Showing papers on "Absorption (logic) published in 1986"

Theory of electron band tails and the Urbach optical-absorption edge.

Abstract: The density of states $\ensuremath{\rho}(E)$ in the tail for an electron in a correlated Gaussian random potential in three dimensions is constructed from first principles by means of a simple physical argument. This yields a linear exponential dependence of $\ensuremath{\rho}$ on $E$ which, for reasonable values of the rms potential fluctuation and correlation length, spans many decades, and occupies most of the experimentally observable energy range. This is suggested as the origin of the fundamental Urbach optical-absorption edge.

Excitation of the 1s5,1s4, 1s3, and 1s2 levels of argon by low-energy electrons.

Abstract: Excitation rate coefficients for the 1${s}_{5}$, 1${s}_{4}$, 1${s}_{3}$, and 1${s}_{2}$ levels of argon by collisions with low-energy electrons have been measured using a drift-tube technique. Time dependences of the absolute population densities of the excited levels were measured by an absorption method with a tunable diode laser as a light source. The absorption data were analyzed according to the rate equations for these levels and the excitation rate coefficient per unit length of electron drift and per argon-atom density was obtained for each level as a function of the electric field to gas density ratio E/N. The values for the 1${s}_{5}$ level vary from 2.0\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}19}$ to 2.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}17}$ ${\mathrm{cm}}^{2}$ as E/N increases from 5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}17}$ to 5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}15}$ V ${\mathrm{cm}}^{2}$. In comparison with these values, those for the 1${s}_{3}$ level are about one-fifth, those for the 1${s}_{4}$ level are about the same, and those for the 1${s}_{2}$ level are slightly larger in respective measured E/N ranges. In order to estimate the cascading effects from the higher-lying levels, excitation rate coefficients for the 2p and 3p levels have also been measured from absolute intensities of the line emissions. From a comparison of all the measured values of the excitation rate coefficients with those calculated from the Boltzmann analysis, a recommended set of cross sections for these levels has been deduced.

Photogeneration of confined soliton pairs (bipolarons) in polythiophene.

Abstract: From photoinduced absorption and light-induced electron-spin resonance, we demonstrate that the dominant photocarriers generated in polythiophene with excitation above the energy gap (${E}_{g}$) are charged bipolarons (spin 0). The observation of bipolarons (${B}^{2\ifmmode\pm\else\textpm\fi{}}$) rather than polarons (${P}^{\ifmmode\pm\else\textpm\fi{}}$) as the dominant photoexciations demonstrates that the Coulomb contribution to the bipolaron energy (${U}_{B}$) is sufficiently small that ${P}^{\ifmmode\pm\else\textpm\fi{}}$+${P}^{\ifmmode\pm\else\textpm\fi{}}$\ensuremath{\rightarrow}${B}^{\ifmmode\pm\else\textpm\fi{}2}$. From the analysis of the spectra, we find ${U}_{B}$/${E}_{g}$\ensuremath{\simeq}0.12. .AE

Optical dephasing of homogeneously broadened two-dimensional exciton transitions in GaAs quantum wells.

Abstract: We report optical dephasing of two-dimensional excitons in $\mathrm{GaAs}\ensuremath{-}{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ single quantum wells by means of time-resolved degenerate-four-wave mixing and transmission experiments in the temperature range below 80 K. The observed absorption linewidths correspond directly to the excitonic phase coherence times, confirming the homogeneous broadening of the excitonic absorption. A linear temperature dependence of the homogeneous linewidth indicates acoustic-phonon scattering as the broadening mechanism.

Nonlinear-optical interactions in fluorescein-doped boric acid glass.

