Extrinsic Loss Research Articles

Inelastic processes in extended x-ray-absorption fine structure.

Inelastic processes in extended x-ray-absorption fine structure (EXAFS) are studied using an extension of the semiclassical model of dynamical screening of a core hole and a photoelectron. This treatment effectively includes both extrinsic and intrinsic inelastic losses, as well as interference between them. The local-density approximation is used to derive a complex, energy-dependent final-state potential which includes dynamical corrections to the static exchange-correlation potential. A local relaxation method is developed to calculate the core-hole Green's function, from which the EXAFS amplitude-reduction factor can be determined. Applications to EXAFS amplitudes for the diatomic molecule ${\mathrm{Br}}_{2}$ and for metallic Cu yield results in reasonable agreement with experiment.

Quantitation of surface electron spectroscopies: problems in correct description of intrinsic lineshape and extrinsic energy loss processes

Methods for improved description of lineshapes and energy loss backgrounds in the surface electron spectroscopies. Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS), are discussed. For simple homogeneous non-metallic materials in the near surface region, the energy loss profiles are a small contribution to the total spectral intensity and can be adequately described by simple background functions. For many pure metals or metal alloys, with or without thin adsorbate overlayers, it is necessary to use more realistic models for the energy loss profiles. These can be generated from functions which describe the electron emission event on a microscopic scale, and/or from electrons beams which are scattered from the surface at the kinetic energy of the photoemission or Auger process of interest. Deconvolution schemes (particularly using Fast Fourier Transform approaches) and non-linear least squares fitting routines are compared for preparing the XPS or AES lineshapes for further analysis.

Extrinsic loss removal in AES/XPS: JAD Matthew,Department of Physics, University of York, Heslington, York YO1 5DD, UK

Extrinsic loss removal in AES/XPS: JAD Matthew,Department of Physics, University of York, Heslington, York YO1 5DD, UK

Extrinsic loss removal in AES/XPS: VM Dwyer,Department of Physics, University of Warwick, Coventry CV4 7 AL, UK

Extrinsic loss removal in AES/XPS: VM Dwyer,Department of Physics, University of Warwick, Coventry CV4 7 AL, UK

Universal single-mode dispersion-flattened fluoride fibre designed for optimum performance from 1.5 to 2.9μm

A simple step-index with depressed-cladding single-mode fluoride fibre gives excellent dispersion flattening over a broad spectral range, 1.5–2.9μm. The extrinsic losses due to macrobending, microbending and splicing are also minimised. This design enables the use of fluoride fibres at ̃1.55μm and later can be upgraded to 2.55μm, where predicted losses are significantly less than those of silica fibres at 1.55μm.

Low-loss fluoride optical fibers for midinfrared optical communication

Attempts to improve transmission loss characteristics in fluoride fibers are described. Optical loss in current fibers is dominated by two major extrinsic loss factors, defect scatterers and impurities. Scatterer analysis using a Raman microprobe has revealed that the majority of them are ZrO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> crystallites. These crystallites dominate the fiber scattering characteristics, having both wavelength independent and Rayleigh wavelength <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> dependencies according to their size. Excess loss due to OH groups which causes absorption at around 2.9 μm is quantified as 2000-5000 dB/km/ppm, depending on glass composition. These results suggest that further efforts in glass synthesis should concentrate on eliminating oxide and hydroxide impurities, and on the further purification of the raw materials. The key for realizing high-quality, low-loss, and long-length fluoride fibers is currently related to whether or not oxide scatterers can be completely eliminated.

Pecan Nut Loss from Pollination to Harvest

Cohorts of wild, native, and improved pecan nuts were studied at various locations in Texas from pollination to harvest over a 4-year period. An abiotic factor, disappearance, was a major source of nut mortality. Biotic factors that caused nut loss were pecan nut casebearer, Acrobasis nuxvorella Neunzig; pecan weevil, Curculio caryae (Horn); and a disease, pecan scab, whose causal agent is an ascomycete, Cladosporium caryigenum (Ell. et Lang.) Gottwald. Twenty other minor sources of mortality were also identified. Disappearance typically accounted for loss of ca. 50% of the cohorts regardless of ecological condition, geographical location, or application of fertilizer or pesticides. Disappearance was ascribed to poor pollination and nut abortion. Extrinsic losses from key insect pests were inversely related to nut density, whereas losses from disease were more affected by climatic factors. Both types of losses were reduced by pesticide treatments. Pecan management strategies can be modified to be more effective by eliminating most insecticide treatments for pecan nut casebearer in years of high nut production, carefully timing treatments for this insect in years of low production, and managing pecan scab when environmental conditions favor disease development.

