose between the theories.
Photoabsorption coefficients of molecular nitrogen in the 300-700 AA region
Authors: W S Watson  , J Lang  , D T Stewart
Published: July 2002

Photoabsorption coefficients of molecular nitrogen have been obtained to +or-3% accuracy in the 380-700AA region using a synchrotron radiation source and channel electron multiplier photodetection. Broad structure in the continuum is clearly observed with an absorption maximum at 540AA and a shoulder at 430AA.
Fluorescence quenching and nonradiative energy transfer in solutions
Authors: A Kawski  , E Kuten  , J Kaminski
Published: July 2002

Intermolecular radiationless energy transfer between unlike fluorescence molecules in fluid and viscous solutions was studied by the quenching of fluorescence. The experimental results have been compared with the Forster quenching theory (1948) and with the 'multi-shell model' of a luminescent centre. Good agreement was found between the values of R'0(critical distance for energy transfer) determined from the Forster and the 'multi-shell model' equations. In fluid solutions the values of R'0differs noticeably from the values R0obtained from the absorption and fluorescence spectra of the investigated mixed solutions. This is due to the effect of diffusion on Forster dipole- dipole transfer.
Angular distribution of L3M2,3M2,3(1S0) Auger electrons of argon
Authors: B Cleff  , W Mehlhorn
Published: July 2002

The angular distribution of L3M2,3M2,3(1S0) Auger electrons of argon has been measured by impact ionization with electrons of primary energy Ep=497 eV, 994 eV and 3976 eV corresponding to the two-, four- and sixteen-fold of the L3binding energy of argon. The anisotropy coefficient A2of the theoretical angular distribution function W( theta )=1+A2P2(cos theta ) of the L3M2,3M2,3(1S0) Auger electrons of argon has been deduced to be 0.05+or-0.01 for Ep=2E(L3) and 4E(L3), and 0.01+or-0.01 for Ep=16E(L3). In the first two cases the Auger electrons are emitted with nonisotropic distribution whereas in the latter case the distribution is isotropic within the experimental error. These results agree well with theoretical results.
Comment on bounds for second-order perturbation theory
Authors: I Shimamura
Published: July 2002

It is pointed out that the complementary variational bounds recently proposed by Burrows (1974) are completely equivalent to the Rebane bounds. The Rebane theory is redeveloped in a generalized form in such a way that its relation to the Hylleraas variational method is clearly demonstrated. A condition is given for the Hylleraas-type method, which is much simpler than the Rebane method, to provide an upper bound on the second-order perturbation energy of an excited state and lower bounds on the dynamic polarizabilities at frequencies above resonance. This condition allows the bound property of a previous calculation by Karplus and Kolker (1963) to be established by analysis of only their results and without the need for further calculations.
Vibrational excitation in e-HCl collisions at low energies
Authors: K Rohr  , F Linder
Published: July 2002

The crossed-beam technique has been used to measure differential cross sections for vibrational excitation in e-HCl collisions from threshold (Eth=0.357 eV for v=1) up to 8 eV. The excitation function for the v=1 level shows a sharp peak close to threshold, a second maximum of larger width around 2.5-3 eV and a monotonic decrease above 3 eV. The angular distribution measurements give nearly isotropic scattering for this process indicating the dominant role of a resonance mechanism via the HCl-2Sigma+state in this energy range. The absolute cross section in the threshold peak, integrated over all angles, is estimated to 1.3 * 10-15cm2+or- 50%.
Measurements of non-thermal electron energy distributions in high temperature laser produced plasmas
Authors: M H Key
Published: July 2002

Measurement of the electron energy distribution in high temperature laser produced plasmas is discussed. The problem arises in thermonuclear fusion research with lasers, where strong enhancement of the high energy tail of the distribution may occur. It is shown that a method based on X-ray bremsstrahlung intensity, recorded as a function of metallic absorber thickness, is applicable in the energy range 1 to 40 keV. Two stages of deconvolution to obtain first the X-ray spectrum and second the electron energy distribution are considered. Simple semi-analytic methods of sufficient accuracy for experimental work are outlined and their regime of validity discussed.
Escape depths of X-ray (MgKα)-induced photoelectrons and relative photoionization cross sections for the 3p subshell of the elements of the first long period
Authors: S Evans  , R G Pritchard  , J M Thomas
Published: July 2002

