Showing papers on "Absorption (logic) published in 1999"

Search for Time Variation of the Fine Structure Constant

Abstract: An order of magnitude sensitivity gain is described for using quasar spectra to investigate possible time or space variation in the fine structure constant $\ensuremath{\alpha}$. Applied to a sample of 30 absorption systems, spanning redshifts $0.5lzl1.6$, we derive limits on variations in $\ensuremath{\alpha}$ over a wide range of epochs. For the whole sample, $\ensuremath{\Delta}\ensuremath{\alpha}/\ensuremath{\alpha}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(\ensuremath{-}1.1\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$. This deviation is dominated by measurements at $zg1$, where $\ensuremath{\Delta}\ensuremath{\alpha}/\ensuremath{\alpha}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(\ensuremath{-}1.9\ifmmode\pm\else\textpm\fi{}0.5)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$. For $zl1$, $\ensuremath{\Delta}\ensuremath{\alpha}/\ensuremath{\alpha}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}(\ensuremath{-}0.2\ifmmode\pm\else\textpm\fi{}0.4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$. While this is consistent with a time-varying $\ensuremath{\alpha}$, further work is required to explore possible systematic errors in the data, although careful searches have so far revealed none.

Electromagnetically induced absorption

Abstract: A large increase in atomic absorption due to coherent interaction with resonant radiation is predicted for a closed transition between two degenerate atomic levels verifying $0l{F}_{g}l{F}_{e}{(F}_{g}$ and ${F}_{e}$ are the total angular momentum of the ground and the excited levels, respectively). In good agreement with the theoretical prediction, a total absorption enhancement by a factor 1.7 was obtained on the ${D}_{2}$ line of ${}^{85}\mathrm{Rb}\mathrm{}$ in a vapor cell experiment.

Albedo and Reflection Spectra of Extrasolar Giant Planets

Abstract: We generate theoretical albedo and reflection spectra for a full range of extrasolar giant planet (EGP) models, from Jovian to 51-Pegasi class objects. Our albedo modeling utilizes the latest atomic and molecular cross sections, a Mie theory treatment of extinction by condensates, a variety of particle size distributions, and an extension of the Feautrier radiative transfer method which allows for a general treatment of the scattering phase function. We find that due to qualitative similarities in the compositions and spectra of objects within each of five broad effective temperature ranges, it is natural to establish five representative EGP albedo classes: a ``Jovian'' class (T$_{\rm eff} \lesssim 150$ K; Class I) with tropospheric ammonia clouds, a ``water cloud'' class (T$_{\rm eff} \sim 250$ K; Class II) primarily affected by condensed H$_2$O, a ``clear'' class (T$_{\rm eff} \gtrsim 350$ K; Class III) which lacks clouds, and two high-temperature classes: Class IV (900 K $\lesssim$ T$_{\rm{eff}}$ $\lesssim$ 1500 K) for which alkali metal absorption predominates, and Class V (T$_{\rm{eff}}$ $\gtrsim$ 1500 K and/or low surface gravity ($\lesssim$ 10$^3$ cm s$^{-2}$)) for which a high silicate layer shields a significant fraction of the incident radiation from alkali metal and molecular absorption. The resonance lines of sodium and potassium are expected to be salient features in the reflection spectra of Class III, IV, and V objects. We derive Bond albedos and effective temperatures for the full set of known EGPs and explore the possible effects of non-equilibrium condensed products of photolysis above or within principal cloud decks. As in Jupiter, such species can lower the UV/blue albedo substantially, even if present in relatively small mixing ratios.

Mechanistic studies of the reaction of reduced methane monooxygenase hydroxylase with dioxygen and substrates