SEMICLASSICAL TREATMENT FOR INELASTIC PROCESSES IN EXAFS

Inelastic processes in EXAFS (extended x-ray absorption fine structure) are studied using an extension of the semiclassical model of dynamical screening of a core hole and a photoelectron. This treatment effectively includes both extrinsic and intrinsic inelastic losses as well as interference between them. A complex, energy-dependent potential which accounts for these inelastic effects is derived. This potential is compared with static final state potentials and with empirically determined mean free paths. An local relaxation method is developed to calculate the core hole Green's function in an inhomogeneous system. Application is made to the EXAFS amplitude of the diatomic molecule Br2.

Theory of Intrinsic and Extrinsic Plasmon Excitation in Deep Core XPS Spectra

Intrinsic and extrinsic plasmon loss effects are discussed based on the theory proposed by the present author, both for surface and bulk plasmon loss observed in XPS spectra from deep cores. Two types of the incident X-ray polarization are considered here; one is normal to the surface, another is parallel to it. The angular distribution is also discussed for electrons emitted from the cores at various depth. Model calculations are carried out for the photoemission from Al K-shell, which show the importance of interference between intrinsic and extrinsic effects for any polarization, and any direction especially for the excitation by low energy photons. In this case loss spectra are featureless and would lost in the background.

Relationship between the Auger line shape and the electronic properties of graphite.

Abstract : The experimental carbon Auger lineshape corrected for the effects of the secondary-electron background and extrinsic losses and placed on an absolute energy scale through the use of photoelectron measurements. The resulting lineshape is compared to a model which consists of the self-convolution of the graphite one-electron density of states, including atomic values for the symmetry determined Auger matrix elements. A poor comparison results from this simple model which is considerably improved by the inclusion of dynamic initial-state screening effects. Further improvements results from accounting for final-state hole-hole interactions. The final state is characterized by effective hole-hole interaction energies of 2.2 eV, corresponding to two holes in the sigma band, 1.5 eV for one hole in the sigma and one in the pi band, and 0.6 eV for both holes in the pi band. The remaining discrepancies in our model comparison are suggested to be due to a plasmon emission intrinsically coupled to the Auger final state. Keywords: Auger spectroscopy, Graphite, Localization, and Screening.

Charge transfer, polarization, and relaxation effects on the Auger line shapes of Si.

The Auger line shapes of Si are quantitatively interpreted noting particularly the core-hole screening effects as exhibited through charge transfer, polarization, and atomic relaxation. The ${\mathrm{KL}}_{1}$V, ${\mathrm{KL}}_{2}$,3V, and ${L}_{1}$${L}_{2}$,3V line shapes reflect a core-hole-screened density of states (DOS) consistent with the core hole in the final state of these processes. A DOS appropriate for the screened core hole is obtained by distorting the theoretical DOS for the ground state utilizing the Green's function for a tight-binding Hamiltonian and a central-cell potential. Comparison of the ${L}_{2}$,3VV and KVV line shapes reveal large differences. These differences are discussed in the context of surface effects, intrinsic and extrinsic plasmon losses, and final-state shakeoff. The ${L}_{2}$,3VV and KVV line shapes also suggest some distortion effects due to final-state hole correlation. The ${\mathrm{KL}}_{2}$,3${\mathrm{L}}_{2}$,3 line shape is interpreted in the context of similar line shapes for Na, Mg, Al, and P; all show plasmon losses and, except for P, initial-state shakeoff contributions.

Heavy metal halide glass fiber lightwave systems

Heavy metal halide glass fibers have the potential of optical loss between 0.001 and 0.01 dB/km in the 2-10 \mu m region. We have evaluated some of the system aspects of these fibers in order to determine the ultimate performance limits and to assist in defining waveguide design and fiber processing techniques. Extrinsic waveguide-related losses and limitations including microdeformation, optical nonlinearities, dispersion characteristics, and source and detector capabilities become more significant as the intrinsic losses decrease. Two representative halide glass systems are discussed: a heavy metal fluoride operating at \simeq 2 \mu m and a heavy metal chloride glass at \simeq 6 \mu m. The results indicate that repeater spacings ≳ 1200 and 3600 km at \lsim 1 Gbit/s may be possible for chlorides and fluorides, respectively.

