Showing papers on "Extended X-ray absorption fine structure published in 1971"

New Technique for Investigating Noncrystalline Structures: Fourier Analysis of the Extended X-Ray—Absorption Fine Structure

Abstract: We have applied Fourier analysis to our point-scattering theory of x-ray absorption fine structure to invert experimental data formally into a radial structure function with determinable structural parameters of distance from the absorbing atom, number of atoms, and widths of coordination shells. The technique is illustrated with a comparison of evaporated and crystalline Ge. We find that the first and second neighbors in amorphous Ge are at the crystalline distance within the accuracy of measurement (1%).

The absorption of radiation near 600 Å by helium

Abstract: Free-bound and free-free radiative absorptions by a pair of atoms are discussed and explicit formulas are derived for the free-bound and free-free absorption cross sections. The theory is applied to the interpretation of helium absorption near 600 a and it is argued that the first two bands observed arise from transitions into quasibound resonating vibrational levels of the A 1Σ u + state. The shape of the A 1Σ u + potential energy curve is derived for internuclear distances between 2 and 3 a. The absorption data analysis lead to a potential barrier of magnitude 0·049 ev. The effects on the absorption structure of replacing 4He by 3He are predicted. An iterative procedure is described that simultaneously locates the energy and evaluates the width of a quasibound-shape resonance state.

Two-photon-absorption laws for coherent and incoherent radiation

Abstract: The differential equation for the attenuation of a light beam transmitted through a two-photon absorber is solved for two cases: 1) nonfluctuating or 2) Gaussian-fluctuating light beams. The two cases obey entirely different absorption laws. It is further shown that the fluctuation behavior changes throughout the absorption process. The fluctuations are characterized by the intensity autocorrelation function g^{(2)}(0) . It is seen that with increasing absorption the fluctuations of the transmitted beam diminish and the spectrum of the light is modified. Two-photon absorption (TPA) is compared with two-step absorption. The latter type is discussed qualitatively for several combinations of parameters. It is shown that for this case the deviation from a linear absorption law is small. Only when the lifetime of the intermediate state is short compared to the duration of the intensity fuctuation, can a change of the fluctuation behavior and of the spectrum result. These changes are much smaller than in the case of direct TPA.

Electroreflectance, Absorption Coefficient, and Energy-Band Structure of CdGe P 2 near the Direct Energy Gap

Abstract: We report electroreflectance and absorption measurements near the direct energy gap of CdGe${\mathrm{P}}_{2}$ at 1.72 eV. The ordering, splittings, and polarization properties of the three closely spaced energy gaps derived from ${\ensuremath{\Gamma}}_{15}\ensuremath{\rightarrow}{\ensuremath{\Gamma}}_{1}$ in zinc-blende crystals are quantitatively explained by a quasicubic model taking into account the built-in uniaxial compression alone. Although the absorption coefficient is \ensuremath{\sim}45 times larger for $\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}}\ensuremath{\parallel}Z$, at low temperatures we observe in the absorption spectrum for $\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}}\ensuremath{\perp}Z$, a free exciton with a binding energy of 7.7 meV near the lowest energy gap. The observation of this exciton is independent evidence that the dichroism of the absorption edge in II-IV-${\mathrm{V}}_{2}$ crystals results entirely from the lowest energy gap having a highly anisotropic oscillator strength. The dichroism of the absorption edge is opposite to that recently assumed by Goryunova et al. in the process of optically orienting single crystals. Hence their conclusion that CdGe${\mathrm{P}}_{2}$ is negatively birefringent is in error, and in fact, CdGe${\mathrm{P}}_{2}$ is positively birefringent (extraordinary index larger than ordinary index). After allowing for this sign error, we show that the dispersion of the birefringence of CdGe${\mathrm{P}}_{2}$ can be readily explained by a theoretical model, taking into account the built-in uniaxial compression alone.

Infrared Absorption of Lattice Modes and the Silicon Local Mode in GexSi1−x Alloys

Abstract: Measurements have been made of the infrared absorption of GexSi1−x alloys for x≤0.12 and x≥0.88. The measurements were made in the range 100–700 cm−1 at temperatures from 10 to 300°K. In Si‐rich alloys (x≤0.12) absorption bands are seen near 485, 400, and 125 cm−1 which are essentially temperature independent, indicating a single‐phonon process. The bands do not correspond well with the predictions of simple models for resonance mode absorption, but are more nearly characteristic of the single‐phonon density of states for Si. In the Ge‐rich alloys (x≥0.88) the Si localized mode is observed as an absorption band at 389 cm−1 for x=0.99, increasing to 394 cm−1 at x=0.88. The absorption cross section αpeak/[Si] is only 4×10−21 cm2, smaller by 102−103 than observations for other local modes in semiconductors. Two‐phonon absorption bands at about 200 and 290 cm−1 in Ge are enhanced by the addition of Si. The over‐all temperature dependence of the absorption strength is large enough that it is not obvious whethe...

Absorption of Infrared Radiant Energy by CO 2 and H 2 O, V. Absorption by CO 2 between 1100 and 1835 cm −1 (9.1–5.5 μm)*

Abstract: Samples of CO2 at pressures up to 21.3 atm and with paths as long as 1067 m have been employed to measure CO2 absorption between 1100 and 1835 cm−1. Most of the absorption is due to the ν1 and 2ν2 bands. These bands are forbidden for the symmetric 16O12C16O and 16O13C16O molecules and, therefore, produce only pressure-induced absorption. The asymmetric 16O12C18O and 16O12C17O molecules produce bands that contain components of both intrinsic and pressure-induced absorption. The integrated-absorption coefficient for the intrinsic absorption is 1.08×10−22 (±10%) molecules −1 cm2 cm−1. The integrated pressure-induced absorption coefficient is 1.68×10−22 (±8%) molecules −1 cm2 atm−1 cm−1.