Abstract: Soluble methane monooxygenase (sMMO) catalyzes the oxidation of methane to methanol. Single-turnover reactions of sMMO from Methylococcus capsulatus (Bath) were studied by stopped-flow optical spectroscopy to examine further the activated dioxygen intermediates and their reactions with hydrocarbon substrates. A diiron(III) peroxo species designated H{sub peroxo} is the first intermediate observed in the reaction between the chemically reduced hydroxylase (H{sub red}) and dioxygen. The optical spectrum of this species determined by diode array detection is presented for the first time and exhibits visible absorption bands with {lambda}{sub max} {approx} 420 nm ({epsilon} = 4,000 M{sup {minus}1} cm{sup {minus}1}) and {lambda}{sub max} = 725 nm ({epsilon} = 1,800 M{sup {minus}1} cm{sup {minus}1}). The temperature dependences of the rate constants for formation and decay of H{sub peroxo} and for the subsequent intermediate, Q, were examined in the absence and in the presence of hydrocarbon substrates, and activation parameters for these reactions were determined. For single-turnover reaction kinetics monitored at 420 nm, the {lambda}{sub max} for Q, a nonlinear Eyring plot was obtained when acetylene or methane was present in sufficiently high concentration. This behavior reflects a two-step mechanism, Q formation followed by Q decay, in which the rate-determining step changes depending on themore » temperature. The rate of H{sub peroxo} formation does not depend on dioxygen concentration, indicating that an effectively irreversible step involving dioxygen precedes formation of the diiron(III) peroxo species. The rate constant observed at 4 C for H{sub peroxo} formation, 1--2 s{sup {minus}1}, is slower than that determined previously by Moessbauer and optical spectroscopy, {approximately}20--25 s{sup {minus}1} (Liu, K. E., et al. J. Am. Chem. Soc. 1995, 117, 4997--4998; 10174--10185). Possible explanations for this discrepancy include the existence of two distinct peroxo species. Intermediate Q exhibits photosensitivity when monitored by diode array methodology, a property that may arise from enhanced reactivity of a transient charge-transfer species. The photoreaction can be avoided by using a monochromator to obtain kinetics data at single wavelengths. The reactions of substrates with intermediate species were studied by single- and double-mixing stopped-flow spectroscopy. The Q decay rate exhibits an approximate first-order dependence on substrate concentration for a wide range of hydrocarbons, the relative reactivity varying according to the sequence acetylene > ethylene > ethane > methane > propylene > propane. In addition, the data indicate that H{sub peroxo} can oxidize olefins but not acetylene or saturated hydrocarbons.« less

Absorption-Limited Generation of Coherent Ultrashort Soft-X-Ray Pulses

Abstract: Coherent growth of high-order harmonic radiation has been extended to propagation lengths comparable to the XUV absorption length at wavelengths as short as 10 nm range for the first time. Absorption-limited production of high harmonics of sub-10-fs near-infrared laser pulses in gas jets gives rise to a harmonic conversion efficiency of $(3--4)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ in the range of 10--13 nm in neon and some 2 orders of magnitude higher at about 30 nm in argon. This kHz-repetition-rate ultrafast soft-x-ray source emits approximately ${10}^{10}$ photons per sec within a 0.9-nm bandwidth of a typical Mo:Si mirror at 13.4 nm in a near-diffraction-limited beam and opens up the way to soft-x-ray nonlinear optics and the measurement of sub-fs soft-x-ray pulses.

High Real-Space Resolution Measurement of the Local Structure of Ga 1-x In x As Using X-Ray Diffraction

Abstract: High real-space resolution atomic pair distribution functions PDFs from the alloy series $\mathrm{Ga}{}_{1\ensuremath{-}x}{ln}_{x}\mathrm{As}$ have been obtained using high-energy x-ray diffraction. The first peak in the PDF is resolved as a doublet due to the presence of two nearest neighbor bond lengths, Ga-As and In-As, as previously observed using x-ray absorption fine structure. The widths of nearest, and higher, neighbor PDF peaks are analyzed by separating the broadening due to static atom displacements from the thermal motion. The PDF peak width is 5 times larger for distant atomic neighbors than for nearest neighbors. The results are in agreement with model calculations.

Elimination of self-absorption in fluorescence hard-x-ray absorption spectra

Abstract: Fluorescence detection is a convenient way to measure x-ray absorption spectra in situations where samples cannot be made in the required configuration. However, self-absorption effects cause considerable distortion of spectra measured in fluorescence. We describe a straightforward procedure to correct for such distortion in the hard-x-ray region using the known energy dependence of the x-ray absorption coefficients. This procedure is used to obtain the vanadium K-edge spectrum of single crystal ${\mathrm{V}}_{2}{\mathrm{O}}_{3}$ and we demonstrate that self-absorption is properly corrected. This facilitates the use of fluorescence detection even in the hard-x-ray region.