Photoionization cross sections (Mg K alpha ) relative to F 1s=1.00 for the 3p subshell of the elements from K to Zn are deduced from XPS peak area measurements on a wide range of compounds using improved techniques. These cross sections are combined with photoelectron intensity comparisons between cleaned elemental surfaces to yield estimates of the escape depths (inelastic mean free paths) for 3p photoelectrons ( approximately 1200 eV energy) ejected from V, Cr, Fe, Co, Ni, Cu and Zn. The values obtained are compared with theoretical estimates.
Impact ionization thresholds as generalizations of Van Hove singularities
Authors: P T Landsberg  , D J Robbins
Published: July 2002

An analogy is drawn between impact ionization probability integrals and joint density of states integrals in radiative processes by consideration of the class of integrals: Ing(D) identical to integral g(y) delta (D-f(y))dy (- infinity to + infinity ) where D is constant, y an n-dimensional vector, and g and f scalar functions of y. The special case f(y)= Sigma sigmaryr2(r=1 to n), with sigmar=+or-1, is considered, and the critical point concept in radiative transitions is extended to six dimensions, some details of these integrations being given, and explicit expressions for the impact ionization probability per unit time discussed. The approach of Hill and Landsberg (1976) to the calculation of the Auger rate in indirect materials is adapted to the calculation of impact ionization thresholds for any parabolic band structure. The analogy between radiative and Auger processes previously drawn by Landsberg is used to extend the usual classification of van Hove singularities to a classification of impact ionization 'thresholds', which depends upon the effective masses.
Dynamical structure factors in binary liquids. II. Liquid Na-K alloy
Authors: K N Swamy  , P K Kahol  , D K Chaturvedi  , K N Pathak
Published: July 2002

For pt.I see ibid., vol.10, p.4181 (1977). The expressions for the partial dynamical structure factors obtained in pt.I have been used to calculate numerically the number-number, concentration-concentration and number-concentration fluctuations in liquid Na-K alloy at 373K. It is found that the variation of absolute intensity at zero-energy transfer and full width at half maximum for the number and concentration fluctuations is correlated with the corresponding static structure factors. The intensity of concentration fluctuation spectra attains its maximum value when the concentration of Na atoms becomes equal to that of K atoms.
Raman scattering at structural phase transitions
Authors: Y Yacoby  , R A Cowley  , T J Hosea  , D J Lockwood  , W Taylor
Published: July 2002

The Raman scattering of light from the ferroelectric fluctuations of the uniaxial ferroelectric lead germanate is analysed and found to be well described by models for which the self-energy has appreciable dispersion at frequencies comparable with the soft-mode linewidth. A model is developed to calculate the self-energy arising from phonon-density fluctuations of the modes belonging to the same branch. The model shows that this contribution to the self-energy of the soft mode is particularly large if the modes with which it is interacting are overdamped. In detail, however, the model suggests that close to Tcthere is an appreciable contribution to the Raman scattering from the interference terms between the one- and two-phonon scattering.
NMR study on a 2H-NbSe2single crystal: A microscopic investigation of the charge density waves state
Authors: C Berthier
Published: July 2002

Presents a pulsed NMR study on a single crystal of 2H-NbSe2at atmospheric pressure and under hydrostatic pressure of 21 kbar in the temperature range 4.2-273K. Analysis of Knight shift (KS) and nuclear spin-lattice relaxation (NSLR) time data in the normal state are consistent with N(EF)=2.4 states/eV spin and an admixture of 40% of dxyand dx2-y2wavefunctions in the dz2band. The product T1T=500+or-100 ms K was found constant in the temperature range 4.2-77K and pressure independent; measurements at the 'magic angle' indicate an orbital contribution (T1T)-1approximately=0.9+or-0.9 (sK)-1. The results are consistent with the onset of incommensurate charge density waves (ICDW) at T0=33K at P=1 bar and T0=26K at 21 kbar. Below T0, the lineshapes of the m to m-1 transitions agree with a local distribution of KS and EFG respecting the symmetry of a triple ICDW with a maximum value of 9% for the redistribution of conduction electrons within an atomic cell. Above T0, nuclear spin-spin relaxation measurements demonstrate that the observed pretransitional broadening of the lines is due to static CDW fluctuations, induced by impurities.
Proton relaxation by tunnelling in ammonium salts at low temperature
Authors: I Svare  , G Thorkildsen  , H I Andersson  , S M Skjaeveland  , P Trivijitkasem
Published: July 2002

