Showing papers on "Photon energy published in 1986"

Optical constants and associated functions of metastable diamondlike amorphous carbon films in the energy range 0.5–7.3 eV

Abstract: The complex refractive index N(ω)=n+ik and the complex dielectric constant e(ω)=e1+ie2 are presented for diamondlike amorphous carbon (a‐C) films in the photon energy range 0.5–7.3 eV. The effective number of valence electrons neff per carbon atom, the static dielectric constant e0,eff, and the energy loss function Im[−1/e(ω)] are deduced via the use of sum rules and are used to interpret the optical data. The a‐C films were deposited using an unbalanced magnetron gun to sputter a graphite target (effective sputtering area of 20 cm2) in ultrapure argon gas. The magnetron is characterized by a high deposition flux of condensing atoms (1.5×1014–1.2×1016 cm−2 s−1) and a concomitant high ion flux (6×1014–2.5×1016 cm−2 s−1). A series of films were prepared by sputtering at different power levels in the range 5–500 W. Insulating substrates were used which allowed the films to self‐bias negatively with respect to the plasma, so that the films were bombarded during their growth by Ar+ ions of energy 16–13 eV at a...

High-energy x-ray response of photographic films: models and measurement

Abstract: A detailed characterization has been established for the new, high-sensitivity double-emulsion Kodak Direct Exposure Film (DEF). The experimental data base consisted of density-versus-exposure measurements that were duplicated at several laboratories for x radiations in the 1000–10,000-eV region. The absorption and geometric properties of the film were determined, which, along with the density-exposure data, permitted the application of a relatively simple analytical model description for the optical density, D, as a function of the intensity, I (photons/μm2), the photon energy, E (eV), and the angle of incidence, θ, of the exposing radiation. A detailed table is presented for the I values corresponding to optical densities in the 0.2–2.0 range and to photon energies, E (eV), in the 1000–10,000-eV region. Experimentally derived conversion relations have been obtained that allow the density values to be expressed as either diffuse or specular. Also presented here is a similar characterization of the complementary, single-emulsion x-ray film, Kodak SB-5 (or 392). For the 1000–10,000-eV region this x-ray film is appreciably less sensitive but has higher resolution.

The photoelectron spectrum of water in the 30 to 140 eV photon energy range

Abstract: Synchrotron radiation from the German storage ring BESSY has been used to study the photoelectron spectrum of the water molecule. Asymmetry parameters and relative photoionization cross sections have been measured for all four molecular orbitals (2a1, 1b2, 3a1, and 1b1) in the 30–140 eV photon energy range. The results have been compared with the Xα calculations of Roche, Salahub, and Messmer and good agreement was found. Comparison was also made with a number of previous measurements, including those obtained by (e,2e) spectroscopy. Finally, the inner valence (2a1) region has been studied at 60 and 100 eV photon energies and the relative intensities of some of the satellites were found to change as the exciting energy was varied.

Optical behavior of sputter-deposited vanadium pentoxide

Abstract: A series of V–O alloy films were sputter deposited on glass substrates using a vanadium target and rf-excited Ar/O2 discharges containing 2%–8% O2. On the basis of x-ray results, the films were nominally identified as vanadium pentoxide. Optical transmission and reflection characteristics were measured by double-beam spectrophotometry in the 390- to 700-nm-wavelength region. From these measurements, the absorption coefficient α was determined as a function of the incident photon energy hν. The absorption edge of all films showed two distinct regions of behavior: a high photon energy region in which α varied linearly with (hν)2 and a low-energy tail. The behavior of α is discussed in terms of the structural and electronic changes in the films due to nonstoichiometry and compared to results obtained for single crystal V2O5.

Electronic structure of ScN.