Excited-state absorption spectroscopy of Er 3 + -doped Y 3 Al 5 O 12 , YVO 4 , and phosphate glass

Abstract: The spectra of excited-state absorption (ESA) and stimulated-emission (SE) cross sections of ${\mathrm{Er}}^{3+}$ in ${\mathrm{Y}}_{3}{\mathrm{Al}}_{5}{\mathrm{O}}_{12},$ ${\mathrm{YVO}}_{4},$ and phosphate glass have been registered between 0.4 and 3 \ensuremath{\mu}m with a pump and probe technique. The experimental setup, based on the use of cw light sources, is described in detail. The incidence of ESA on the laser properties of these ${\mathrm{Er}}^{3+}$-doped oxide hosts is evaluated particularly around 0.55, 1.6, and 2.8 \ensuremath{\mu}m, corresponding to the ${}^{4}{\mathrm{S}}_{3/2}{\ensuremath{\rightarrow}}^{4}{\mathrm{I}}_{15/2},$ ${}^{4}{\mathrm{I}}_{13/2}{\ensuremath{\rightarrow}}^{4}{\mathrm{I}}_{15/2},$ and ${}^{4}{\mathrm{I}}_{11/2}{\ensuremath{\rightarrow}}^{4}{\mathrm{I}}_{13/2}$ laser transitions, respectively. The migration $[{(}^{4}{\mathrm{I}}_{13/2}{,}^{4}{\mathrm{I}}_{15/2}{)\ensuremath{\rightarrow}(}^{4}{\mathrm{I}}_{15/2}{,}^{4}{\mathrm{I}}_{13/2})]$ and up-conversion $[{(}^{4}{\mathrm{I}}_{13/2}{,}^{4}{\mathrm{I}}_{13/2}{)\ensuremath{\rightarrow}(}^{4}{\mathrm{I}}_{9/2}{,}^{4}{\mathrm{I}}_{15/2})]$ energy transfers involved in some laser operations are analyzed by deducing the energy-transfer microparameters ${C}_{\mathrm{DD}}$ and ${C}_{\mathrm{DA}}$ from ESA and SE measurements. The Judd-Ofelt analysis, usually applied from the ground-state manifold, is then used to evaluate the ESA integrated cross sections and these calculated cross sections are found equal to the measured ones within about 20%.

Self-trapping and multiplication of electronic excitations in Al 2 O 3 and Al 2 O 3 : S c crystals

Abstract: The processes of intrinsic and extrinsic luminescence excitation by synchrotron radiation of 4\char21{}40 eV or electron pulses have been studied in $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ single crystals at 8 K. The intrinsic A (7.6 eV) and E emissions (3.77 eV) can be effectively excited in the region of long-wavelength (8.85\char21{}9.1 eV) and short-wavelength (9.1\char21{}9.3 eV) components of exciton absorption doublet, respectively. Fast (\ensuremath{\sim}6 and \ensuremath{\sim}20 ns) and slow (\ensuremath{\sim}150 ns) components of the A emission correspond to the creation of singlet and triplet ${p}^{5}s$ excitons. The efficiency of the A emission in the region of band-to-band transitions is low. The intensity of A emission sharply increases (approximately quadratically) with a rise of the excitation density by nanosecond electron pulses. In ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}:\mathrm{S}\mathrm{c},$ the 5.6-eV luminescence is caused by the decay of near-impurity electronic excitations (\ensuremath{\sim}8.5 eV) as well as by the electron recombination with holes localized near ${\mathrm{Sc}}^{3+}$ centers. The efficiency of 7.6-, 5.6-, and 3.8-eV emission sharply increases at the energy of exciting photons of $h\ensuremath{ u}g25\mathrm{eV}.$ One photon of 26\char21{}29 and 30\char21{}37 eV causes the ionization of the ${2p}^{6}$ or ${2s}^{2}$ shell of the oxygen ion and provides the creation of two or three electron-hole pairs, respectively. Long-term investigations of $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ crystals did not lead to the detection of immobile self-trapped holes or electrons. The A emission excited at the direct photocreation of excitons or at the recombination of free electrons and free holes is interpreted by us as the radiative decay of self-shrunk excitons. The theoretical model of Sumi allows the existence of such immobile self-shrunk excitons even if an electron and a hole do not separately undergo the self-trapping.