Proton spin-lattice relaxation times T1and T1 rhohave been measured in partly deuterated NH4ClO4, (NH4)2SnCl6, cubic and trigonal (NH4)2SiF6and (NH4)2Ce(NO3)6at several frequencies 4 to 30 MHz and from 1.5 to 4.2K. T1depends upon the deuterium content in all cases, but T1 rhois very short also in some of the undeuterated salts. The true relaxation is a direct process proportional to T but T1is diffusion-limited in some of the more deuterated salts. The relaxation rates are discussed in terms of transition probabilities between tunnel-split energy states slightly mixed by the dipolar interaction. First-order mixing gives relaxation of asymmetric NH3D+, but second-order mixing is required for relaxation of symmetric NH4+. The relaxation is rapid where the unperturbed levels cross, and the authors derive T1varies as omega02for salts with small tunnel frequencies omegat<< omega0. This behaviour is seen in cubic (NH4)2SiF6. The observed behaviour T1 rhovaries as omegat-2cannot be explained by dipolar mixing and spin-rotational interaction is suggested.
Investigation of the ferroelectric-ferroelastic phase transition in KH2PO4 and RbH2PO4 by means of γ-ray diffractometry
Authors: P Bastie  , J Lajzerowicz  , J R Schneider
Published: July 2002

Without application of an external electric field, the temperature dependence of the shear angle uxy, which is proportional to the spontaneous polarisation and therefore to the order parameter, has been measured by means of Bragg scattering of 412 keV gamma -radiation in KH2PO4 and RbH2PO4. Simultaneously, optical observations of the domain structure were performed. The shape of the rocking curve of KDP and RbDP are compared and discussed. The ferroelectric transition in RbDP is of second order in contrast to KDP which exhibits a first-order one. The modified Slater's theory provides for RbDP a good fit to the experimental data uxy(T) in the whole temperature range investigated. The fact that beta =0.5 near Tc can be understood as an effect of the dipolar interaction; so this system exhibits a cross over from a Slater behaviour to a dipolar behaviour.
Dielectric relaxation and dielectric fluctuations
Authors: U M Titulaer
Published: July 2002

The relation between the frequency dependent dielectric constant and the fluctuations in the dielectric polarisation is re-examined from the standpoint of Felderhof's macroscopic theory of electric and magnetic fluctuations. Contributions of the electronic degrees of freedom to the polarisation are included in the formalism; they are treated as an instantaneous and non-fluctuating response to the electric field. The authors results provide a confirmation and extension of results obtained earlier using cavity and reaction fields. As such they should dispel any remaining doubts concerning this subject, which has been controversial for some time.
EPR measurements of interactions between pairs of Tm2+ions in alkaline-earth fluorides
Authors: J M Baker  , D Marsh
Published: July 2002

Measurements of the spin-Hamiltonian parameters of pairs of interacting Tm2+ions in CaF2, SrF2and BaF2are described. Superexchange and magnetic dipole-dipole interactions are shown to be of comparable magnitude and the superexchange is highly anisotropic. As Tm2+has a single hole in the 4f shell, in alkaline-earth fluorides it is at a site of high symmetry, and as so much is known about the system, the possibility that these systems are ideal for quantitative tests of superexchange theories is examined. In particular the applicability of the overlap model, is examined. It is suggested that the g-values indicate small lattice distortions which change the surroundings of the Tm2+ion but preserve the D2hsymmetry of the nearest-neighbour pair.
The contribution of vacancy defects to mass transport in alkali halides-an assessment using theoretical calculations of defect energies
Authors: C R A Catlow  , J Corish  , K M Diller  , P W M Jacobs  , M J Norgett
Published: July 2002

The authors have made detailed calculations of the energies of formation and migration of vacancy defects in those alkali halides with the rock-salt structure, using recently derived potentials based on the shell model. The computed Schottky and cation-vacancy activation energies are in good agreement with experiment. The calculated cation and anion activation energies are very similar; the anion values thus agree more nearly with the results of diffusion measurements than with the larger values deduced by assuming that the separate contributions of the two types of vacancy can determine the curvature of the plot of log( sigma T) against T-1( sigma is the conductivity and T the temperature). The observed curvature of the conductivity plot may therefore indicate that some other process contributes to ion transport.
The approach to classical dynamics of reorienting methyl groups
Authors: S Clough  , A Heidemann
Published: July 2002

The authors have studied the approach towards classical behaviour of a dynamical system which exhibits strong quantum effects at low temperature. By combining nuclear magnetic relaxation and inelastic neutron scattering data, the rate of thermally activated methyl-group reorientation is measured from 10 to 65K for the 4-methyl group of MDBP (4-methyl-2,6-ditertiarybutylphenol). At low temperatures reorientation occurs by thermally assisted quantum tunnelling processes. The reorientation rate converges towards the predictions of classical theory with increasing temperature, but still deviates significantly at 65K. It appears that the classical theory may be used without important errors only at temperatures for which kT exceeds the methyl torsion splitting. In this high-temperature range a simple classical calculation of the reorientation rate is within a factor of 2 or 3 of the measured value.
Gauss-Seidel-Aitken perturbation expansions for interlayer multiple scattering in LEED: bootstrap acceleration method
Authors: D L Adams
Published: July 2002