Optical Absorption Spectra of Gd3+:LaF3 and GdCl3·6H2O

Abstract: The absorption spectra of single crystals of Gd3+:LaF3 and GdCl3·6H2O were examined at low temperatures and high resolution. Stark components belonging to the 6G multiplet near 50 000 cm−1 were observed in both matrices. Theoretical treatment of the data confirmed the importance of including two‐body magnetic interactions in the analysis.

Optical Absorption by Impurities in p-Type Gallium Phosphide

Abstract: We present the results of a detailed study of optical absorption due to the oxygen donor and the Zn-O and Cd-O nearest-neighbor impurity complexes in $p\ensuremath{-}\mathrm{G}\mathrm{a}\mathrm{P}$. In solution-grown crystals doped for optimum luminescence efficiency we find that the absorption coefficient below the band gap over the visible region ($1.7 \mathrm{eV}lh\ensuremath{ u}l2.3 \mathrm{eV}$) is typically in the range 2-4 ${\mathrm{cm}}^{\ensuremath{-}1}$. Although a major fraction of the absorption in this region results from the O and the Zn-O or Cd-O centers, we find that other inadvertent impurities (e.g., Si, Cu, S, and C) also contribute to the absorption. The temperature dependence of the individual absorption bands due to Zn-O and Cd-O are found to be well described by a semiclassical single-linear-mode model for phonon-coupled impurity absorption. In contrast, the highly skewed oxygen absorption band requires (on theoretical grounds) a hybridization of the standard configuration-coordinate approach with a model which takes into account the continuum nature of the initial (valence band) state of the free-to-bound absorption. Qualitative agreement is found between the hybridized model and the shape of the oxygen band. Because of the large degree of lattice coupling associated with the oxygen transitions, we find that the oscillator strength in absorption is a factor 4-12 times stronger than that in emission. This effect is attributed to differences in the lattice configuration depending on whether the oxygen donor is in a neutral or ionized state. For the Zn-O transitions, a factor-of-3 enhancement of absorption to emission is found.

Optical Absorption of Lead in Glass

Abstract: The uv absorption spectra caused by Pb in bulk silicate glasses and in a film sputtered from 1.08 PbO·SiO2 glass are measured. Lead causes a strong absorption band between 5 and 6 eV. Vacuum uv measurements on the sputtered film reveal a second, much larger, absorption band that has a maximum near 7.6 eV and that is tentatively attributed to excitation of an O ion bonded to a Pb2+ ion.

Anisotropic X-Ray K Absorption in a Single Crystal of Gallium

Abstract: Results are given of a previous experimental investigation of $K$ absorption of unpolarized x rays in a gallium single crystal. The absorption turned out to be anisotropic as regards the amplitudes of the variations in the extended absorption fine structure; no shift of the fine-structure positions was observed. A brief survey of orientation experiments on x-ray absorption in single crystals is added. A brief summary is also given of Kronig's and Kostarev's previous absorption fine-structure theories; the inadequacy of Kronig's theory of solids in the case of a metallic single-crystal absorber is shown. The extended $K$-absorption fine structure of a single crystal in combination with linearly polarized x rays is calculated and applied to gallium, evaluating the variations of transition probability caused by elastic scattering of the electron wave by separate atoms surrounding an absorbing atom. In agreement with the experiment, an orientation dependence results, affecting primarily the amplitudes of the absorption variations, the energy positions of the maxima and minima remaining practically unchanged. The number of absorption maxima and minima up to 300 eV from the main edge is the same as in the experimental gallium $K$ spectra in the same region; the calculated energy positions agree satisfactorily with the experimental values.

INFRARED ABSORPTION STUDIES IN NEUTRON- AND ELECTRON-IRRADIATED GaAs.

Abstract: Mechanically and chemically polished samples of GaAs were irradiated with fission neutrons to fluences varying from 2 × 1016 to 1.4 × 1018 neutrons/cm2 and with 1.5 MeV electrons with doses varying from 1017 to 1018 el/cm2. Infrared absorption in irradiated crystals was measured at 300°K and 85°K. In neutron-irradiated samples absorption bands centered at 0.47 eV and at 0.25 eV are observable. It is suggested that the 0.47 eV band is connected with transitions from a level at (Ec −0.5) eV reported earlier from electrical measurements. The absorption coefficient was found to vary linearly with the square of the photon energy over a limited energy range. For electron-irradiated GaAs, the absorption in the band gap had an exponential variation with energy. The possible reasons for such a behaviour are discussed. The absorption spectrum at 85°K in electron-irradiated samples shows the presence of a band at 1.0eV. Isothermal and isochronal annealing behaviour of this absorption band was studied. An an...

Optical Absorption Induced by a Laser Pulse in II-VI Semiconductors

Abstract: Multi-photon spectroscopy on some II-VI semiconductors was carried out by monitoring the transmitted intensity of a probe light under the application of a laser light. Both the increase and decrease of the transmitted intensity were observed after the laser pulse. They correspond to the change of an absorption coefficient by the order of 10 -3 cm -1 . In comparison with the studies of the recombination radiation of excitons excited by two photons of the laser, these effects are considered to be due to the carriers created by two-photon absorption and captured by the trap states in the band gap. These states which are impurity levels or lattice defects are generally located at the lower energy side of the free-exciton recombination states. Small change of the optical absorption in a thick sample is detected by this method.