Showing papers on "Absorption (logic) published in 1993"

Short-time behavior of the diffusion coefficient as a geometrical probe of porous media.

Abstract: We investigate the time-dependent diffusion coefficient, D(t)=〈${\mathit{r}}^{2}$(t)〉/(6t), of random walkers in porous media with piecewise-smooth pore-grain interfaces. D(t) is measured in pulsed-field-gradient spin-echo (PFGSE) experiments on fluid-saturated porous media. For reflecting boundary conditions at the interface we show that for short times D(t)/${\mathit{D}}_{0}$ =1-${\mathit{A}}_{0}$(${\mathit{D}}_{0}$t${)}^{1/2}$+${\mathit{B}}_{0}$${\mathit{D}}_{0}$t+O[(${\mathit{D}}_{0}$t${)}^{3/2}$], where ${\mathit{A}}_{0}$=4S/(9 \ensuremath{\surd}\ensuremath{\pi} ${\mathit{V}}_{\mathit{P}}$) and ${\mathit{B}}_{0}$=-HS/(12${\mathit{V}}_{\mathit{P}}$)-${\mathit{tsum}}_{\mathit{i}}$(${\mathit{L}}_{\mathit{i}}$/${\mathit{V}}_{\mathit{P}}$)f(${\mathrm{\ensuremath{\varphi}}}_{\mathit{i}}$). Here ${\mathit{D}}_{0}$ is the diffusion constant of the bulk fluid, S/${\mathit{V}}_{\mathit{P}}$ is the surface area to pore volume ratio, H is the mean curvature of the smooth portions of the surface, ${\mathit{L}}_{\mathit{i}}$ is the length of a wedge of angle ${\mathrm{\ensuremath{\varphi}}}_{\mathit{i}}$, and the function f(\ensuremath{\varphi}) is defined below. More generally, we consider partially absorbing boundary conditions, where the absorption strength is controlled by a surface-relaxivity parameter \ensuremath{\rho}. Here, the density of walkers (i.e., the net magnetization) decays as M(t)=1-\ensuremath{\rho}St/${\mathit{V}}_{\mathit{P}}$+..., and D(t) is defined as 〈${\mathit{r}}^{2}$(t)${\mathrm{〉}}_{\mathit{s}}$/(6t), where 〈${\mathit{r}}^{2}$(t)${\mathrm{〉}}_{\mathit{s}}$ is the mean-square displacement of surviving walkers. When \ensuremath{\rho}\ensuremath{ e}0 we find that the coefficient ${\mathit{A}}_{0}$ of the \ensuremath{\surd}${\mathit{D}}_{0}$t term in the above equation is unchanged, while the coefficient of the linear term changes to ${\mathit{B}}_{0}$+\ensuremath{\rho}S/(6${\mathit{V}}_{\mathit{P}}$). Thus, data on D(t) and M(t) at short times may be used simultaneously to determine S/${\mathit{V}}_{\mathit{P}}$ and \ensuremath{\rho}. The limiting behavior of D(t) as \ensuremath{\rho}\ensuremath{\rightarrow}\ensuremath{\infty} is also discussed.

Oxygen 1s x-ray absorption of tetravalent titanium oxides: A comparison with single-particle calculations.

Abstract: The oxygen 1s x-ray-absorption spectra of ${\mathrm{SrTiO}}_{3}$ and ${\mathrm{TiO}}_{2}$, in both the rutile and anatase crystal structure, are analyzed using the oxygen p-projected density of states of ground-state band-structure calculations. Good agreement is found and it is concluded that multielectron effects, transition matrix elements, and the core-hole potential present only small, largely undetectable, influences on the spectral shape. From the site- and symmetry-projected density of states the rutile peaks could be assigned to the 3d band (4--8 eV), antibonding oxygen 2p states (10--18 eV), and the titanium 4sp band (20--25 eV). For anatase the titanium 4sp band is shifted to lower energy by about 5 eV, which can be related to the lower density of anatase. From differences in the crystal structure it is argued that the core-hole potential is considerably more effective in perovskite ${\mathrm{SrTiO}}_{3}$ than in both ${\mathrm{TiO}}_{2}$ crystal structures. This is in accordance with the experimental findings.

X-ray-absorption studies of zirconia polymorphs. II. Effect of Y2O3 dopant on ZrO2 structure.