Two commonly used perturbation schemes for calculation of interlayer multiple scattering in low-energy electron diffraction are alternatively derived as applications of the Gauss-Seidel-Aitken (GSA) double-sweep iterative method for solution of systems of linear equations. In a reciprocal-space representation, the GSA procedure leads directly to Pendry's 'renormalised forward scattering' method. In an angular-momentum-space representation, the GSA procedure leads to a method related to, but faster than, the original formulation of Zimmer and Holland's 'reverse-scattering perturbation' method. It is further shown that the rates of convergence of the iterative procedures can be increased by altering the normal starting conditions. The bootstrap acceleration procedure consists of using as starting values for the iterative schemes, self-consistent, converged, layer-scattering amplitudes rather than single-scattering amplitudes. The self-consistent amplitudes are obtained from a previous calculation carried out, for example, for a slightly different value of the incident electron energy.
NMR study for tunnelling NH4+ions: non-exponential relaxation in the rotating frame
Authors: K Morimoto
Published: July 2002

The quantum effect of NH4+rotation on spin-lattice relaxation in the rotating frame has been investigated in NH4IO4and NH4ReO4. It is shown that the relaxation becomes of non-exponential type for Lambda0>> omega1and Lambda0taut>1, because the effect of the spin diffusion is not enough to prepare a common spin temperature, where Lambda0is the tunnelling frequency of NH4+, omega1the Larmor frequency in the rotating frame tautthe correlation time. The three-bath model was adopted in the explanation, in good accordance with experiment. The tunnelling frequencies Lambda0/2 pi =1.5 MHz at 68K and 800 kHz at 65K have been determined in NH4IO4and NH4ReO4, respectively.
Linear irreversible thermodynamics and the phenomenological theory of liquid helium II
Authors: P Hawaii  , J Tilley
Published: July 2002

The general treatment of a reacting binary mixture by means of Onsager's linear irreversible thermodynamics is applied to the two-fluid model of liquid helium II. The expression for the total heat flux is derived, comprising one term from thermal conduction and another from internal convection. The alternative single-continuum theory of liquid helium II due to Lebon and Jou (1979) is examined. It is argued that the suggested parallels between the latter theory and the two-fluid model are invalid. In particular, the introduction of an extra thermodynamic degree of freedom in order to obtain the heat flux is shown to be unnecessary. Furthermore, it is shown that when this additional degree of freedom is properly applied in the two-fluid model, the result is the appearance in the heat flux of a third term, proportional to the cube of the relative velocity between the two fluids.
Critical frontiers associated with the bond-diluted Ising ferromagnet on triangular and honeycomb lattices
Authors: A C N de Magalhaes  , G Schwachheim  , C Tsallis
Published: July 2002

Within a real space renormalisation group framework (12 different procedures, all of them using star-triangle and duality-type transformations) the authors calculate accurate approximations for the critical frontiers associated with the quenched bond-diluted first-neighbour, spin-1/2Ising ferromagnet on triangular and honeycomb lattices. All of them provide, in both pure bond percolation and pure Ising limits, the exact critical points and exact or almost exact derivatives in the p-t space (p is the bond-independent occupancy probability and t identical to tanh(J/kBT)). The authors best numerical proposals lead to the exact derivative in the pure percolation limit (p=pc) and, in what concerns the pure Ising limit (p=1) derivative, to a 0.15% error for the triangular lattice and to a 0.96% error for the honeycomb one; in the intermediate region (pc<p<1), where the exact critical frontiers are still unknown, the worst error in the variable t (for fixed p) is estimated to be less than 0.27% for the triangular lattice and 0.14% for the honeycomb one.
On the experimental verification of Landau's theory of superfluid helium
Authors: R J Atkin  , N Fox
Published: July 2002

The theory of superfluid helium proposed by Landau has been the object of much experimental verification. However, the theory involves a number of independent assumptions, and the authors attempt to identify the particular assumptions which have been verified in a number of classic experiments. In some cases they find that the experimental evidence is not entirely unequivocal. In particular, a slightly different assumption concerning energy flux could explain the effects of heat flow in capillaries without involving the difficulties associated with mutual friction terms.