Abstract: The optical reflectivity has been measured over more than four decades of photon energy on chemically well-defined single crystals of ScN, in addition to the electrical conductivity and the Hall effect. From a Kramers-Kronig analysis the dielectric functions have been derived. A plasmon-phonon coupling (plasmaron) has been observed in the far-infrared region which could be decomposed and the phonon part of which compared with a Raman measurement of the phonon density of states. The interpretation of the optical results in a two-band model together with other data confirms recent theoretical estimates of the electron and hole masses and of the charge-carrier density in this compound, which we find is a compensated semimetal.

Optical Spectra of Y2O3 Single Crystals in VUV

Abstract: Optical absorption in the tail region and reflectivity at room temperature in the VUV region up to 42 eV are measured on Y 2 O 3 single crystals. Various optical constants as functions of photon energy are derived therefrom by use of the Kramers-Kronig relation. The Urbach rule parameters are also determined experimentally for the tail. A tentative interpretation is given on the spectral structures found in the optical constants, referring to the energy levels of Y 2+ and Y 3+ free ions.

Refractive indices of In0.49Ga0.51−xAlxP lattice matched to GaAs

Abstract: The refractive indices of In0.49Ga0.51P, In0.49Al0.51P, and In0.49Ga0.29Al0.22P, lattice matched to GaAs grown by molecular‐beam epitaxy, are determined from double‐beam reflectance measurements for photon energies ranging from 0.6 to 1.3 eV. Variation of the In0.49Ga0.51−xAlxP, refractive index with Al composition x and photon energy is calculated according to the single‐effective‐oscillator model. These analytical results are then compared with experimental data.

Soft x‐ray beam line for surface EXAFS studies in the energy range 60≤hν≤11 100 eV at the Daresbury SRS

Abstract: The characteristics of the beam line used for surface EXAFS (extended x‐ray absorption fine structure) studies at the Daresbury Synchrotron Radiation Source (SRS) are described. Monochromatic photons are available in the energy range 60≤hν≤11 100 eV. This has been achieved using a monochromator with a combination of crystal and grating optics. The radiation is focused through the monochromator by a toroidal premirror at 0.5° grazing angle. Three pairs of crystals and a plane grating with focusing mirror are available which can be interchanged under ultrahigh vacuum (UHV). The plane grating/mirror monochromator has two ranges of zero‐order angles, 6.4° and 2°, giving photon energy ranges of 60–550 eV and 600–2000 eV, respectively, with a 1200 l mm−1 grating. The crystals in use are InSb(111), Ge(111), and Ge(220) having photon energy ranges of 1745–7360 eV, 2000–8430 eV, and 3260–11 100 eV. The performance of the gratings and crystals in respect to intensity, resolution, scattered light, and higher orders ...

On the Origin of the Infrared and X-Ray Continua of Active Galactic Nuclei

Abstract: The continuum spectra of radio-quiet active galactic nuclei, which are steeper in the near-IR and flatter in the X-ray regime, can be produced in a synchrotron self-Compton model with a broken power law electron distribution that is flat at low, and steep at large, Lorentz factor values. While the steeper part generates the IR spectrum via synchrotron process, both parts together produce the flatter X-ray spectrum via Compton scattering. The intersection of the two spectra corresponds to equipartition between the magnetic field and synchrotron photon energy densities, giving approximately the same luminosities in the synchrotron and Compton components. The photon spectra are self-consistently computed here, taking the effects of cooling, e(+)e(-) pair production, and higher order Compton scattering into account.

Multi-component flow measurement and imaging

Abstract: A method of measurement of mass flow rates of multicomponent systems comprises directing gamma radiation from a collimated source onto extended scattering means irradiating the flowing medium with single-scattered gamma photons, detecting scattered photons with wide angle photon energy detectors and computing the flow rates from the component velocities and densities.