Abstract: Extended x-ray-absorption fine structure and x-ray-absorption near-edge structure spectra at both the Zr and Y K edge have been obtained at 10 K and room temperature for solid ${\mathrm{ZrO}}_{2}$-${\mathrm{Y}}_{2}$${\mathrm{O}}_{3}$ solutions Zr has very different local structures in the tetragonal and cubic solid solutions in terms of the oxygen-bond distance and coordination number In contrast, the Y local structure is nearly the same in all of the solid solutions Charge-compensating oxygen vacancies caused by Y doping are preferentially located next to Zr ions, leaving eightfold oxygen coordination for the Y ions The nearest-neighbor and next-nearest-neighbor distances in these solid solutions do not follow the virtual-crystal approximation with reference to x-ray-diffraction lattice parameter The distortion of the Zr-cation shell in cubic solid solutions is especially severe so that the long-range fluoritelike order implied by global cubic symmetry is not manifested in the local atomic environment

X-ray magnetic dichroism of antiferromagnet Fe2O3: The orientation of magnetic moments observed by Fe 2p x-ray absorption spectroscopy

Abstract: We report strong magnetic linear dichroism at the Fe ${\mathit{L}}_{2,3}$ edge of the antiferromagnet ${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ (hematite). The relative difference in absorption for light polarized parallel and perpendicular to the magnetic moment is as high as 40% at the Fe ${\mathit{L}}_{2}$ edge. The spectra are in excellent agreement with calculations of magnetic dichroism for ${\mathrm{Fe}}^{3+}$ (3${\mathit{d}}^{5}$). The magnetic origin of this dichroism is demonstrated by the Morin transition at \ensuremath{\approxeq}-10 \ifmmode^\circ\else\textdegree\fi{}C, where the moments in ${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$ rotate by 90\ifmmode^\circ\else\textdegree\fi{}. Magnetic linear dichroism may be applied to measure the spin orientation in thin films and multilayers, and to image magnetic domains.

Precision measurement of the photon recoil of an atom using atomic interferometry

Abstract: The atomic recoil from the absorption of up to 60 photons is measured using an atom interferometer. We determine \ensuremath{\Elzxh}/${\mathit{m}}_{\mathrm{Cs}}$ with a precision of ${10}^{\mathrm{\ensuremath{-}}7}$ in 2 h of integration time.

Model of the photon-avalanche effect

Abstract: The photon-avalanche effect, which involves both absorption from a metastable intermediate level and cross-relaxation energy transfer, introduces a new trend in the achievement of efficient up-conversion lasers. In this paper, we report a quite general theoretical treatment of this process which is successfully applied to the case of ${\mathrm{LiYF}}_{4}$:${\mathrm{Nd}}^{3+}$. This model shows clearly that the avalanche effect may occur, above a pumping threshold, only if the cross-relaxation energy-transfer probability is higher than the relaxation rate from the up-converted excited state to the levels located below the metastable intermediate state.

Paramagnetic Meissner effect in high-temperature superconductors

Abstract: We have studied the low-field Meissner effect of polycrystalline Bi high-temperature superconductors using a special superconducting-quantum-interference-device magnetometer. In certain samples a surprising feature was observed: Instead of the usual diamagnetic moment a paramagnetic moment develops in the field cooling mode below ${\mathit{T}}_{\mathit{c}}$ for fields H1 Oe. The data are consistent with orbital paramagnetic moments due to spontaneous currents. Such currents may originate in so-called \ensuremath{\pi} contacts in the weak-link network of polycrystalline material. In some of these samples also an anomaly in the low-field microwave absorption was observed, which is obviously correlated with the existence of spontaneous currents.

Mid-infrared optical absorption in undoped lamellar copper oxides.

Abstract: Common optical absorption features are observed near 0.5 eV in four undoped single-crystal copper oxides: ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4}$, ${\mathrm{Nd}}_{2}$${\mathrm{CuO}}_{4}$, ${\mathrm{Pr}}_{2}$${\mathrm{CuO}}_{4}$, and ${\mathrm{Sr}}_{2}$${\mathrm{CuO}}_{2}$${\mathrm{Cl}}_{2}$. These absorption bands are shown to be weakly electric dipole allowed excitations of the ${\mathrm{CuO}}_{2}$ layers. The features are ascribed to an exciton near 0.4 eV, probably of crystal field origin, with strong multimagnon sidebands. Our observations provide an important basis for understanding the doped materials.