Abstract: Fluorescein-doped boric acid glass is a material characterized by an extremely low saturation intensity of \ensuremath{\sim}15 mW${\mathrm{?}}^{\mathrm{\ensuremath{-}}2}$ and a nonlinear susceptibility ${\ensuremath{\chi}}^{(3)}$ as large as \ensuremath{\sim}1 esu. The saturated absorption of this material is shown both theoretically and experimentally to depend on the state of polarization of the saturating beam, even though the unsaturated absorption is polarization insensitive. Phase-conjugate reflectivities as large as 0.6% have been obtained through use of degenerate four-wave mixing in this material. These measured reflectivities are in good agreement with the predictions of a theory that includes the effects of excited-state absorption and grating washout. In addition, two-beam coupling due to the nonlinearity of saturable absorption has been demonstrated in this material. The magnitude of the coupling is maximized by inducing a frequency shift between the two beams of \ensuremath{\sim}0.1 Hz.

Multiple-scattering regime and higher-order correlations in x-ray-absorption spectra of liquid solutions

Abstract: By making a comparison between the Mn K-edge absorption of (${\mathrm{MnO}}_{4}$${)}^{\mathrm{\ensuremath{-}}}$ and [Mn(${\mathrm{H}}_{2}$O${)}_{6}$${]}^{2+}$ complexes in aqueous solution we obtain an experimental determination of the energy extent of the type-II multiple-scattering (MS) regime that is substantially wider than expected. Theoretical calculations based on the MS formalism support this conclusion. We also recognize three energy regions in the absorption spectra of these complexes: a full MS region, where numerous or an infinite number of MS paths of high order contribute (depending on whether the MS series converges or not), an intermediate MS region, where only a few MS paths of low order are relevant, and a single-scattering region where the photoelectron is backscattered only once by the ligands [extended x-ray-absorption fine-structure (EXAFS) regime]. Theoretical considerations show that this must be a general situation in x-ray-absorption spectra and opens the way to a unified scheme for their interpretation. The energy extent of the three regions is obviously system dependent. We also show how to generalize to MS contributions the usual EXAFS analysis using curved-wave propagators and indicate how to extract geometrical information from the spectra of the two clusters investigated. In particular the method is used to derive the Mn---O---O---Mn path length in the (${\mathrm{MnO}}_{4}$${)}^{\mathrm{\ensuremath{-}}}$ complex.

Fundamental radiation-induced defect centers in synthetic fused silicas: Atomic chlorine, delocalized E' centers, and a triplet state.

Abstract: A series of synthetic fused silicas of diverse OH contents was subjected to 100-keV x irradiations at 77 K and investigated by electron-spin-resonance techniques at \ensuremath{\sim}110 K or higher temperatures. Spectra were recorded at X-band frequencies (\ensuremath{\sim}9.2--9.3 GHz) both as the first derivative of absorption and in the high-power second-harmonic mode in order to bring out features not fully accessible by using one of these methods alone. In addition to the previously known ${E}_{\ensuremath{\alpha}}^{\mathcal{'}}$, ${E}_{\ensuremath{\gamma}}^{\mathcal{'}}$, and oxygen-associated hole centers, three new defects were detected and characterized by computer line-shape simulation methods. These were atomic chlorine, a delocalized E' center (denoted ${E}_{\ensuremath{\delta}}^{\mathcal{'}}$), and the first biradical to be reported in a-${\mathrm{SiO}}_{2}$. A sample-to-sample correlation of the radiation yields of these three new centers has been noted, leading to the suggestion that all three find their origins in specific chlorine-decorated precursor sites in the unirradiated glasses. Although significant chlorine impurities (g100 ppm) may be ubiquitous in both type-III (high OH) and type-IV (low OH) fused silicas, the occurrence of chlorine-associated radiation-induced defects appears to be anticorrelated with the OH contents of the materials. Some possible technological implications of these findings are discussed.

Carrier confinement and special crystallite dimensions in layered semiconductor colloids.

Abstract: Microcrystallites of the layered semiconductor ${\mathrm{PbI}}_{2}$ were prepared in colloidal form and examined by transmission electron microscopy and optical absorption. The crystallites have lateral dimensions grouped around 12, 18, and 29 A\r{} corresponding to close-packed clusters with bulk ${\mathrm{PbI}}_{2}$ symmetry. Blue-shifted absorption features can be explained by carrier confinement in these differently sized crystallites, each a single layer (\ensuremath{\sim}7 A\r{}) thick.

