Showing papers in "Physica Scripta in 1990"

The international temperature scale of 1990 (ITS-90)

Abstract: On 1st January 1990 the International Temperature Scale of 1990 (ITS-90) will come into force. This brief article describes the new scale and gives the differences between temperatures measured on ITS-90 and those measured on the current international scale, the International Practical Temperature Scale of 1968 (IPTS-68).

Temperature dependence of the electronic structure of oxides: MgO, MgAl2O4 and Al2O3

Abstract: We have studied the room temperature optical reflectivity of MgO, MgAl2O4, and α-Al2O3 from 5 to 40eV using a novel spectrophotometer with a laser plasma light source. Structure in the imaginary component of the dielectric response is analysed using critical point line shapes, and the origins of the major transitions in MgO and MgAl2O4 are determined using an ab initio pseudofunction band structure calculation of MgO. The exciton reflectivity has been studied in the three materials at temperatures between 300 and 1500 K, and exciton-phonon coupling appears to increase from MgO to α-Al2O3. The temperature dependence of the higher lying interband transitions in MgO has been determined to 1100 K, and we find that while the temperature dependence of the onset transitions at Γ and X are nearly identical (− 1.22meV/K at Γ), higher lying transitions have very different temperature dependences. Furthermore with increasing temperature the X point valence band separation increases at a rate of 0.38meV/K, while the conduction band separation at X decreases at −0.41meV/K.

Temperature dependence of the electronic structure of oxides: {MgO}, {MgAl}

Abstract: We have studied the room temperature optical reflectivity of MgO, MgAl2O4, and α-Al2O3 from 5 to 40eV using a novel spectrophotometer with a laser plasma light source. Structure in the imaginary component of the dielectric response is analysed using critical point line shapes, and the origins of the major transitions in MgO and MgAl2O4 are determined using an ab initio pseudofunction band structure calculation of MgO. The exciton reflectivity has been studied in the three materials at temperatures between 300 and 1500 K, and exciton-phonon coupling appears to increase from MgO to α-Al2O3. The temperature dependence of the higher lying interband transitions in MgO has been determined to 1100 K, and we find that while the temperature dependence of the onset transitions at Γ and X are nearly identical (− 1.22meV/K at Γ), higher lying transitions have very different temperature dependences. Furthermore with increasing temperature the X point valence band separation increases at a rate of 0.38meV/K, while the conduction band separation at X decreases at −0.41meV/K.

Polarized XANES studies of oriented polyethylene and fluorinated polyethylenes

Abstract: Polarization dependent XANES spectra were obtained by use of synchrotron radiation for oriented polyethylene and fluorinated polyethylenes: (1) elongated polyethylene (PE) film, (2) elongated poly(vinylidene fluoride) (PVDF) film, (3) poly(tetrafluoroethylene) (PTFE) film, (4) oriented evaporated film of hexatriacontane. CH3(CH2)34CH3 as a model compound of PE, (5) oriented evaporated film of perfluoroeicosane, CF3(CF2)18CF3 as a model compound of PTFF. Pronounced polarization dependence was observed for each compound, which allows unambiguous assignments of the XANES spectrum. Fluorination effects on the XANES spectra was also discussed

X-ray absorption spectra of poly-p-phenylenes and polyacenes: localization of π orbitals

Abstract: Carbon K-edge X-ray absorption near edge structure (XANES) spectra of evaporated films of poly-p-phenylenes and polyacenes were measured by use of synchrotron radiation. Only one dominant resonance associated with a Cls-to-π* transition was observed for each poly-p-phenylene, although there should exist several π* unoccupied orbitals which are energetically well separated from each other. These features were characterized by means of semi-empirical molecular orbital (MO) calculations based on the equivalent core approximation, which leads to a satisfactory agreement between the observed and calculated spectra. The calculations show that the π* orbitals, which are delocalized in the ground state, become localized by the creation of a Cls core hole. Spectral features of polyacenes, which are somewhat different from those of poly-p-phenylenes in their π* regions, indicate a lesser degree of π* localization.