SO2 uptake on ice spheres: Liquid nature of the ice‐air interface

Abstract: The amount of SO{sub 2} gas absorbed by ice of known surface area at equilibrium was used to estimate the volume of liquid water present at the ice-air interface at temperatures from {minus}1 to {minus}60{degrees}C. Calculations were based on Henry`s law and acid dissociation equilibrium. The liquid volume is lowest at lower temperatures and ionic strength and under most conditions was greater than the volumes calculated based on freezing-point depression. The equivalent layer thickness, assuming that liquid water is uniformly distributed around the grains, ranged from 3-30 nm at {minus}60{degrees}C to 500-3000 nm at {minus}1{degrees}C. Corresponding ionic strengths for the two temperatures were 1.7-0.0012 M and 0.005-0.00009 M. Lower values were for ice made from distilled water, and higher values were for ice made from 10{sup {minus}3} M NaCl. Estimated pH values were from 2.9 at {minus}60{degrees}C to 4.1 at {minus}1{degrees}C. Results demonstrate that gas absorption can be used to estimate an equivalent liquid volume and thickness for the ice-air interfacial region. While not directly comparable to physical measurements, the estimated values should be directly applicable to modeling uptake of SO{sub 2} and other trace gases by ice. Lack of good thermodynamic data for temperature below 0{degrees}C is the mainmore » limitation to applying this method. 23 refs., 3 figs., 1 tab.« less

Angle-dependent X-ray emission and resonance absorption in a laser-produced plasma generated by a high intensity ultrashort pulse.

Abstract: Plasmas were generated by 400 fs KrF laser pulses at intensities of \ensuremath{\sim}${10}^{17}$ W ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$ on aluminum targets. Reflectivity and x-ray emission were measured as a function of laser polarization, angle of incidence, and intensity. For the same absorbed intensity, p-polarized laser light is up to a factor of 5 more efficient in generating x rays (g0.5 keV) than s-polarized light. These results show the importance of an additional absorption process, besides collisional absorption, for short scale length plasmas that is effective for p-polarized light only and has the characteristics of resonance absorption.

Ultrahigh energy neutrino absorption by neutrino dark matter.

Abstract: If the dark matter in the Universe consists of massive neutrinos (${m}_{{\ensuremath{ u}}_{i}}\ensuremath{\simeq}30\ensuremath{-}100$ eV, $i=\ensuremath{\mu}, \ensuremath{\tau}$), a cosmic ray ${\ensuremath{ u}}_{i}$ of ultrahigh energies may be absorbed at large redshifts, or even while crossing galactic halos, due to their annihilation with the dark matter neutrinos. These processes can become very important for ${E}_{\ensuremath{ u}}\ensuremath{\simeq}\frac{{M}_{Z}^{2}}{2{m}_{\ensuremath{ u}}}\ensuremath{\simeq}5\ensuremath{-}10\ifmmode\times\else\texttimes\fi{}{10}^{19}$ eV, for which the $Z$-exchange cross section is resonant, and may lead to a dip in the cosmic ray neutrino spectrum. I compute all the relevant contributions to the $\ensuremath{ u}$ cross section and use them to evaluate the absorption redshift at ultrahigh energies and to analyze the absorption probability at large redshifts as well as in halos of galaxies. If the determination of the neutrino spectrum at those energies were to become experimentally feasible, the observation of the features discussed here would provide a clear indication of the nature of the dark matter.

Analysis of SiH and SiN vibrational absorption in amorphous SiNx:H films in terms of a charge-transfer model.

Abstract: Amorphous ${\mathrm{SiN}}_{\mathit{x}}$:H films were deposited at 300 \ifmmode^\circ\else\textdegree\fi{}C by rf glow discharge of ${\mathrm{SiH}}_{4}$-${\mathrm{NH}}_{3}$ mixtures, and the SiH and SiN stretching absorption were investigated as a function of the N content x From the x dependence of the absorption intensity, the bonding structure was decomposed into five bonding units without H atoms and those with H atoms, on the basis of a modified random-bonding model For each bonding unit, the oscillator strength of the SiH absorption and the SiH and SiN bond lengths were estimated from the values of the additional partial charge on the Si, N, and H atoms by using Sanderson's model The peak wave numbers of the SiH and SiN absorptions were calculated by linking them to their respective bond-length values, a procedure which is based on an assumption that the force constant will be mainly controlled by the bond length The effective SiH and SiN peak wave numbers for a given value of x, obtained as a statistical average of the calculated ones for individual bonding units, agreed well with the experimentally determined values Also, peak shifts of the SiH absorption for various bonding units and those of the SiN absorption due to incorporation of SiH, SiN, and NH bonds are discussed in terms of the above charge-transfer model

Unified formulation of excitonic absorption spectra of semiconductor quantum wells, superlattices, and quantum wires.