Kinetics of hydrogen absorption by Pd(110)

Abstract: A coupled high-pressure--ultrahigh-vacuum technique is employed to make the first kinetic measurements of high-pressure hydrogen absorption through an atomically clean palladium surface, Pd(110). The hydrogen uptake kinetics in the \ensuremath{\alpha}-phase region are found to be rate limited by diffusion of atomic H into the bulk rather than by dissociative chemisorption of ${\mathrm{H}}_{2}$ at the surface as previously reported. Our findings strongly suggest that previous high-pressure measurements of hydrogen absorption by palladium may be suspect due to uncharacterized surface contamination. The bulk diffusivity of hydrogen in palladium determined in this study has an activation barrier of 5.39\ifmmode\pm\else\textpm\fi{}0.30 kcal/mol with a preexponential factor of (2.83\ifmmode\pm\else\textpm\fi{}0.05)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}3}$ ${\mathrm{cm}}^{2}$/s.

C 1s excitation studies of diamond (111). II. Unoccupied surface states

Abstract: Unoccupied electronic states at the diamond (111) surface are studied by measuring both bulk- and surface-sensitive C 1s partial-yield soft-x-ray absorption spectra. Several absorption features are observed in the bulk band gap below the 289.19-eV bulk-C 1s absorption edge. They are associated with transitions from the C 1s surface core level to unoccupied surface states by their sensitivity to chemisorbed species and changes in their intensity as the electron escape depth is varied. These states have been detected previously with electron-energy-loss spectroscopy but no structure was resolved. The close proximity of the observed surface absorption (onset at h\ensuremath{ u}=284 eV) with the h\ensuremath{ u}=285.35 eV, C 1s\ensuremath{\rightarrow}${\ensuremath{\pi}}^{\mathrm{*}}$ transition in graphite indicates the existence of unoccupied ${\ensuremath{\pi}}^{\mathrm{*}}$ bands at the diamond (111) surface. An interpretation of these results in terms of the \ensuremath{\pi}-bonded chain model for the diamond (111) 2\ifmmode\times\else\texttimes\fi{}1 reconstructed surface is given.

Far-infrared spectroscopy on oriented films of dry and hydrated DNA

Abstract: Far-infrared measurements, between 3 and 450 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, of absorption spectra of highly oriented films of Li-DNA and Na-DNA in the temperature range 5\char21{}300 K are reported. Five low-frequency infrared-active vibrational modes are observed. The lowest infrared-active mode at 45 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for Li-DNA and 41 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for Na-DNA, as observed for the first time, is found to soften upon sample hydration. Studies of the hydration-induced absorption at the low-frequency end of the spectrum also show a pronounced absorption band at about 10 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ that is attributed to relaxation processes. Based on a simple lattice-dynamics model, the description of low-lying vibrational modes of DNA is presented. Eigenvectors of the lowest infrared-active and Raman-active modes are calculated. In addition, the model semiquantitatively describes the influence of hydration on mode frequencies.

Solitons, polarons, and phonons in the infinite polyyne chain.

Abstract: The infinite polyyne chain, ( C?C ${)}_{\mathrm{\ensuremath{\infty}}}$, or polyyne, is interpreted as a Peierls distorted one-dimensional metal with strong intrinsic electron-phonon coupling at the half-filled band level. Because there are two degenerate \ensuremath{\pi}-electron bands in polyyne the effective internal degeneracy (N) of the electrons is 4, rather than 2 as in the model of polyacetylene. This leads to a rich variety of kink-soliton and polaron states and, in the limit of a continuum description, to the interesting result that polyyne is an approximate physical realization of an N=4 Gross-Neveu model relativistic field theory. The low-lying electronic excitations of the polymer are kink-solitons with charges 0, \ifmmode\pm\else\textpm\fi{}e, and \ifmmode\pm\else\textpm\fi{}2e. The lowest-lying ionization states are a polaron, a bipolaron, and a tripolaron, with charges \ifmmode\pm\else\textpm\fi{}e, \ifmmode\pm\else\textpm\fi{}2e, and \ifmmode\pm\else\textpm\fi{}3e, respectively. Photoexcitation of the polymer leads to a neutral polaron consisting of an electron and a hole bound by lattice distortion (``polarexciton''). Both the soliton and the polarons involve the appearance of localized intragap levels that are not present in the ground state. Moreover, a localized excited state exists for the polaron, while photoexcitation of the bipolaron and the tripolaron result in soliton-antisoliton pair production. Photoinduced absorption measurements on long finite polyynes in solution should be able to confirm the photogeneration of the polarexciton. Formation of the polaron states can be expected if doping with strong electron withdrawing species is possible. An ?dd?membered polyyne is predicted to contain a soliton in its ground-state configuration, the doubly ionized closed-shell chain being particularly stable. Soliton- or polaron-bearing acetylenic chains might be of relevance in astrophysics. The ground-state acoustic and optical branches of the phonon spectrum of the discrete chain are also calculated and an expression derived for the reduction of the speed of sound due to the electron-phonon coupling.