A New Approach to the Theory of Photoemission from Solids

By neglecting certain terms in the Hamiltonian (of importance only in the immediate neighbourhood of the threshold energy) we are able to write an exact expression for the photoemission intensity which involves the standard time inverted LEED state and broadened hole-states. If inelastic extrinsic losses are neglected our expression is similar to the one used by e.g. Feibelman and Eastman [1], Pendry [2] and Spanjaard et al. [3].Our theory also allows inelastic losses (including interference between intrinsic and extrinsic losses) to be evaluated. The expressions are similar to those obtained by Inglesfield [4], but the damping effect no longer has to be put in by hand. Our theory further can handle not only core electron but also valence electron photoemission and it is not restricted to metals but applies to all types of solids.We have also traced why and when Inglesfield's one step theory agrees with a semiclassical calculation.

Model calculations of the micrometer to millimeter intrinsic absorption for simple ceramics

A systematic study has been made of the strength of the intrinsic absorption, or loss, for millimeter to micrometer electromagnetic radiation propagating in simple ceramic materials. This study employed a number of model diatomic systems whose simplicity enabled the basic two-phonon decay process, responsible for this intrinsic loss, to be computed with precision, even at the lowest frequencies, on a dedicated microcomputer. It was found that, at least at low frequencies, the loss was a relatively structureless universal function of the atomic mass ratio, in general agreement with earlier results for real alkali halides. In particular, it was found that, if one atomic mass was markedly increased, the low-frequency damping dropped dramatically and a "window" appeared in the loss function. This behavior and its origins are discussed and illustrated by predictions of the absolute values of the intrinsic low-frequency losses for MgO and SrO at room temperature. The values for MgO are almost an order of magnitude larger, and in reasonable accord with values measured for typical ceramics, when estimated extrinsic losses are subtracted. The present studies also provide a natural explanation of the general observation that the loss at the fundamental lattice resonance frequency is, at least in centrosymmetric systems, dominated by three-phonon decay.

Aluminum collective excitations: Reflection electron energy loss results

Reflection electron energy loss investigation on Al samples has been carried out with an average angle collection system from 30 to 1000 eV of primary electron energy. Plasmonic structures occur always at the same energy while their lineshape broadening increases decreasing the electron kinetic energy. The ratio between surface and bulk contributions has been exploited and reproduced with a phenomenological model. The extrinsic loss mechanism for the creation of the bulk plasmon has been investigated and compared with available theories.

Theory of intrinsic and extrinsic excitation effects in deep core XPS spectra studied by the many-body scattering theory

Intrinsic and extrinsic energy loss structures observed in XPS spectra from a deep core are discussed based on the many-body scattering theory proposed by Fukutome and Low. Within this theory these two effects are naturally interpreted and represented in a simple way. Both the intrinsic and extrinsic transition amplitudes are evaluated in the RPA-boson approximation. It is also shown that the extrinsic effects play an important role in the resonant X-ray photoemission spectra.

Analysis of temperature sensitive operation in 1.6-μm In0.53Ga0.47As lasers

Temperature sensitive operations in 1.6-μm In0.53Ga0.47As lasers are analyzed assuming an extrinsic optical loss due to impurities. As the intrinsic loss mechanism, the intervalence-band absorption is considered for photons and the Auger recombination for excited carriers. The hot carrier effect is taken into account in terms of the hole population in the spin splitoff band. Experimental data can be well explained in terms of the extrinsic optical loss, which is most likely to occur in the cladding region outside the active region, with an activation energy of 0.19–0.28 eV.