Circular Dichroism and Spin Polarization in Photoemission from Adsorbates and Non-Magnetic Solids

Abstract: Spin-resolving techniques in photoemission are presently attracting wide interest because they can provide a detailed insight into specific aspects of electronic structure and the dynamics of the photoexcitation process. Recent experimental work in the field of non-magnetic systems is reviewed with emphasis on the complementary nature of the information gained by exploiting the photon- and/or electron-spin. A very promising achievement is the verification of circular dichroism (CDAD) in the angular distribution of photoelectrons. It arises without spin-orbit interaction through an interference of final-state partial waves. Every aligned or oriented target (molecular adsorbates, single crystals) should in principle show this effect in photoemission with circularly polarized light. Electron-spin polarization (ESP) in photoemission is induced by circularly polarized light as a direct consequence of spin-orbit interaction. This type of spectroscopy thus provides a unique method for determining the symmetries of electronic bands of high-Z materials. Recent examples of both categories are discussed.

Electronic structure of poly(tetrafluoroethylene) studied by UPS, VUV absorption, and band calculations

Abstract: The electronic structure of poly(tetrafluoroethylene) (PTFE) was studied by UPS, VUV absorption, and ab-initio MO calculations. The UPS spectra give a photoemission threshold energy of 10.6eV, with deeper valence band features consistent with the reported XPS and the oligomer vapour UPS spectra. The UPS spectra are also consistent with the density of states derived from the calculated band structure, which indicates that the uppermost part of the valence band is formed from the C and F 2p orbitals with C-C bonding and C-F antibonding combinations. The VUV absorption spectrum shows an intense peak at 7.7eV, which most probably corresponds to the valence excitation from the top of the valence band to the bottom of the conduction band. With these data, the structure of the occupied and vacant states are deduced.

Stimulated scattering of large amplitude waves in the ionosphere

Abstract: The nonlinear coupling of a large amplitude electromagnetic pump wave with low-frequency collisional modes in the ionosphere is considered. It then turns out that the threshold values for stimulated electromagnetic emissions are exceeded in ionospheric heating experiments.

High Resolution Soft X-Ray Spectroscopies with the Dragon Beamline

Abstract: Novel spectral features and new information regarding core-excitation processes are revealed in the photoabsorption and photoemission experiments performed with a newly constructed synchrotron radiation beamline. This beamline, dubbed Dragon, has an unprecendented resolving power and superior transmission in the soft x-ray region. Several experimental results on gas phase, solid and biological systems are presented to exemplify the scientific applications of high resolution soft x-ray spectrocopies. The possible application of photoabsorption spectroscopy as a tool for chemical analysis is also discussed and a few examples are given.

X-ray/VUV transmission gratings for astrophysical and laboratory applications

Abstract: This paper describes the techniques used to fabricate deep-submicron-period transmission gratings for astrophysical and laboratory applications, with special attention given to the major steps involved in the transmission grating fabrication. These include the holographic lithography procedure used to pattern the master transmission grating, the fabrication of X-ray mask, the X-ray lithography step used to transfer the X-ray mask pattern into a substrate, and the electroplating of the substrate to form the final grating pattern. The various ways in which transmission gratings can be used in X-ray and VUV spectroscopy are discussed together with some examples of experiments reported in the literature.

Core Excitation and Ionization of Molecules

Abstract: A progress report is given on recent experimental studies of core excitation and inoization of free-molecules in the VUV and soft x-ray (50 to 2600 eV) by electron impact and x-ray photoabsorption (XAS) techniques. The current capabilities of inner shell electron energy loss spectroscopy (ISEELS) and XAS for gas phase studies are compared, with focus on: Dipole versus non-dipole excitations, electric dipole oscillator strength determination, and bond length correlations. Recent studies of ionic fragmentation of molecules induced by tuned x-rays are discussed with regard to the possibility of site-selective fragmentation via core excited state. Using organometallic compounds as an intermediate, connections are made between the core excitation spectra of free molecules and those of surface absorbed molecules, as measured by near-edge x-ray absorption fine structure (NEXAFS) or reflection near-edge energy-loss fine structure (NEELFS) spectroscopies.