Synchrotron radiation study of Cd1-xMnxTe (0 <= x <= 0.65).

Abstract: The electronic structure of ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te (0\ensuremath{\le}x\ensuremath{\le}0.65) has been investigated by photoemission in the photon energy range from 20 to 140 eV. A sharp (\ensuremath{\approxeq}1 eV full width at half maximum) peak located 3.4 eV below the valence-band maximum (VBM) is assigned to emission from Mn 3${d}_{\ensuremath{\uparrow}}$ states with e symmetry. The ${t}_{2}$ components hybridize significantly with the Te 5p states and contribute therefore nearly uniformly to the top 6 eV of the valence bands. Weak structures below 6 eV of mixed Mn 3d--Te 5p character occur also due to the p-d hybridization. Cd and Te 4d core-level binding energies remain constant over the whole range of Mn concentrations when measured relative to the VBM. This implies that there are negligible chemical shifts and that the VBM is not affected by the replacement of Cd by Mn. The increase in the optical gap with x is thus due to an increase of the conduction-band energy, in agreement with a shift in the Te 4d absorption threshold as measured by partial-yield spectroscopy. A maximum of the Te 5p component in the density of empty conduction states is identified \ensuremath{\approxeq}2 eV above threshold. The Mn 3p\ensuremath{\rightarrow}3${d}_{\ensuremath{\downarrow}}$ excitations are atomiclike. The results are interpreted in terms of a schematic linear-combination-of-atomic-orbitals level scheme for the band structure of ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te.

Theoretical calculation of two-photon absorption cross sections in atomic oxygen

Abstract: Two-photon absorption cross sections for transitions from the O${(\mathrm{}}^{3}$P) ground electronic state to the 3p ${}^{3}$P, 4p ${}^{3}$P, and the autoionizing ${\mathrm{O}}^{+}$(${\mathrm{}}^{2}$D)3p' and ${\mathrm{O}}^{+}$(${\mathrm{}}^{2}$P)3p'' states have been calculated by explicit evaluation of the perturbation-theory summation using matrix elements calculated from configuration-interaction wave functions and sum rules to correct for truncation. The role of different contributions in the summation has been analyzed. For the 3p ${}^{3}$P state, a cross section of ${\mathcal{J}}_{J\mathcal{'}}$${\ensuremath{\sigma}}_{0}^{(2)}$(J'\ensuremath{\leftarrow}J) of (1.319\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}35}$ ${\mathrm{cm}}^{4}$ has been obtained, which is in excellent agreement with the experimental value for the quantity ${\mathcal{J}}_{J\mathcal{'}}$${\ensuremath{\sigma}}_{0}^{(2)}$(J'\ensuremath{\leftarrow}2)${G}^{(2)}$ =(2.66\ifmmode\pm\else\textpm\fi{}0.80)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}35}$ ${\mathrm{cm}}^{4}$, given in the companion paper, if the photon statistical factor, ${G}^{(2)}$, is evaluated in the chaotic field limit (${G}^{(2)}$=2).

Correlation effects on the oscillator strength of optical absorption: Sum rule for the one-dimensional Hubbard model.