High energy X-ray measurements during lower hybrid current drive on the Alcator C tokamak

Abstract: High energy X-ray emission (E?> 20 keV) from superthermal plasma electrons during lower hybrid current drive on the Alcator C tokamak has been measured using sodium iodide (NaI) scintillation spectroscopy The X-ray spectra are generally linear on a semi-log plot of count rate versus photon energy and extend out to several hundred kiloelectronvolts For the range of densities (e?(03?08) ? 1014 cm?3) over which current drive was performed on Alcator, there was negligible emission before the injection of radiofrequency wave power The radial profiles of the emission were also measured and indicate that the current carrying high energy electrons exist primarily within the inner half (r/a < 1/2) of the plasma column Plasma parameter scans produced variations in the X-ray emission profiles that are consistent with changes in the launched Fourier power spectrum and the conditions imposed by lower hybrid wave accessibility In addition, the velocity space distribution function of the energetic tail electrons has been determined using the angular variation in the X-ray emission

Dosimetry and quality specification of high energy photon beams.

Abstract: A number of quality descriptors are defined characterizing the photon attenuation and lepton contamination properties of high energy photon beams for radiation therapy. The dependence of the quality parameters on the design of the clinical beams such as the incident electron energy, target and filter thicknesses, field size and depth in the phantom are analyzed in some detail using analytical and Monte Carlo techniques. It is shown that the mean attenuation coefficient of the beam for a standard field size of 10 cm × 10 cm is related very accurately to the mean stopping power ratio for ionizing chamber dosimetry but also approximately to the equilibrium absorbed dose in the beam for a given photon energy fluence. This means that accurate photon dosimetry can be performed without knowing the acceleration potential, target design or filter thickness for the beam in use. Furthermore, the mechanism behind beam hardening and softening in the phantom are quantitized and suitable quality parameters for the lepto...

10 eV photon detector for inverse photoemission

Abstract: An energy-selective photon detector for inverse photoemission spectroscopy is described. The bandpass is effected by the transmission cut-off of a CaF2 window and the ionisation threshold of an acetone filling. The detected mean photon energy is E0=9.9 eV with an energy resolution Delta E=0.4 eV (FWHM). With this detector an inverse photoemission spectrum of silver has been measured; it yields a total energy resolution of the apparatus (electron source and photon detector) of Delta E=0.55 eV (FWHM).

Magneto‐optical properties of substituted cobalt ferrites: CoFe2−xMexO4 (Me=Rh3+, Mn3+, Ti4++Co2+)

Abstract: The Faraday rotation has been calculated at 1.55‐eV photon energy (800 nm wavelength) for various substitutions in CoFe2O4 from the optical constants and the Kerr rotation and ellipticity spectra of polycrystalline bulk samples. The Faraday rotation consists of contributions both from the Co2+→Fe3+ charge transfer transition on octahedral sites as well as from the Co2+ 4A2→4T1 (P) crystal‐field transition on tetrahedral sites, both centered around 2‐eV photon energy. For the various substitutions the contribution of the crystal field transition to the Faraday rotation at 1.55 eV is determined by the amount of Co2+ on tetrahedral sites as well as the shift in energy of this transition upon substitution. A significant contribution of the 4A2→4T1 (P) transition to the Faraday rotation at 1.55 eV is reached in Rh3+ substituted CoFe2O4.

Equilibrium for radiation in a homogeneous plasma

Abstract: Entropy increase and comparison principles are found for the Fokker–Planck equation for the radiation field in a homogeneous plasma with constant electron temperature. When emission– absorption is neglected, this is used to find equilibrium distributions that have the form of a Planck distribution plus a δ function at zero photon energy. For distributions below the Planck and with emission–absorption included, a rate of entropy increase is obtained. Numerical results confirm these conclusions.