Threshold Behaviour and Resonances in the Photoionization of Atoms and Molecules

Abstract: Threshold and near threshold satellite intensities are examined with respect to threshold behaviour, specifically for certain electron correlations contributing to the intensity of these satellites. It is assumed that the different dominant electron correlations give rise to at least four types of threshold behaviour, making it possible to differentiate to a first approximation among them. These correlations are subdivided phenomenologically into two classes: "intrinsic" and "dynamic" correlations. Experimental evidence is presented for both inner- and outer-shell photoionization, substantiating this semiempirical concept. Our results are also compared with first ab initio calculations. The effect of resonances is emphasized as part of the threshold behaviour but also with respect to their deexcitation into various decay channels. Multielectron processes such as resonant shake-off, as well as two step autoionization and spectator Auger decay, are shown to play important roles in the decay of resonances and satellite states.

Statistical Gamma-Decay at Low Angular Momentum

Abstract: This contribution summarizes results on nuclear structure of rare earth nuclei at high intrinsic excitation energy and low spin obtained with the (3He, 4He) and (3He, 3He') reactions. For Ex > 4.5 MeV we find that the γ-spectra from the two reactions are similar indicating that equilibrium is present prior to γ-emission. An unresolved γ-ray bump at 1 MeV is observed in the decay. This structure is ascribed the depopulation of the vibrational regime, defined around Ex = 1-2 MeV, to the ground state band. The technique of extracting first-generation γ-ray spectra is described and surprising low-energy γ-rays are found at high excitation energy. Preliminary results from the new CACTUS multidetector system are presented.

Laser plasma interaction

Abstract: Two topics are reviewed: laser generation of homogeneous plasmas with prescribed density and laser induced high amplitude electron plasma waves. A special emphasis is put on various forms of stimulated Raman scattering. Applications to elementary particle acceleration are discussed.

Wavebreaking of longitudinal plasma oscillations

Abstract: A general treatment for obtaining the maximum amplitude for nonlinear plasma oscillations is given. Both temperature and relativistic effects are included. The maximum or wavebreaking amplitude is derived for cold plasmas and nonrelativistic fluid velocities, cold plasmas and relativistic fluid velocities, warm plasmas and nonrelativistic fluid velocities and, warm plasmas and relativistic fluid velocities. The various derivations which appear in the literature are unified.

Solution of the Hartree-Fock equations for atoms and diatomic molecules with the finite element method

Abstract: The finite element method (FEM) is now developed to solve two-dimensional Hartree-Fock (HF) equations for atoms and diatomic molecules. The method and its implementation is described and results are presented for the atoms Be, Ne and Ar as well as the diatomic molecules LiH, BH, N2 and CO as examples. Total energies and eigenvalues calculated with the FEM on the HF-level are compared with results obtained with the numerical standard methods used for the solution of the one dimensional HF equations for atoms and for diatomic molecules with the traditional LCAO quantum chemical methods and the newly developed finite difference method on the HF-level. In general the accuracy increases from the LCAO - to the finite difference - to the finite element method.

The spectrum of singly ionized oxygen, O II

Abstract: The spectrum of singly ionized oxygen, O II, emitted by a spark-generated, electrodeless discharge light source, has been investigated in the wavelength range 2100-11 700 A. About 740 lines, 359 of which have not been reported earlier, were measured. As a result the energies of about 50 new levels have been determined, and the O II term system now presented includes 254 levels. The quartet system is firmly connected to the doublet system by 56 identified intersystem lines. The ionization limits have been determined by applying the polarization formula to the 4f, 5f, 5g and 6g levels. The experimentally determined value for the lowest limit, 2s22p2 3P0, is 283 270.9 ± 0.5 cm−1 above the 2s22p3 4S3/2 ground state.