Abstract: The problem of hydrogenic systems placed into strongly anisotropic media is solved exactly by using a metric space with a noninteger dimension \ensuremath{\alpha} (1). This appraoch is an elegant and convenient way to treat the case of Wannier-Mott excitons confined in semiconductor superlattices, quantum wells, and quantum-well wires. Indeed, the relative motion of the electron-hole pair which constitutes such excitons can never be considered strictly one dimensional (1D), 2D, or 3D. In this paper, we propose a quantitative analysis of the shape of the optical-absorption edge near an excitonic energy gap, for any arbitrary value of \ensuremath{\alpha}. We present an exact generalization of the calculations performed in the effective-mass approximation for allowed transitions by Elliot [Phys. Rev. 108, 1384 (1957)] in the three-dimensional case, and by Shinada and Sugano [J. Phys. Soc. Jpn. 21, 1936 (1966)] for two-dimensional media: this model includes contributions of bound states and of the so-called unbound states, which are responsible for an enhanced absorption continuum above the interband energy gap. At high energies, this continuum tends to behave like the \ensuremath{\alpha}-dimensional valence-to-conduction joint density of states. The versatility of this approach should be particularly useful for modeling and improving the dynamic properties of optical modulators, for which not only the energy gap, but also the dimensionality of the excitonic absorption onset is modulated.

Synchrotron-radiation photoemission and x-ray absorption of Fe silicides.

Abstract: The valence-band and core-electron states of the iron silicides FeSi, \ensuremath{\alpha}-${\mathrm{FeSi}}_{2}$, and \ensuremath{\beta}-${\mathrm{FeSi}}_{2}$, were measured by synchrotron-radiation photoemission and x-ray absorption. The analysis of the Fe 3s, Fe 3p, and Si 2p core-level photoemission line shape is made by comparing the silicide data with the data for bcc Fe, hcp Co, fcc Cu, ${\mathrm{CoSi}}_{2}$, and Si(111)7\ifmmode\times\else\texttimes\fi{}7 surfaces. The Fe 3p spin-orbit splitting in silicides is resolved spectroscopically. The results are discussed in connection with the electronic properties of the silicides and also provide reference standards for the very active spectroscopic research on the metastable interface-silicide phases of iron.

Spectra of one-dimensional excitons in polysilanes with various backbone conformations

Abstract: Linear and nonlinear optical spectra (one-photon and two-photon absorption, photoluminescence, and electroabsorption spectra) have been investigated for solid thin films of polysilanes with a variety of conformations, such as trans-planar, alternating trans-gauche (TGTG' type), 7/3 helical, and disordered backbone structures. Optical spectra have revealed features of one-dimensional exciton states characteristic of the respective conformations. In particular, the lowest allowed and second parity-forbidden (or two-photon allowed) transitions can be assigned to the ground ${(}^{1}$${\mathit{B}}_{1\mathit{u}}$) and first excited ${(}^{1}$${\mathit{A}}_{\mathit{g}}$) states of one-dimensional Wannier-type exciton, respectively. Furthermore, the higher excited exciton states and subsequent band-to-band transitions show up in the electroabsorption spectra for polysilanes with various conformations of the Si backbones. Quantitative analysis based on a microscopic theory for the one-dimensional excitons is also presented.

γ rays and neutrinos from a powerful cosmic accelerator

Abstract: Possibly the powerful radio quasar 3C273 will reveal its nature as an efficient proton accelerator up to energies of order ${10}^{11}$ GeV in the near future. It is shown in this paper that the shock-accelerated protons expected to be present in the quasar's plasma jet induce an unsaturated synchrotron cascade with electromagnetic radiation emerging in the x-ray and \ensuremath{\gamma}-ray ranges. While (including the synchrotron emission from the accelerated primary electrons) the broadband nonthermal emission from 3C273 can be explained over the observed 18 orders of magnitude, a flattening of the spectrum at the highest observed energies (a few GeV) is predicted that could be falsified by the Energetic Gamma Ray Experiment Telescope on board the Compton Gamma Ray Observatory. Above \ensuremath{\approxeq}100 GeV the cascade spectrum dramatically steepens again due to the absorption of the \ensuremath{\gamma}-ray photons by the host galaxy's strong infrared photon field from extended dust clouds, in accordance with the nondetection of 3C273 by Cher\'enkov telescopes. However, neutrinos from the hadronic interactions initiating the cascade are not damped and reach terrestrial experiments without any modification of their injected flux. In contrast with the neutrino flux from pp interactions, which are energetically unimportant in jets, p\ensuremath{\gamma} interactions generate a flat neutrino flux. Therefore it is emphasized that one must not simply normalize the expected neutrino flux by the observed \ensuremath{\gamma}-ray flux. Hence it is shown that the expected neutrino flux in the energy range relevant for underwater or under-ice detectors is much lower than assumed by many authors. On the other hand, with an increasing number of cosmic \ensuremath{\gamma}-ray sources at known positions, their neutrino detection should be feasible when it is realized that the angular resolution is the crucial design property for neutrino detectors.