Physical basis of power conversion of energy fluctuations of hot electrons

The physical basis is given for the design of the reversible energy fluctuations (REF) converter to enable the energy fluctuations of hot nonequilibrated (HNE) photogenerated electrons in the first layer to be converted into power with high efficiency. The design of an idealized model with physical components is described and analyzed as a performance comparison standard for a practical model. The radiation losses, the electron thermalization losses and the transfer time to the third layer for the hot electron energy fluctuations are computed as a function of the level of voltage fluctuations in the first layer. The relation between the master equation of the REF model for equilibrated and HNE electrons for both two-level and N-level models shows that, in principle, the limiting efficiency of 93% for the reversible thermodynamic cycle for solar energy can be approached for these models. The equations for the rectified current and efficiency also show for these REF models that the performance is not limited by the circuit capacitance so that circuit sizes orders of magnitude larger than the smallest physically achievable can approach this limiting efficiency. Also, the limit on maximum output power as a function of circuit size and as a function of the voltage fluctuations in the first layer can far exceed 10 7 W m −2 as the circuit size is decreased below 0.1 μm. A design of a thin film REF test model is given and experiments are described to identify and measure all intrinsic and extrinsic loss sources as a function of input radiation wavelength and intensity, lattice temperature, layer thickness and material, and first layer optical design. The results using the room temperature REF test model having a minimum or no temperature differential across the second or thermal barrier layer can determine whether the predicted output efficiency of over 80% can be achieved for the conversion of solar energy by a practical and cost-effective REF converter.

Anomalous Auger-electron spectra of metallic calcium

A1 Kα-excited L 2, 3 MM and L 2, 3 MV Auger-electron spectra of Ca have been measured in ultrahigh vacuum from a metallic sample evaporated onto an Ag substrate. An interpretation of the spectra is made by applying a line-fitting procedure. The lineshape and the solid-state—free-atom kinetic-energy shift are also studied. The extrinsic loss structure in the L 2, 3 MM Auger-electron emission is found to be similar to that in 2p photoelectron emission. Spin—density-functional (SDF) calculations for the singularity index describing the intrinsic lineshape give a value of ∼ 0.35 for both processes. Thus the experimental 2 p 3/2 photoelectron line broadened from 1.2 to ∼ 5 eV FWHM has been used as a standard line in the line fitting of the L 2, 3 MM transitions. The term splitting of the L 2, 3 M 2, 3 M 2, 3 transition is larger than in the corresponding free-atom spectrum. This result is also supported by the SDF calculations. The L 2, 3 M 2, 3 V spectrum is anomalously sharp, probably both because of the structure of the local density of states at the site of the core-ionized atom and because of differences in the transition probabilities into the different parts of the band. The experimental solid-state shift is 20.3 eV for the L 2, 3 M 2,3 M 2, 3: 1 D transition, and the binding-energy shifts are 8.3 and 6.1 eV for the 2 p and 3 p levels, respectively. The SDF shifts for the above transitions are 19.9 (configurational average), 9.4 and 8.0 eV, consecutively, in agreement with the experimental values. The calculations also show a localized d-type (atomic-like) structure for the screening of the initial- and final-state core hole (s). This is the origin of the large values of both the singularity index and the solid-state shift.

Surface effects on core-level binding energies and valence in thulium chalcogenides

Vacuum-cleaved (100) surfaces of mixed-valent TmSe, divalent TmTe, and trivalent TmS were studied by high-resolution, angle-integrated photoelectron spectroscopy with the use of synchrotron radiation in the energy range $30\ensuremath{\le}h\ensuremath{\nu}\ensuremath{\le}110$ eV. In the topmost surface layers of TmSe and TmTe the $4f$ levels are found to be shifted to higher binding energies by 0.32\ifmmode\pm\else\textpm\fi{}0.04 and 0.41\ifmmode\pm\else\textpm\fi{}0.05 eV, respectively. In both TmSe and TmS the topmost surface layers are divalent. In the case of TmSe a separation of the ${\mathrm{Tm}}^{2+}$ $4{f}^{12}$ spectral feature into surface and bulk contributions allows a determination of the bulk mean valence $\overline{v}=2.55\ifmmode\pm\else\textpm\fi{}0.05$. While a shift of the Se $3d$ levels to lower binding energy is observed for Se atoms in the topmost surface layer of TmSe, no such shift can be resolved for the Te $4d$ levels of TmTe. The surface-derived divalent spectral features can be quenched completely in all three cases by exposure of the surfaces to submonolayer amounts of oxygen, resulting in the formation of trivalent surface oxides. Values for the electron mean free path $l$ are derived from the observed surface- and bulk-spectral intensities, with $l$ decreasing with decreasing electron kinetic energy down to \ensuremath{\cong}45 eV. Smaller singularity indices $\ensuremath{\alpha}$ of the Doniach-Sunji\ifmmode \acute{c}\else \'{c}\fi{} line shapes as well as smaller extrinsic losses are observed for photoemission from the divalent surface layers as compared to the bulk.