Photoinduced two-photon absorption and second-harmonic generation in As 2 Te 3 -CaCl 2 -PbCl 2 glasses

Abstract: Photoinduced nonlinear optical phenomena in amorphous ${\mathrm{As}}_{2}{\mathrm{Te}}_{3}{\ensuremath{-}\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{l}}_{2}{\ensuremath{-}\mathrm{P}\mathrm{b}\mathrm{C}\mathrm{l}}_{2}$ glasses have been studied using experimental and theoretical quantum chemical and molecular-dynamics methods. Especially photoinduced two-photon absorption (TPA) and second-harmonic generation (SHG) were measured in the IR region from 5.5--21 \ensuremath{\mu}m. CO laser (\ensuremath{\lambda}=5.5 \ensuremath{\mu}m) and parametrically generated wavelengths (5.2--13.7 \ensuremath{\mu}m) were used as a source of pumping light. We have found that with an increase of photoinducing power, the SHG for probe ${\mathrm{CO}}_{2}$ laser (for the double frequency \ensuremath{\lambda}=5.3 \ensuremath{\mu}m) signal increases and achieves its maximum value at photoinducing power 1.45 ${\mathrm{G}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$ per pulse. The absolute values of the SHG were more than one order less comparing to ${\ensuremath{\chi}}_{222}$ tensor for ${\mathrm{Ag}}_{3}{\mathrm{AsSe}}_{3}$ single crystals. With decreasing temperature, the SHG signal strongly increases within the 16--24 K temperature range. Femtosecond probe-pump measurements indicate on an existence of SHG maximum at pump-probe time delay about 25 ps. Spectral positions of the TPA maxima are strongly dependent on the pump power. Contrary to the SHG behavior, for the TPA we observe at least two time delayed maxima: at 20--27 and 65 ps. We explain these dependencies within a framework of the quantum chemical approach taken into account with photoinduced anharmonic electron-vibration interaction. We have revealed that As-Te tetrahedra play a key role in the observed photoinduced nonlinear optics effects. The obtained results show that the mentioned effects can be used as a powerful tool for investigations of picosecond IR nonlinear optics processes. Simultaneously the investigated glasses are promising materials for IR femtosecond quantum electronics.

Quadrupolar vibrational mode of silver clusters from plasmon-assisted Raman scattering

Abstract: Absorption and low-frequency Raman-scattering experiments have been performed on thin films consisting of small silver clusters embedded in a porous alumina matrix. When the Raman excitation wavelength is close to the maximum (\ensuremath{\approx}420 nm) of the Mie band (dipolar surface plasmon resonance) the Raman spectra exhibit a strong band located around 10 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, the maximum of which depends on the cluster diameter D at the maximum of the cluster-size distribution in the sample according to the approximate law ${\ensuremath{\omega}}_{\mathrm{vib}}\ensuremath{\propto}{D}^{\ensuremath{-}1}.$ The Raman band corresponds to the excitation of the quadrupolar vibration mode of the clusters, via the plasmon-phonon interaction. Moreover, the maximum of the Raman band shifts towards lower frequencies when the excitation light is shifted to the red. This feature, as well as the rather large Mie-band width, is shown to reflect the ellipsoidal shape distribution of part of the embedded clusters.

Influence of hydrostatic pressure and temperature on two-photon absorption of a C 60 -2-thioxo-1,3-dithiole cycloadduct

Abstract: Two-photon absorption (TPA) at $\ensuremath{\lambda}=532\mathrm{nm}$ versus applied hydrostatic pressure (p) (up to 20 GPa) and temperature (T) (in the 77--300 K range) was used to study intermolecular dynamics in powderlike fullerene-1,3-dithiolic adduct and ${\mathrm{C}}_{60}$ systems. For ${\mathrm{C}}_{60}$-1,3-dithiole adduct, we observed a sharplike maxim of the TPA versus $p\ensuremath{-}T$ below 230 K. Comparing the obtained results with those of ${\mathrm{C}}_{60},$ we have unambiguously shown that the observed behaviors are connected with the adding of the 1,3-dithiole core. To explain the experimental results, we performed ab initio molecular-dynamics geometry optimization, taking into account the superposition of all possible molecular conformations with appropriate weighting factors and higher-order intermolecular multipole interactions. We have shown an increase of the absolute value of the TPA coefficient in the ${\mathrm{C}}_{60}$ cycloadduct by more than 30%. This fact is mainly caused by the asymmetry of intramolecular electronic charge-density asymmetry. The modulatedlike dependence of the TPA versus p and T have been observed. The latter one reflects the vibration and rotational contributions to the TPA values.