Characterization of multilayer x-ray analyzers: models and measurements

Abstract: A procedure is described for a detailed characterization of multilayer analyzers that can be effectively applied to their design, optimization, and application for absolute x-ray spectrometry in the 100 to 10,000 eV photon energy region. An accurate analytical model has been developed that is based upon a simple modification of the dynamical Darwin-Prins theory to extend its application to finite multilayer systems and to the low energy x-ray region. Its equivalence to the optical E&M solution of the Fresnel equations at each interface is demonstrated by detailed comparisons for the reflectivity of a multilayer throughout the angular range of incidence of 0° to 90°. A special spectrograph and an experimental method are described for the measurement of the absolute reflectivity characteristics of the multilayer. The experimental measurements at three photon energies in the 100 to 2000 eV region are fit by the analytical modified Darwin-Prins equation (MDP) for 1(0), generating a detailed characterization of two state-of-the-art multilayers: sputtered tungsten-carbon with 2d 70 A and a molecular lead stearate with 2d 100 A. The fitting parameters that are determined by this procedure are applied to help establish the structural characteristics of these multilayers.

Many-electron effects in multiphoton ionization : Screening effects in single-electron ionization

Abstract: We study the influence of many-electron effects in multiphoton ionization within the framework of diagrammatic many-body perturbation theory. We renormalize the electron-dipole coupling by summing to infinite order both many-electron interactions using the random-phase approximation and higher-order intensity terms. We introduce an effective intensity, which takes into account the screening of the field by the electrons and represents the intensity really seen by an electron. The theory is applied to a calculation of the two-photon one-electron ionization rate of helium in the weak-field limit, using a local-density approximation one-electron basis set. The influence of many-electron effects strongly depends on the field frequency. The two-photon ionization rate of helium is lowered at low frequency (by a factor up to 1.4) and increased at high frequency when the photon energy is above the ionization threshold. Finally, the importance of many-electron effects in multiphoton ionization (e.g., regarding inner-shell ionization) is qualitatively discussed in connection with experiments.

Normal incidence grating spectrometer designed for inverse photoemission studies in the range 10–30 eV

Abstract: We describe a normal incidence grating spectrometer for use in inverse photoemission studies of surfaces We show that by operating in a configuration where the source and image are displaced from the Rowland Circle it is possible to construct a useful spectrometer that can be attached to any existing surface science chamber The device covers the range 10–30 eV in photon energy with a typical energy resolution of 150 meV Capable of operating with relatively low incident currents, this instrument will be an excellent choice for spin‐polarized inverse photoemission experiments

Experimental and theoretical study of configuration interaction states of CO + 2

Abstract: Configuration interaction states of CO+2 have been observed as satellites in a high resolution photoelectron spectrum of CO2 using an angle resolved electron spectrometer and far UV synchrotron radiation in the 30–55 eV photon energy range. Ten satellites have been identified in the 22–40 eV binding energy region and classified as the result of outer and inner valence ionization processes on the basis of their asymmetry parameter ( β) values. Theoretical calculations of configuration interaction states of CO+2 using an ab‐initio SCF‐CI method show that three‐hole‐two‐particle excited configurations are necessary to explain the line positions and intensities. The calculated satellite spectrum, corrected for transition moments at 45 and 1254 eV, shows an excellent agreement with experiment and allows assignments of both outer and inner valence satellite lines, with Πg, Πu, Σ+g, and Σ+u symmetries.

A possible use of the soft X-ray standing wave method for surface and interface structure analysis

Abstract: The standing wave method in the soft X-ray region is described, in which the incident photon energy is scanned and Auger electrons from the sample crystal are detected in a UHV chamber. A preliminary application to the Si(111) (7×7) surface shows that this method will be a promising tool for surface and interface structure analysis of semiconductors.

Angle‐resolved photoelectron spectroscopy of the valence orbitals of SiCl4 as a function of photon energy from 14 to 80 eV

Abstract: Angle‐resolved photoelectron spectroscopy coupled with synchrotron radiation have been used to measure partial cross sections and angular distribution parameters, β, from a photon energy of 14 to 80 eV for SiCl4. Parallel to these measurements, calculations have been made using the continuum multiple scattering Xα method. The results have been examined, primarily in terms of the phenomena of the Cooper minimum and shape resonances. Minima in both the cross sections and β values were found for each of the first five orbitals of SiCl4: 2t1, 8t2, 2e, 7t2, and 7a1. These minima were examined for their energy positions and, in the case of the β values, the depth of the minimum. Shape resonances were calculated in the photoionization of each of the orbitals, and a number of experimental features due to shape resonances are identified. The results, both experimental and theoretical, are compared with earlier work on CC14.