Abstract: The optical sum rule which is related to the mean kinetic energy of band electrons is evaluated for the one-dimensional Hubbard Hamiltonian. The case of a half-filled band is treated exactly using the Bethe ansatz solution of Lieb and Wu. The kinetic energy decreases monotonically as a function of U and tends to zero like 1/U for U\ensuremath{\rightarrow}\ensuremath{\infty}. For other band fillings the Gutzwiller ansatz is used. Correlation effects increase quadratically with the density n of electrons (or holes) for n\ensuremath{\ll}1 and exhibit a maximum at n=1 (i.e., for the half-filled band).

Far-infrared nonlinear optics. II. χ ( 3 ) contributions from the dynamics of free carriers in semiconductors

Abstract: We report the first bulk frequency tripling in the far-infrared. The experiments are carried out at 20 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in the doped semiconductors Ge, Si, and GaAs. Good power conversion efficiency is obtained (${10}^{\mathrm{\ensuremath{-}}3}$). Our arrangement allows an absolute determination of the nonlinear susceptibility ${\ensuremath{\chi}}^{(3)}$(3\ensuremath{\omega},\ensuremath{\omega},\ensuremath{\omega},\ensuremath{\omega}). We show that in the far-infrared, ${\ensuremath{\chi}}^{(3)}$ is dominated by free-carrier contributions. Hence, our measurement gives new insight into nonlinear transport properties such as the momentum dependence of the effective mass and of the relaxation time. Furthermore strong changes of ${\ensuremath{\chi}}^{(3)}$ (as well as of the absorption) are found to occur at high laser intensity above 100 kW/${\mathrm{cm}}^{2}$, which we attribute to carrier heating and hot carrier transfer into higher energy-band minima.

Ferromagnetic resonance in a system composed of a ferromagnetic substrate and an exchange-coupled thin ferromagnetic overlayer

Abstract: The theory of Rado and Ament and the general exchange boundary conditions of Rado and Weertman have been used to discuss the magnetic field dependence of the absorption of microwave radiation by a composite system consisting of a thin ferromagnetic overlayer exchange coupled to a thick ferromagnetic substrate. The overlayer and substrate are assumed to interact through a surface exchange interaction of the form ${E}_{\mathrm{ex}=\mathrm{\ensuremath{-}}\mathrm{J}{\mathrm{M}}_{A}\mathrm{\ensuremath{\cdot}}{\mathrm{M}}_{B}}$. In the limit of weak to moderate coupling strengths, the overlayer ferromagnetic resonance (FMR) is shifted by an effective field of the form ${\mathrm{JM}}_{B}$/d, where ${M}_{B}$ is the substrate equilibrium magnetization and d is the overlayer thickness. Pronounced effects on the strength and position in magnetic field of the substrate absorption occur when the coupling parameter J is such that the overlayer and substrate FMR's occur at nearly the same value of applied magnetic field: The microwave absorption exhibits two peaks having comparable strengths. In the limit of very strong coupling (J\ensuremath{\sim}${10}^{\mathrm{\ensuremath{-}}4}$ cm) the magnetizations in the overlayer and in the substrate precess together to yield one absorption peak at a field value which is shifted from that corresponding to the isolated substrate FMR for an unpinned surface. The shift in peak position, as well as changes in the linewidth, is caused by an effective surface pinning due to the presence of the overlayer. This pinning can be described by an effective surface energy which contains contributions from the surface pinning energy at the free surface plus contributions which are proportional to the overlayer thickness and which depend on the difference in magnetization and on the difference in volume magnetocrystalline anisotropy fields between the substrate and overlayer materials.

Intraband and interband processes in the infrared spectrum of metallic aluminum.