Optical properties of argon clusters in the VUV

Abstract: Optical properties of argon clusters and microcrystals containing between 2 and 500 000 atoms have been studied in the VUV using fluorescence excitation spectroscopy with Synchrotron Radiation. Below the ionisation limit the fluorescence yield roughly corresponds to the absorption cross section. Surface and bulk excitations as well as Rydberg states have been observed. The evolution with cluster size shows strong variations of the band intensities whereas the energetic positions are only slightly shifted compared to the solid. It turns out that the Wannier excitons appear only in clusters if the radius of the cluster is approximately four times larger than the radius of the exciton.

The painlevé analysis of the Zakharov-Kuznetsov equation

Abstract: In this paper we make a Painleve analysis of the Zakharov-Kuznetsov equation (which governs nonlinear ion-acoustic waves in a magnetized plasma) and show that it possesses the Painleve property though there is no other evidence that this equation is integrable.

Soft X-ray reflectometer with a laser-produced plasma source

Abstract: A soft X-ray reflectometer with a laser-produced plasma source driven by a Q-switched Nd: YAG laser or a XeCl excimer laser was constructed. By comparing the reflectances of a multilayer mirror with the data obtained using synchrotron radiation, its usefulness for reflectance measurements in the soft X-ray region (40-130 eV) is demonstrated.

Transitions in Al-like, Mg-like and Na-like Kr and Mo, observed in the JET Tokamak

Abstract: Spectra of highly ionized Kr and Mo, emitted from the JET tokamak plasmas, have been recorded in the region 30-355 ?. Detailed analysis of the n = 3, ?n = 0 transitions in Kr XXIV-XXVI and Mo XXX-XXXII has resulted in a number of new spectral classifications. Some isoelectronic regularities for Al-like and Mg-like ions are discussed.

Measurements of the normal-incidence X-ray reflectance of a molybdenum-silicon multilayer deposited on a 2000 l/mm grating

Abstract: Normal-incidence grating optics coated with appropriate multilayers show great promise as a means of achieving high spectral resolution at x-ray wavelengths. Multilayer-coated mirrors have been made and tested successfully, but comparatively little work on such multilayer-coated gratings has been reported. We describe the results of reflectance meansurements made on a superpolished flat mirror and a Ferranti-Astron ion-etched 2000 l/mm laminar grating, which were coated simultaneously at the Lawrence Livermore National Laboratory with a 25-period Mo-Si multilayer. The multilayer was designed so that at normal incidence the mirror would have a maximum reflectance of 31% at a wavelength of 176 A. The measurements were performed using a reflectometer and monochromator installed on the Naval Research Laboratory X24C beamline at the Brookhaven National Synchrotron Light Source.

Spatially-resolved X-ray Absorption Near-edge Spectroscopy of Silicon in Thin Silicon-oxide Films

Abstract: The electronic structure of oxide thin films on silicon single crystals has been measured with high spatial resolution by x-ray absorption near edge spectroscopy (XANES) using a photoelectron microscope and monochromatic synchrotron radiation. Native oxide overlayers on Si(100) and Si(111) crystals were studied at various stages during thermal desorption of the oxide. The spatial resolution capability of the microscope is used to perform selected-area XANES measurements of silicon and silicon oxides, which reveal for the first time the absorption characteristics of intermediate oxidation states of silicon (Si2+ and Si3+), and identify the spin-orbit splitting of the silicon 2p core-exciton structure in Si2+, Si3+, and SiO2. Our results confirm photoemission measurements which find Si2+ and Si3+ states existing at vertical interfaces between Si and SiO2. Unlike the photoemission spectra, however, selected-area XANES can resolve the spectroscopic signatures of these intermediate oxidation states in laterally inhomogeneous interfaces.

Performance of a tungsten/carbon multilayer-coated, blazed grating from 150 to 1700 eV

Abstract: The advantages of multilayer coated blazed gratings are discussed, and multilayer/grating matching conditions are considered. Measurements of a W/C multilayer-coated, 2000 g/mm 3.8° blaze angle ion-etched grating and superpolished flat are reported. These include reflectometer measurements as well as operational performance of the pair as multilayer-multilayer grating diffraction elements in a two-crystal type monochromator. The results are promising for a variety of new spectrometer types.