Emission wavelength of AlGaAs‐GaAs multiple quantum well lasers

Abstract: We have recorded spontaneous emission spectra from multiple quantum well lasers grown by molecular beam epitaxy with 25‐A‐wide GaAs wells by opening a window in the top contact stripe. These spectra have a low‐energy tail and consequently the gain spectra derived from them show that laser emission occurs at a lower photon energy than the lowest energy confined particle transition. The observed laser wavelength and threshold current are consistent with the position of the peak in the gain spectrum.

Spectroscopic photoresponse of the passive film formed on iron

Abstract: Spectroscopic photoresponse of the passive film formed on iron in neutral borate solution was studied. The spectroscopic three parameter reflectometry and the photoelectrochemical technique were employed to obtain the spectra of absorption coefficient and photocurrent. From these spectra, a conversion efficiency, η, from the flux of photons absorbed in the film to the photocurrent was calculated as a function of photon energy. The result shows that the photoexcitation process of electron‐hole‐pair formation includes two transition mechanisms; a direct allowed transition and an indirect allowed transition. The direct transition, which occurs in the photon energy higher than 2.6 eV, is attributed to the excitation between the valence (O‐2p) and the conduction (Fe‐3d) bands of iron oxide, whereas the indirect transition, which occurs in the photon energy higher than 0.8 eV, is associated with electronic levels of impurities or d‐d transition in the passive film. The maximum value of η observed at a photon energy of 3.1 eV is about 20%, and this high efficiency suggests that the high electric field exists in the passive film of iron.

Polarization effects in elastic photon-atom scattering

Abstract: We have calculated two linear polarization effects in the elastic scattering of x rays and ..gamma.. rays (in the energy range a few keV to little above 1 MeV) from targets with Z-italic ranging from 13 to 92, using Rayleigh scattering amplitudes obtained numerically with the procedures of Kissel e-italict-italic a-italicl-italic. We consider both polarization of an unpolarized beam in scattering and asymmetries in the scattering of an initially polarized beam. Contrary to the simple form-factor predictions these polarization effects depend on atomic number (Z-italic) and photon energy (E-italic). Results obtained using only K-italic-shell Rayleigh amplitudes are not too different from those using total-atom amplitudes. In some cases polarization properties are more sensitive to Delbrueck amplitudes than the differential cross sections. Theoretical predictions are compared with the limited experimental data for both types of measurements.

Energy of microwave-emitting electrons and hard X-ray/microwave source model in solar flares

Abstract: We present a new method of estimating the energy of microwave-emitting electrons from the observed rate of increase of the microwave flux relative to the hard X-ray flux measured at various energies during the rising phase of solar flares. A total of 22 flares observed simultaneously in hard X-rays (20–400 keV) and in microwaves (17 GHz) were analyzed in this way and the results are as follows: (1) The observed energy of X-rays which vary in proportion to the 17 GHz emission concentrates mostly below 100 keV with a median energy of 70 keV. Since the mean energy of electrons emitting 70 keV X-rays is ≲130 keV or ≲180 keV, depending on the assumed hard X-ray emission model (thin-target and thick-target, respectively), this photon energy strongly suggests that the 17 GHz emission comes mostly from electrons with an energy of less than a few hundred keV. (2) Correspondingly, the magnetic field strength in the microwave source is calculated to be 500–1000 G for the thick-target case and 1000–2000 G for the thin-target case. Finally, judging from the values of the source parameters required for the observed microwave fluxes, we conclude that the thick-target model in which precipitating electrons give rise to both X-rays and microwaves is consistent with the observations for at least 16 out of 22 flares examined.