Abstract: The dielectric function of polycrystalline metallic aluminum derived by Shiles et al. [Phys. Rev. B 22, 1612 (1980)] from room-temperature ultrahigh-vacuum reflectance measurements has been analyzed into intraband and interband components and compared with theoretical predictions of Szmulowicz and Segall [Phys. Rev. B 24, 892 (1981)]. For this analysis, a new approach was developed, which utilizes experimental data over a larger energy range than in previous studies. To within experimental uncertainty, the resulting intraband component is consistent with a Drude model having a plasma frequency ${\ensuremath{\Omega}}_{p}$=12.5\ifmmode\pm\else\textpm\fi{}0.3 eV and relaxation time \ensuremath{\tau}=(1.06\ifmmode\pm\else\textpm\fi{}0.12)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}14}$ sec. These values are in accord with the bulk static conductivity which in turn is in agreement with the inertial sum rule on the real part of the dielectric function. The interband contribution was found to consist of a broad background with two superimposed peaks: the well-known ``0.8 \ensuremath{\mu}m'' absorption at \ensuremath{\sim}1.5 eV and a weaker absorption in the vicinity of 0.4 eV. This interband spectrum is in good agreement in regard to both peak position and oscillator strength with the one-electron augmented-plane-wave model calculations for the optical conductivity by Szmulowicz and Segall. The total experimental intraband and interband oscillator strengths for the conduction electrons are 1.9 and 1.2 electrons/per atom, respectively.

Electron-Phonon Interaction in Optical Absorption at the Si(111)2 × 1 Surface

Abstract: The temperature dependence of the optical absorption associated with surface states on the Si(111)2 \ifmmode\times\else\texttimes\fi{} 1 surface is presented. The results indicate a behavior characteristic of localized excitations with a strong electron-lattice interaction, that is, a nearly Gaussian absorption that broadens roughly as the square root of the absolute temperature and shifts to lower energies upon increasing temperature. The frequency of the surface phonons that couple to the transition is smaller than hitherto assumed for the 2 \ifmmode\times\else\texttimes\fi{} 1 surface.

Raman scattering and optical-absorption studies of the metastable alloy system GaAs x Sb 1 − x

Abstract: The optical properties of ${\mathrm{GaAs}}_{\mathrm{x}}$${\mathrm{Sb}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$ alloys grown across the entire concentration range by multitarget sputter deposition are reported. X-ray diffraction, optical absorption, and Raman spectra show the samples to be single-crystal, single-phase alloys for all x, including those in the miscibility gap. Alloy lattice constants are found to vary linearly with concentration. The direct \ensuremath{\Gamma}-point energy gaps, determined from optical-absorption measurements, show significant negative bowing. Contrary to previous reports, the Raman spectra exhibit two-mode behavior throughout, including a local mode of As in GaSb and a resonant mode of Sb in GaAs. An analysis of peak frequencies and line shapes versus concentration is given in the context of disorder effects. We observe broadenings much less severe and asymmetric than those seen in similar systems and usually interpreted in terms of k\ensuremath{ e}0 density-of-states activation. The observed adherence to zone-center selection rules for all x, suggests a more accurate interpretation to be one involving the k\ensuremath{\approxeq}0 spectral projection of the density of states.

Pulsed-laser irradiated silicon studied by time-resolved x-ray absorption (90-300 eV).

Abstract: We report the first x-ray absorption spectra of pulsed-laser irradiated amorphous Si around the Si L edge, recorded with a time resolution of 18 hs At irradiances above 017 J/${\mathrm{cm}}^{2}$ significant differences are found with the spectrum of amorphous Si The disappearance of the characteristic Si ${\mathrm{L}}_{\mathrm{I}\mathrm{I},\mathrm{I}\mathrm{I}\mathrm{I}}$ edge structure at 100 eV and the decrease in overall absorption are interpreted in terms of the metallic character of liquid Si and the formation of droplets, respectively