Search for invisible decay of orthopositronium

Abstract: Orthopositronium (o-Ps) decay or conversion into ``invisible'' final states is searched for by measuring the energy deposited in a hermetic photon detector for each positron produced from ${\mathrm{\ensuremath{\beta}}}^{+}$ decay of $^{22}\mathrm{Na}$ and stopped by an aerogel target. No invisible event is detected for a total of ${10}^{8}$ stopping positrons, thereby giving an upper limit of 2.8\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}6}$ on the o-Ps branching ratio, which is 200 times more stringent than previous limits. This experiment excludes an invisible decay as the origin of the reported discrepancy on o-Ps lifetime, provides a limit of \ensuremath{\epsilon}1.5\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}8}$ on the photon--mirror-photon mixing, and rules out millicharged particles lighter than 500 keV.

Synthesis of poly(1-aminonaphthalene) and poly(1-aminoanthracene) by chemical oxidative polymerization and characterization of the polymers

Abstract: Poly(1-aminonaphthalene) and poly(1-aminoanthracene) have been synthesized in high yields (90--96%) by chemical oxidative polymerization of 1-aminonaphthalene and 1-aminoanthracene using H{sub 2}O{sub 2} in the presence of Fe catalyst. The oxidative polymerization of 1-aminonaphthalene and 1-aminoanthracene proceeds via successive coupling that gives the polymer structure similar to polyaniline. The polymers are brown powders and soluble in dimethyl sulfoxide (DMSO), dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), H{sub 2}SO{sub 4}, and HCOOH. The obtained poly(1-aminonaphthalene) and poly(1-aminoanthracene) show electrical conductivities of 1.7 {times} 10{sup {minus}6} and 1.6 {times} 10{sup {minus}4} S cm{sup {minus}1}, respectively, which increase to 3.8 {times} 10{sup {minus}4}-1.5 {times} 10{sup {minus}3} S cm{sup {minus}1} on doping with HCl or I{sub 2}. Light scattering measurements of the polymers in NMP shows a large degree of depolarization ({rho}{sub v} = 0.33), indicating that the polymers have a linear and stiff structure. The number-average molecular weights (M{sub n}) of poly(1-aminonaphthalene) and poly(1-aminoanthracene) obtained are determined as 4,300 and 4,500, respectively, with narrow molecular weight distribution by gel permeation chromatography (vs polystyrene). The weight-average molecular weight (M{sub b}) of poly(1-aminoanthracene) determined by the light scattering method is 7,000. {sup 1}H NMR spectra of PNA and PAA in DMSO-d{sub 6} give rise to the absorption of themore » NH hydrogen in the region of {delta} 5--6 ppm, and the NH hydrogen of PNA is exchangeable with the water hydrogen contained in DMSO-d{sub 6} on the NMR time scale with an activation energy of 27 kJ mol{sup {minus}1}. PNA and PAA do not show a distinct absorption peak in the visible region.« less

Molecular-dynamics simulation of the trivalent europium ion doped in sodium disilicate glass: Electronic absorption and emission spectra.

Abstract: We report the simulation of the optical absorption and emission spectra of the ${\mathrm{Eu}}^{3+}$ ion doped in a sodium disilicate glass. A model of this glass was previously simulated by the molecular-dynamics technique. We employ a full treatment, including J mixing, of the point-charge crystal-field method developed for doped crystalline materials in order to simulate (1) the $^{5}$${\mathit{L}}_{6}$${,}^{5}$${\mathit{D}}_{3,2,1,0}$${\ensuremath{\leftarrow}}^{7}$${\mathit{F}}_{0,1}$ absorption spectrum and (2) the $^{5}$${\mathit{D}}_{0}$${\ensuremath{\rightarrow}}^{7}$${\mathit{F}}_{0--6}$ emission spectrum of the ${\mathrm{Eu}}^{3+}$ ion. This produces simulated spectra with correct energies and relative intensities. A comparsion to experimental room-temperature absorption and fluorescence spectra of the corresponding laboratory glass is presented. By combining the simulated structural model with the calculated optical spectra, we are able to investigate spectra-structure correlations of doped inorganic glasses.

Infrared absorption in superlattices: A probe of the miniband dispersion and the structure of the impurity band.

Abstract: Infrared spectroscopy is used to study optical transitions between the two lowest minibands in strongly coupled n-type GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As superlattices. Two absorption maxima, related to the critical points of the mini-Brillouin zone, are clearly observed. The asymmetric shape of the spectrum can be reproduced theoretically, when the variation of the oscillator strength over the mini-Brillouin zone is taken into account. In a superlattice which is doped not too heavily, but metallic, a third absorption line appears at low temperatures. It can be identified with the 1s-2${\mathit{p}}_{\mathit{z}}$ donor transition and provides a sensitive probe to investigate the density of states near the metal-insulator transition. The existence of this transition implies that the impurity band has not merged with the conduction band despite the metallic behavior.

Structure and dynamics of surface crystallization of liquid n-alkanes.