Laser-plasma sourced, temperature dependent, VUV spectrophotometer using dispersive analysis

Abstract: We have developed a vacuum ultraviolet spectrophotometer with wide energy and temperature range coverage, utilizing a laser-plasma light source (LPLS), CO2-laser sample heating and time-resolved dispersive analysis. Reflection and transmission spectra can be taken from 1.7 to 40eV (31-700nm) on samples at 15-1800K with a time resolution of 20-400ns. These capabilities permit the study of the temperature dependence of the electronic structure, encompassing the effects of thermal lattice expansion and electron-phonon interaction, and changes in the electronic structure associated with equilibrium and metastable phase transitions and stress relaxation. The LPLS utilizes a samarium laser-plasma created by a Q-switched Nd:YAG laser (500mJ/pulse) to produce high brightness, stable, continuum radiation. The spectrophotometer is of a single beam design using calibrated iridium reference mirrors. White light is imaged off the sample in to the entrance slit of a l-m polychromator. The resolution is 0.1 to 0.3nm. The dispersed light is incident on a focal plane phosphor, fiber-optic-coupled to an image-intensified reticon detector. For spectroscopy between 300 and 1800K, the samples are heated in situ with a 150 Watt CO2 laser. The signal to noise ratio in the VUV, for samples at 1800 K, is excellent. From 300 K to 15 K samples are cooled using a He cryostat.

Core level photoemission from monosize mass selected Pt clusters deposited on SiO2 and amorphous carbon

Abstract: We here report core level photoemission spectra of mass selected monodispersed Pt clusters. The samples were produced from a mass selected cluster beam and deposited onto various substrates such that each sample contained clusters having an identical number of atoms. In this series of experiments we have followed the development of the Pt 4f core electron binding energy for different size clusters having between one and six Pt atoms. By comparing the results for two different substrate materials, naturally oxidized Si wafers and amorphous C films, we can evaluate the support interaction for these small clusters.

3d → 4f resonant photoemission in rare earth systems

Abstract: 3d → 4f resonant photoemission (PE) data are presented for Sm and Gd metal as well as for the intermetallic compounds CeAl2, EuPd3, TmAl2, and YbAl2. The observed effects are generally similar as in the corresponding 4d → 4f resonances, they are, however, considerably different in magnitude. For Tm3+ and Yb3+, enhancement factors of 40 and 48, respectively, are derived for the 4f PE cross-section. In case of Gd3+, quintet states are observed that resonate at the M5 threshold through a separate spin-flip channel. Differences between constant-initial-state and total-electron-yield spectra of Tm, Eu, and Sm reveal that resonant decay channels involving 4f electrons are blocked at certain excitation energies.

An assessment of multiple-scattering effects in Auger-electron diffraction and photoelectron diffraction

Abstract: Multiple-scattering effects in Auger electron diffraction and photoelectron diffraction are assessed using a new spherical-wave multiple scattering formalism which is based on a separable approximation to the scattering Green's function. Results obtained in both single-scattering and fully-converged multiple-scattering limits are presented for linear chains of Cu atoms. Simulations of some Auger electron diffraction and photoelectron diffraction experimental results with this new theory are also presented for the first time.

The modulational instability of colinear waves

Abstract: A brief review is given of the modulational instability of a single wave. Some aspects of the modulational instability of two colinear waves are then studied. In general, the waves are modulationally unstable with a maximal growth rate which is larger than the modulational growth rate of either wave alone. Moreover, waves which are modulationally stable by themselves are often unstable in the other's presence. This is true for both copropagating and counterpropagating waves. An important property of an instability is whether it is absolute or convective in nature. The modulational instability of two equal-amplitude copropagating waves is usually, but not always, convective. The modulational instability of two equal-amplitude counterpropagating waves is always absolute. Some applications of current interest are discussed.