Sulfur 1s core-level photoionization of SF6

Abstract: Photoelectron spectra have been taken between 2460 and 2600 eV photon energy across the discrete and continuum resonances in the vicinity of the sulfur K edge in gaseous ${\mathrm{SF}}_{6}$. Results at the below-threshold S 1s\ensuremath{\rightarrow}6${t}_{1u}$ resonance indicate that ``highly excited'' S 2p and S 2s satellites (with two core holes) are the primary autoionization final states of ${\mathrm{SF}}_{6}$${\mathrm{}}^{+}$. An observed asymmetric profile in the S(LVV) Auger angular distribution suggests interference effects in the alignment of these resonantly produced ${\mathrm{SF}}_{6}$${\mathrm{}}^{+}$ ions. Decay of the low-energy S 1s continuum resonances near 2507 eV photon energy into S 2p, S 2s, and/or valence photoemission channels indicates autoionizing character. These features are assigned as doubly excited states, leading to S 1s satellite thresholds observed here for the first time. At higher photon energies, between 2520 and 2570 eV, large oscillations in the S 1s cross section are reproduced well by multiple-scattering X\ensuremath{\alpha} calculations, but are not explained adequately by single-scattering plane-wave extended x-ray absorption fine structure effects. We speculate that improvements in the description of both the electron scattering process and the molecular potential are necessary to model the diffractive and nondiffractive (barrier interaction) effects in this energy region.

Optical properties of copper indium diselenide near the fundamental absorption edge

Abstract: In this work we report on an optical absorption study near the band gap of n-type ${\mathrm{CuInSe}}_{2}$ at 7 K. From the analysis of the results the energy gap is found to be 1.02\ifmmode\pm\else\textpm\fi{}0.01 eV. The binding energy of the exciton and the ionization energy of acceptors and donors are determined to be 18, 54, and 26 (\ifmmode\pm\else\textpm\fi{}5) meV, respectively. It is suggested that ${\mathrm{In}}_{\mathrm{Cu}}$ antisite donors and ${V}_{\mathrm{Cu}}$ acceptors are the predominant active intrinsic defects in ``In-rich'' ${\mathrm{CuInSe}}_{2}$.

Spectroscopy of excited states in In 0.53 Ga 0.47 As-InP single quantum wells grown by chemical-beam epitaxy

Abstract: Photoluminescence excitation (PLE) spectra are reported for six single quantum wells with thicknesses between 130 and 23 \AA{} grown on the same wafer by chemical-beam epitaxy. The very strong, narrow line emission from these high-quality quantum wells enabled us to perform PLE with a lamp-monochromator combination as the excitation source. All of the observed excitonic absorption peaks are assigned. Good fits to the spectra can be made with band offsets of ${Q}_{e}\ensuremath{\approx}60%$ and ${Q}_{h}\ensuremath{\approx}40%$ and masses ${m}_{e}^{*}=0.041{m}_{0}$, ${m}_{\mathrm{hh}}^{*}=0.465{m}_{0}$, and ${m}_{\mathrm{lh}}^{*}=0.085{m}_{0}$. Energy-dependent corrections for ${m}_{e}^{*}$ due to conduction-band nonparabolicities are essential for these fits and yield ${\ensuremath{\gamma}}_{e}=3.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$ for the ${\ensuremath{\gamma}}_{e}{k}^{4}$ correction term in the energy dispersion.

Exciton linewidth in semiconducting quantum-well structures.

Abstract: The exciton linewidth has been calculated in semiconducting quantum-well structures as a function of the width of the well when the electron-hole pairs which are bound into the exciton are scattered by polar optical phonons. The linewidth for this scattering mechanism is found to increase as the width of the well decreases; therefore confinement of the exciton in a quantum-well structure will increase the linewidth of the exciton peaks when the exciton is scattered by polar optical phonons. The dominant contribution to the linewidth comes from processes in which the exciton is ionized into a free electron-hole pair by the absorption of a polar optical phonon. The numerical value we calculate for the linewidth is in good agreement with the experimentally observed value in GaAs/${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Al}}_{\mathrm{x}}$As quantum-well structures.

Absorption edge of Zn1-xMnxTe under hydrostatic pressure.

Abstract: We present room-temperature measurements of the pressure dependence of the absorption edge of ${\mathrm{Zn}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$${\mathrm{Mn}}_{\mathrm{x}}$Te performed in a diamond-anvil cell. A blue shift of the direct gap with pressure, which decreases with manganese concentration, is observed. The absorption at energies below the direct gap is attributed to manganese and shows a red shift under pressure. They dominate the absorption structure at sufficiently high pressures and concentrations. A first-order phase transition limits the available pressure range: The transition pressure decreases drastically with manganese concentration.