Abstract: Formation of a crystalline monolayer at the surface of an absorbed liquid n-heptadecane film above the bulk solidification temperature, with the molecules in the layer hexagonally packed and their axes tilted by \ensuremath{\sim}25\ifmmode^\circ\else\textdegree\fi{} from the surface normal, is found using molecular-dynamics simulations, corroborating recent observations. A crystallized adsorbed thin film (\ensuremath{\sim}40 \AA{}) of hexadecane is found to consist of four layers of molecules oriented parallel to the substrate and a topmost hexagonally packed layer with molecules oriented toward the normal.

Raman- and infrared-active phonons in YBaCuFeO 5 : Experiment and lattice dynamics

Abstract: The results of polarized Raman scattering, far-infrared (FIR) absorption, Kramers-Kronig analysis of FIR reflectance, and lattice-dynamical calculations of the perovskitelike compound ${\mathrm{YBaCuFeO}}_{5}$ (space group P4mm) are reported. All 16 Raman- and infrared-active \ensuremath{\Gamma}-point phonons (6${\mathit{A}}_{1}$+2${\mathit{B}}_{1}$+8E) have been identified and assigned to definite atomic vibrations in close comparison with the structurally similar tetragonal ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6}$ system. The appearance of an additional strong Raman line at 576 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ is discussed in terms of disorder-induced Raman scattering. The normal-mode frequencies are in reasonable agreement with the results of lattice-dynamical calculations in the framework of a shell model with parameters extracted from several metallic oxides and perovskite compounds.

Observation of nonisotropic Auger angular distribution in the C(1s) shape resonance of CO.

Abstract: Angle-resolved high-resolution C(KVV) Auger spectra of CO were taken in the vicinity of the C(1s) ${\mathrm{\ensuremath{\sigma}}}^{\mathrm{*}}$ shape resonance. These spectra show clear evidence for the theoretically predicted anisotropic K-shell Auger emission in molecules. Complementary results from angle-resolved photoion spectroscopy show that the small size of the observed effect is, besides the varying intrinsic anisotropy of the Auger decay, also due to a smaller anisotropy in the primary absorption process than originally predicted but in good agreement with more recent calculations. Contrary to this, satellite Auger transitions show unexpectedly large anisotropies.

Transient and steady-state photoconductivity of a solid C60 film.

Abstract: We report the experimental results of the subnanosecond time-resolved transient photoconductivity (PC) of solid ${\mathrm{C}}_{60}$ film at various photon energies, and the spectral response of the steady-state PC in the energy range between 1.5 and 4.5 eV. The initial fast transient PC response decays exponentially in the subnanosecond time regime, followed by a weak slower component. Decay time at \ensuremath{\Elzxh}\ensuremath{\omega}=2.0 eV is longer than that at \ensuremath{\Elzxh}\ensuremath{\omega}=2.6 and 2.9 eV. At \ensuremath{\Elzxh}\ensuremath{\omega}=2.0 eV, transient PC peak shows a superlinear intensity dependence, suggesting carrier generation via exciton-exciton collision ionization, consistent with the recent results of fast photoinduced absorption. However, the linear intensity dependence of the transient PC at \ensuremath{\Elzxh}\ensuremath{\omega}=2.6 and 2.9 eV and a sharp increase of the transient and steady-state PC response at \ensuremath{\Elzxh}\ensuremath{\omega}\ensuremath{\approxeq}2.3 eV indicate direct photogeneration of free electrons and holes at \ensuremath{\Elzxh}\ensuremath{\omega}g2.3 eV.

Correlation of preexisting diamagnetic defect centers with induced paramagnetic defect centers by ultraviolet or vacuum-ultraviolet photons in high-purity silica glasses