Correlation of x-ray absorption and x-ray photoemission spectroscopies in amorphous carbon nitride

Abstract: The nature of bonding in amorphous carbon nitride was studied by synchrotron-radiation photoemission spectroscopy and near-edge x-ray-absorption fine structure. The experimental data are compared with theoretical calculations of chemical shifts, and the relative importance of initial and final state effects is discussed. Bond lengths are determined from the position of the ${\ensuremath{\sigma}}^{*}$ resonance and the obtained results suggest ${\mathrm{sp}}^{2}$ hybridization. The experimental and theoretical evidence presented in this paper supports another interpretation of the photoemission spectra of amorphous carbon nitride.

Luminescence and defect formation by visible and near-infrared irradiation of vitreous silica

Abstract: We report the observation of photoluminescence (PL) originating from defects created by the visible and near-infrared irradiation of vitreous silica. Three PL bands at 283, 468, and 558 nm exhibited excitation spectra peaking at 250 nm, which corresponds to the oxygen vacancy defect ${V}_{\mathrm{O}}$ absorption. The observed PL bands can be attributed to the following transitions: the singlet-singlet transition of oxygen vacancy for 283 nm, and the radiative recombination of separated carriers on the vacancy-interstitial pairs ${(V}_{\mathrm{O}}{;\mathrm{O}}_{i})$ and $({V}_{\mathrm{O}}{;(\mathrm{O}}_{2}{)}_{i})$ for 468 and 558 nm, respectively.

Infrared absorption study of Fermi resonance and hydrogen-bond symmetrization of ice up to 141 GPa

Abstract: The infrared absorption spectra of ${\mathrm{H}}_{2}\mathrm{O}$ and ${\mathrm{D}}_{2}\mathrm{O}$ ices were measured with a diamond-anvil cell up to 141 GPa at 298 K. The high-pressure spectra corrected for diamond absorption provided the substantial absorption profile of ices over the whole pressure range measured. A sequence of pressure-induced Fermi resonance was observed between the softening stretch mode and the combinations of rotational and bending modes below 55 GPa as reported previously. One rotational mode showed an abrupt increase in the peak frequency at $58\ifmmode\pm\else\textpm\fi{}3\mathrm{GPa}$ in ${\mathrm{H}}_{2}\mathrm{O}$ ice and $68\ifmmode\pm\else\textpm\fi{}3\mathrm{GPa}$ in ${\mathrm{D}}_{2}\mathrm{O}$ ice. This can be attributed to a transition from ice VII to proton-tunneling-disordered ice VII. A further transition into proton-disordered ice X was inferred from the spectral change to occur roughly at 65--75 GPa for ${\mathrm{H}}_{2}\mathrm{O}$ and 80--90 GPa for ${\mathrm{D}}_{2}\mathrm{O}.$

On the Construction and Support Properties of Measure-Valued Diffusions on $D \subseteq \mathbb{R}^d$ with Spatially Dependent Branching

Abstract: In this paper, we construct a measure-valued diffusion on $D\subseteq \mathbb{R^d}$ whose underlying motion is a diffusion process with absorption at the boundary corresponding to an elliptic operator $$L = 1/2 abla \cdot a abla + b \cdot abla \text{ on } D \subseteq \mathbb{R}^d$$ and whose spatially dependent branching term is of the form $\beta(x)z-\alpha(x)z^2,x \in D$,where $\beta$ satisfies a very general condition and $\alpha> 0$. In the case that $\alpha$ and $\beta$ are bounded from above, we show that the measure-valued process can also be obtained as a limit of approximating branching particle systems. We give criteria for extinction/survival, recurrence/transience of the support, compactness of the support, compactness of the range, and local extinction for the measure-valued diffusion. We also present a number of examples which reveal that the behavior of the measure-valued diffusion may be dramatically different from that of the approximating particle systems.

Experimental identification of “vacuum heating” at femtosecond-laser-irradiated metal surfaces

Abstract: Aluminum and iron targets were irradiated by intense ( $I\ensuremath{\le}{10}^{15}\mathrm{W}/{\mathrm{cm}}^{2}$), 120 fs laser pulses with sufficiently high contrast such that the surface expanded no more than the peak electron quiver amplitude during excitation. Under these experimentally verified conditions, obliquely incident, $p$-polarized pulses uniquely experienced anomalous absorption, proportional to $(I{\ensuremath{\lambda}}^{2}{)}^{0.64}$, and as high as $20%$. This extra absorption was distinguished from competing pump-induced linear mechanisms by fs-time-resolved reflectivity, and agreed quantitatively with essential features of Brunel's ``vacuum heating,'' in which light is absorbed by drawing electrons into the vacuum and sending them back into the plasma with approximately the quiver velocity.