Antisite-related defects in plastically deformed GaAs.

Abstract: Optical absorption measurements on plastically deformed GaAs show that the total extrinsic absorption increases with deformation, while the quenchable EL2 absorption stays constant. The nonquenchable extrinsic absorption is observed to be proportional to the EPR measured ${\mathrm{As}}_{\mathrm{Ga}}$ containing defects produced during deformation. Since the ${\mathrm{As}}_{\mathrm{Ga}}$-related defects produced by plastic deformation anneal at T\ensuremath{\sim}650 \ifmmode^\circ\else\textdegree\fi{}C, the implication for any correlation between EL2 and ${\mathrm{As}}_{\mathrm{Ga}}$ antisites is that only those ${\mathrm{As}}_{\mathrm{Ga}}$-related EPR centers which are stable up to at least 950 \ifmmode^\circ\else\textdegree\fi{}C can possibly be responsible for the EL2 level.

Slow electrons in condensed matter

Abstract: The irreversible thermalization of energy lost by an electron in amorphous condensed matter makes successive losses incoherent. This allows us to extend a stochastic treatment of electron degradation down to the thermal range. The probability of a given momentum and energy transfer is viewed as a macroscopic property of each material, which reduces to the structure factor $S(\mathbf{q},\ensuremath{\omega})$ in the Born approximation. The identification of such bulk properties presents novel challenges for experimentalists.

Detection of soliton shape modes in polyacetylene.

Abstract: We observed soliton shape modes by photoinduced absorption in partially isomerized ${(\mathrm{CH})}_{x}$ and ${(\mathrm{CD})}_{x}$, coexisting with the soliton translational modes. The shape modes' ir activity, their dependence on the degree of isomerization, and the photoinduced spectra are explained in detail by a symmetry-breaking interaction which allows coupling of shape modes and translational modes. We also observed soliton shape-mode absorption in fully isomerized samples as phonon sidebands associated with optical transitions of neutral solitons above 1.5 eV.

X-ray-absorption study of CuBr at high pressure

Abstract: The x-ray-absorption spectrum of cuprous bromide has been measured as a function of pressure. The x-ray-absorption near-edge structure proved to be an excellent indicator of high-pressure phase transitions in this material. The normalized ``white-line'' peak heights at both the Cu and Br K edges decreased on entering the tetragonal phase and increased in going to the NaCl structure. The zinc-blende to tetragonal phase transition took place over a very narrow pressure range centered at 46\ifmmode\pm\else\textpm\fi{}5 kbar. The transformation from the tetragonal to the NaCl structure, on the other hand, showed a broad mixed-phase region, suggesting a nucleation-and-growth mechanism for the transition. The mixed-phase region was centered at 75\ifmmode\pm\else\textpm\fi{}6 kbar. No evidence of a phase between the zinc-blende and tetragonal phases was observed, presumably because it does not exist. Analysis of the extended x-ray-absorption fine-structure (EXAFS) clearly showed that there is no change in coordination in going from the zinc-blende to the tetragonal phase although the nearest-neighbor distance increases slightly. A much larger increase in ${R}_{1}$ occurs at the transition to the NaCl structure, where the coordination increases from 4 to 6. The mean-square deviation in the nearest-neighbor bond length, ${\ensuremath{\sigma}}_{1}^{2}$, appears to be a fairly smooth function of nearest-neighbor distance, decreasing (or increasing) as ${R}_{1}$ decreases (or increases) more or less independent of structure. Evidence from the literature was presented to suggest that the zinc-blende to tetragonal transition in CuBr (and also CuCl) should occur by shear deformation. The zinc-blende lattice becomes unstable under pressure due to a decrease in the bond-bending force constant. This mechanism is consistent with the results of our EXAFS study. Unfortunately, the information obtained from the EXAFS, even at low temperature, was not sufficient to solve completely the structure of the tetragonal phase.