Abstract: Electron-spin-resonance (ESR) and vacuum-ultraviolet (vuv) absorption measurements were performed on a series of high-purity silica glasses exposed to 6.4-eV photons, 7.9-eV photons from excimer lasers, and to \ensuremath{\gamma} rays. The concentration of defect centers varies from ${10}^{14}$ to ${10}^{16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ depending on the method of material fabrication and photon energy of the irradiating lasers. Variation of the defect species with both the incident-photon energy and manufacturing condition is observed by ESR measurements. E' centers (\ifmmode^\circ\else\textdegree\fi{}Si\ensuremath{\cdot}) are observed in all types of silicas. Nonbridging-oxygen hole centers (NBOHC's \ifmmode^\circ\else\textdegree\fi{}Si-O\ensuremath{\cdot}) in high-OH silica ([OH]\ensuremath{\approxeq}1000 ppm) and peroxy radicals (PR's, \ifmmode^\circ\else\textdegree\fi{}Si-O-O\ensuremath{\cdot}) in oxygen-surplus silica ([OH]1 ppm) are observed, only when the samples are exposed to 7.9 eV photons. The defect centers observed in \ensuremath{\gamma}-irradiated silica are qualitatively in agreement with those in 7.9 eV laser-irradiated silica. The variation of defect species with manufacturing methods indicates that the observed paramagnetic centers are created from preexisting defects. Concentration of the defects induced by either 6.4 or 7.9 eV laser photons is proportional to the square of the pulse energy, indicating that two-photon absorption process dominates in the defect formation. The defect formation process can be understood in terms of the creation of an electron-hole (e-h) pair by two-photon excitation and the subsequent hole trapping or decay of an e-h pair at the site of preexisting defects. Dependence of the induced-defect species on incident-photon energy can be explained by the variation in the energy level of preexisting defects or in the defect formation energy. Results of vuv-absorption measurements reveal the conversion of diamagnetic precursor defects (e.g., \ifmmode^\circ\else\textdegree\fi{}Si-Si\ifmmode^\circ\else\textdegree\fi{}, \ifmmode^\circ\else\textdegree\fi{}Si-O-O-Si\ifmmode^\circ\else\textdegree\fi{}, and \ifmmode^\circ\else\textdegree\fi{}Si-OH) introduced during the manufacturing process, into paramagnetic defect centers, the E' centers, NBOHC's, and PR's.

Electro-optic properties of CdS embedded in a polymer.

Abstract: CdS microcrystals under weak confinement conditions embedded in a polyvinyl alcohol polymer film have been optically characterized by linear absorption and time-resolved luminescence spectroscopy. They are studied in the presence of an external electrical field at low and room temperature and different densities of photoexcited carriers. By applying a voltage of 400 V corresponding to an external electric-field strength of 5\ifmmode\times\else\texttimes\fi{}${10}^{4}$ V/cm, the observed absorption change in this material is \ensuremath{\Delta}\ensuremath{\alpha}/\ensuremath{\alpha}\ensuremath{\approxeq}0.07. The electrical field produces a redshift of the absorption band which is explained in terms of the quantum-confined Stark effect. In addition, a restoring of the oscillator strength is observed and explained by screening effects of internal fields in the interface region by the photogenerated electron-hole pairs. At high laser excitation an electric-field-induced change of the nonlinear absorption spectrum of \ensuremath{\Delta}\ensuremath{\alpha}/\ensuremath{\alpha}=0.25 is achieved, giving a considerable modulation of the absorption edge. The electrical field separates the laser excited carriers and the change in the absorption is attributed to a compensation of the many-particle interaction by the external electrical field.

Model of self-trapped excitons in alkali halides.

Abstract: We have carried out an ab initio many-electron variational calculation of the adiabatic potential-energy surface (APES) for the lowest triplet state of the self-trapped exciton (STE) in LiCl, NaCl, and KCl. Calculations of the H center in these crystals show that the 〈111〉 orientation is favored, in agreement with experimental results for NaCl but not for KCl, in which it is oriented along a 〈110〉 direction (no measurement exists for LiCl), and hence most detailed calculations for STE's are carried out for NaCl. It is found that the APES minimum for each crystal occurs when the ${\mathrm{Cl}}_{2}^{\mathrm{\ensuremath{-}}}$ molecular ion is displaced along its molecular axis from its symmetrical position (${\mathit{D}}_{2\mathit{h}}$) nearly halfway to the nearest halogen lattice point. The calculated transition energies for the optical absorption and luminescence at this configuration agree with the experimental values for the triplet STE, although the calculated stretching vibration frequency of the ${\mathrm{Cl}}_{2}^{\mathrm{\ensuremath{-}}}$ molecular ion in NaCl is much smaller than that for the H center, contradictory to recent resonant Raman studies. Other minima are found at the nearest F-H pair configuration, in which the ${\mathrm{Cl}}_{2}^{\mathrm{\ensuremath{-}}}$ molecular ion is reoriented by 90\ifmmode^\circ\else\textdegree\fi{} from the initial orientation and next-nearest F-H pair. Extremely small luminescence energy at these configurations excludes the possibility that they are the candidates for the luminescent state of the STE. It is found that, after the displacement of the ${\mathrm{Cl}}_{2}^{\mathrm{\ensuremath{-}}}$ molecular ion beyond the first minimum of the APES towards the nearest F-H pair configuration, the total energy is lowered by reorientation, inducing an anomaly on the APES. The results of a recent experimental investigation, including existence of several types of relaxed configuration of the STE in alkali halides, the stretching vibration frequency, and the femtosecond oscillation on APES, are discussed on the basis of the results of the calculation.

Exciton radiative decay and homogeneous broadening in CdTe/Cd0.85Mn0.15Te multiple quantum wells.