Disorder, defects, and optical absorption in a-si and a-si:h

Abstract: In this paper we present the optical properties of various structural models for $a\ensuremath{-}\mathrm{Si}$ and $a\ensuremath{-}\mathrm{S}\mathrm{i}:\mathrm{H}.$ We discuss how topological disorder, hydrogen content, and different types of defects (dangling bonds and floating bonds) influence the shape of the optical-absorption spectrum and the position of the Urbach edge. The absorption behavior is characterized by the joint density of states. The band gaps are obtained via the Tauc plot. The principal structure of the optical absorption spectrum is determined mainly by the degree of topological disorder and the amount of hydrogen. The presence of defects gives rise to tail absorption. Dangling bonds affect the optical properties more than floating bonds do.

Effect of Xe 2 * light (7.2 eV) on the infrared and vacuum ultraviolet absorption properties of hydroxyl groups in silica glass

Abstract: Changes in the optical-absorption properties of hydroxyl group (\ensuremath{\equiv}SiOH) in silica glasses were studied to determine the specific form of \ensuremath{\equiv}SiOH whose absorption takes place at around the intrinsic absorption edge of silica glass (\ensuremath{\sim}8 eV). In the experiments, four kinds of fused silica were irradiated with 7.2 eV light from a xenon dielectric-barrier discharge lamp or they were heated at 1273 K in vacuum. In addition to the near-edge absorption, changes in the intensity, peak position and the shape of the infrared absorption bands due to a bond-stretching mode of \ensuremath{\equiv}SiOH s at 3673 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (2.72 \ensuremath{\mu}m) and the first overtone of these stretching vibrations at 7230 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (1.38 \ensuremath{\mu}m) were monitored. The results of absorption spectral observation after the irradiation are summarized as two points: (i) a blue shift of the effective absorption edge took place (increasing in the transmissivity); (ii) the 3673 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ band broadened and shifted to the red for all the samples except the OH-free silicas, and similarly, the absorption intensity of the 7230 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ band decreased and the band shifted to the red, too. It was found through analysis of these changes that the blue shift of the absorption edge corresponded with the decrement of a particular form of \ensuremath{\equiv}SiOH and the red shift of the 3673 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ band was caused by the decrement of the isolated \ensuremath{\equiv}SiOH. The \ensuremath{\equiv}SiOH absorption located at more than 7.4 eV region is tentatively ascribed to \ensuremath{\equiv}SiOH existing as the isolated form in silica glass. It is argued that the isolated \ensuremath{\equiv}SiOH was converted into the hydrogen bonded \ensuremath{\equiv}SiOH by irradiation of the ${\mathrm{Xe}}_{2}^{\mathrm{*}}$ band emission.

Correlation between rare-earth oscillator strengths and rare-earth–valence-band interactions in neodymium-doped Y M O 4 ( M = V , P, As), Y 3 Al 5 O 12 , and LiYF 4 matrices

Abstract: ${\mathrm{Nd}}^{3+}{:\mathrm{Y}\mathrm{V}\mathrm{O}}_{4}$ is one of the more promising laser hosts for micro and diode-pumped solid-state lasers. At room temperature, ${\mathrm{Nd}}^{3+}$ ions in this matrix exhibit strong absorption cross sections sixfold higher than in ${\mathrm{Y}}_{3}{\mathrm{Al}}_{5}{\mathrm{O}}_{12}.$ The neodymium oscillator strengths are measured in $\mathrm{Y}M{\mathrm{O}}_{4}$ ($M=\mathrm{V},$ P, As), ${\mathrm{Y}}_{3}{\mathrm{Al}}_{5}{\mathrm{O}}_{12},$ and ${\mathrm{LiYF}}_{4}$ hosts, and they increase in the sequence ${\mathrm{Y}}_{3}{\mathrm{Al}}_{5}{\mathrm{O}}_{12}{l\mathrm{LiYF}}_{4}{l\mathrm{YAsO}}_{4}{l\mathrm{YPO}}_{4}{l\mathrm{YVO}}_{4}.$ This paper is an attempt to correlate these variations with covalent interactions between neodymium $4f,5d$ levels and valence-band levels. The strength of orbital interactions between $4f$ levels and valence-band states is estimated from the analysis of $3d$ x-ray photoemission spectra of ${\mathrm{Nd}}^{3+}$ ions. A two-step model is derived, in which $5d$ admixture into the $4f$ levels of the rare earth occurs via the valence-band levels. This model shows that the oscillator strengths increase with the Nd $4f$-valence-band interactions.

Ligand field parameters obtained from and chemical shifts observed at the Cr L 2,3 edges

Abstract: Using synchrotron radiation, we have measured the Cr ${L}_{2,3}$ absorption edges for the three most common Cr oxidation states. Significant chemical shifts (0.7 eV for a one electron change) are observed between the ${L}_{2,3}$ edges of hexavalent, trivalent, and divalent Cr. The absorption edges are also compared to ligand field atomic multiplet calculations. The different oxidation states clearly show the ``fingerprints'' of final states belonging to the different ${d}^{n}$ ground states. The ligand field parameters obtained for hexavalent ${\mathrm{K}}_{2}{\mathrm{CrO}}_{4}$ $(10Dq=\ensuremath{-}1.6\mathrm{eV})$ and trivalent ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ $(10Dq=2\mathrm{eV})$ are quite comparable to ligand field parameters determined from the forbidden $3d\ensuremath{-}3d$ optical transitions. A large tetragonal splitting of 3.1 eV is obtained for the ${e}_{g}$ electrons in the Jahn-Teller ${d}^{4}$ compound ${\mathrm{CrF}}_{2}.$ The $10Dq$ value obtained is 1.1 eV, which indicates that the ordering of the $3d$ orbitals has changed. The ${d}_{{z}^{2}}$ and ${d}_{\mathrm{xy}}$ orbitals have changed position, with the ${d}_{\mathrm{xy}}$ orbital being the highest occupied orbital. The ligand field splittings obtained for ${\mathrm{CrF}}_{2}$ are inconsistent with values obtained in two other studies based on the forbidden $3d\ensuremath{-}3d$ optical transitions. The positions of the first two optical transitions are consistent with $3d\ensuremath{-}3d$ excited states calculated with our parameters. The ${\mathrm{CrF}}_{2}$ system shows that a clear ligand field analysis is possible for distorted octahedral systems using ${L}_{2,3}$ edges absorption experiments.

Geometric and electronic structure effects in polarized V K-edge absorption near-edge structure spectra of V 2 O 5

Abstract: Experimental and theoretical polarized V K-edge spectra of ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ are presented. By analyzing experimental spectra, we find that quadrupole transitions are significant, although not dominant at the pre-edge region. Real-space multiple-scattering calculation, relying on a non-self-consistent muffin-tin potential, can account for the basic polarizations trends and for peak heights and positions of the experimental spectra. However, important differences between theory and experiment occur for low-photoelectron energies. Lack of selfconsistency, muffin-tin form of the potential, and complex core-hole effects are identified as possible sources of this discrepancy. We argue that this deficiency of the theory would not get revealed if unpolarized spectra were investigated only. Cluster-size convergence of the calculated spectra is not uniform and it depends on the polarization. By performing model calculations we find, that deviations of the ${\mathrm{V}}_{2}{\mathrm{O}}_{5}$ structure from a perfect horizontal symmetry are much less significant for the shape of the spectrum than deviations from the vertical symmetry. For generating the distinct prepeak in the spectrum, which is polarized along the vertical axis, both short V-O bond length and deviations from inversion symmetry are crucial. The high-energy tail of the spectrum (photoelectron energies more that 30 eV) seems to be generated by scattering from more distant atoms.

An In Situ X‐Ray Absorption Spectroscopic Study of Charged Li ( 1 − z ) Ni ( 1 + z ) O 2 Cathode Material