Abstract: Exciton resonances are observed in the room-temperature absorption and photoluminescence (PL) spectra of CdTe/${\mathrm{Cd}}_{0.85}$${\mathrm{Mn}}_{0.15}$Te multiple-quantum-well structures. Temperature-dependent absorption linewidth and PL decay measurements are reported. Above 45 K, exciton-LO-phonon scattering determines the homogeneous linewidth (${\mathrm{\ensuremath{\Gamma}}}_{\mathit{H}}$). High excitation intensities were used in the PL decay measurements so that delocalized excitons were created. The PL decay time is found to be a measure of the exciton radiative decay time between 45 and 145 K and it increases linearly with ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{H}}$/r(T), where r(T) is the fraction of excitons with transition frequencies in ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{H}}$. From 170 K nonradiative processes dominate the decay so that the PL decay time decreases with temperature.

Characterization of the nitrate complexes of Pu(IV) using absorption spectroscopy, {sup 15}N NMR, and EXAFS

Abstract: Nitrate complexes of Pu(IV) are studied in solutions containing nitrate up to 13 molar (M). Three major nitrato complexes are observed and identified using absorption spectroscopy, {sup 15}N nuclear magnetic resonance (NMR), and extended x-ray absorption fine structure (EXAFS) as Pu(NO{sub 3}){sub 2}{sup 2+}, Pu(NO{sub 3}){sub 4}, and Pu(NO{sub 3}){sub 6}{sup 2{minus}}. The possibility that Pu(NO{sub 3}){sub 1}{sup 3+}, Pu(NO{sub 3}){sub 3}{sup 1+} and Pu(NO{sub 3}){sub 5}{sup 1{minus}} are major species in solution is not consistent with these results and an upper limit of 0.10 can be set on the fraction for each of these three nitrate complexes in nitrate containing solutions. Fraction of the three major species in nitric acid over the 1--13 M range were calculated from absorption spectra data. The fraction of Pu(NO{sub 3}){sub 6}{sup 2{minus}} as a function of nitric acid concentration is in good agreement with the literature, whereas the fraction of Pu(NO{sub 3}){sub 2}{sup 2+} and Pu(NO{sub 3}){sub 4} species differ from previous studies. We have modeled the chemical equilibria up to moderate ionic strength ( < 6 molal) using the specific ion interaction theory (SM. Comparison of our experimental observations to literature stability constants that assume the presence of mononitrate species is poor. Stability constant at zero ionic strength for the dinitrato complex is determined to be log({beta}{sub 2}{sup 0})=3.77 {plus_minus} 0.14 (2{sigma}).

Multiple time scale analysis of interacting diffusions

Abstract: We consider the questions: how can the long term behavior of large systems of interacting components be described in terms of infinite systems? On what time scale does the infinite system give a qualitatively correct description and what happens at large (resp. critical) time scales? LetY N (t) be a solution (y i N (t))i∈[−N,N]of the system of stochastic differential equations (w i (t) are i.i.d. brownian motions) $$dy_i^N (t) = \left( {\frac{1}{{2N + 1}}\sum\limits_{j = - N}^N {y_j^N (t) - y_i^N (t)} } \right)dt + \sqrt {2g(y_i^N (t))} dw_i (t).$$ In the McKean-Vlasov limit,N→∞, we obtain the infinite independent system $$dy_i^\infty (t) = (E(y_i^\infty (t)) - y_i^\infty (t))dt + \sqrt {2g(y_i^\infty (t))} dw_i (t),i \in Z.$$ This infinite system has a one parameter set of invariant measures $$v_\Theta = \mathop \otimes \limits_{x \in Z} \Gamma _\Theta $$ with Γθ the equilibrium measure of $$dx(t) = (\Theta - x(t))dt + \sqrt {2g(x(t))} dw(t)$$ . LetQ s(·,·) be the transition kernel of the diffusion with generator $$u_g (x)\left( {\frac{\partial }{{\partial x}}} \right)^2 $$ with $$u_g (x) = \int {g(y)\Gamma x(dy)} $$ . Then one main result is that asN→∞ $$\mathcal{L}((Y^N (s(2N + 1)))) \Rightarrow \int {Q_s (\Theta ',d\Theta )v_\Theta ,\Theta '} = E(y_0 ).$$ This provides a new example of a phenomenon also exhibited by the voter model and branching random walk. In particular we are also able to modify our model by adding the termcN −1(A−y i N (t))dt to obtain the first example in which the analog ofQ s (·,·) converges to an honest equilibrium instead of absorption in traps as in all models previously studied in the literature. Finally, we discuss a hierarchical model with two levels from the point of view discussed above but now in two fast time scales.