Abstract: The authors have measured in situ the Ni K-edge X-ray absorption spectra of Li{sub (1{minus}z)}Ni{sub (1+z)}O{sub 2} cathode material charged in a nonaqueous cell The material was charged to various states of charge (ie, Li content) which corresponded to x = 00, 012, 024, 037, 049, and 086 in Li{sub (1{minus}x{minus}z)}Ni{sub (1+z)}O{sub 2} The authors have determined variations in the Ni-O and Ni-Ni coordination numbers, bond lengths, and local disorders as well as the Ni K-edge energies as a function of Li content They show that in the pristine state, the composition of the material can be described by the formula Li{sub 086}N{sub 114}O{sub 2} (ie, x = 0 and z = 014) That is, the material consists of Ni{sup 2+} (25%) and Ni{sup 3+} (75%) with half the Ni{sup 2+} atoms residing in Li sites and the other half in the NiO{sub 2} slabs Upon charging, initially Ni{sup 2+} is oxidized to Ni{sup 3} up to a state of charge which corresponds to x = 2z Upon further charging to states corresponding to 2z < x {le} 1 {minus} z, Ni{sup 3} is oxidized to Ni{sup 4+} with fractions being dependent on the values of x and z Analysismore » of the edge energies for NiO, stoichiometric LiNiO{sub 2}, and KNiIO{sub 6} as reference compounds for Ni{sup 2+}, Ni{sup 3}, and Ni{sup 4}, respectively, shows a quadratic dependence for edge energy vs oxidation state This type of correlation is consistent with variations observed in earlier studies for some Mn reference compounds in the same range of oxidation states Oxidation-state determination of Ni in Li{sub (1{minus}x{minus}z)}Ni{sub (1+z)}O{sub 2} as a function of state of charge (ie, Li content or x) on the basis of edge energies yielded results which are in excellent agreement with oxidation state determinations made on the basis of the mole fractions for Ni{sup 2+}, Ni{sup 3}, and Ni{sup 4} extracted from extended X-ray absorption fine structure spectra« less

Large numbers of cold metastable helium atoms in a magneto-optical trap

Abstract: We report loading of $15\ifmmode\times\else\texttimes\fi{}{10}^{9}$ metastable triplet helium atoms in a large magneto-optical trap, using far-red-detuned laser beams We fully characterized this trap by measuring trap losses and absorption of a probe beam From the highly nonexponential trap decay we derive Penning ionization loss rate coefficients for two detunings: $53(9)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{\mathrm{cm}}^{3}/\mathrm{s}$ at $\ensuremath{-}35\mathrm{MHz}$ and $37(6)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}{\mathrm{cm}}^{3}/\mathrm{s}$ at $\ensuremath{-}44\mathrm{MHz}$ Also, we find that the loss rate is maximum at $\ensuremath{-}5\mathrm{MHz}$ detuning, where the rate is $13(3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}{\mathrm{cm}}^{3}/\mathrm{s},$ much larger than recent theoretical and experimental values In the absence of light the $S\ensuremath{-}S$ ionization rate constant is measured to be $13(2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10}{\mathrm{cm}}^{3}/\mathrm{s}$

Optical transition probability of Nd 3 + ions in a α − N d 3 + : L a S c 3 ( B O 3 ) 4 crystal

Abstract: Optical absorption and emission intensities are investigated for ${\mathrm{Nd}}^{3+}$ ions in a $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Nd}}^{3+}{:\mathrm{L}\mathrm{a}\mathrm{S}\mathrm{c}}_{3}{(\mathrm{B}\mathrm{O}}_{3}{)}_{4}$ crystal. Oscillator strengths of transition were measured from absorption spectra recorded at 300 K. Corresponding oscillator strengths for transition between the J manifold were calculated using the Judd-Ofelt theory. Phenomenological intensity parameters were derived from a least-squares-fitting procedure. The root-mean-square deviation of observed and calculated oscillator strengths was \ifmmode\pm\else\textpm\fi{}8.1%. The intensity parameters were also used to calculate the radiative transition probabilities and branching ratios from a level of ${}^{4}{\mathrm{F}}_{3/2},$ which fluoresce in a $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Nd}}^{3+}{:\mathrm{L}\mathrm{a}\mathrm{S}\mathrm{c}}_{3}{(\mathrm{B}\mathrm{O}}_{3}{)}_{4}$ crystal. The stimulated emission cross sections ${\ensuremath{\sigma}}_{p}$ were calculated. It was concluded that $\ensuremath{\alpha}\ensuremath{-}{\mathrm{Nd}}^{3+}{:\mathrm{L}\mathrm{a}\mathrm{S}\mathrm{c}}_{3}{(\mathrm{B}\mathrm{O}}_{3}{)}_{4}$ crystal is a laser material with high-emission intensity.

Local Si-H bonding environment in hydrogenated amorphous silicon films in relation to structural inhomogeneities

Abstract: The hypothesis of the relation between the local environment of polyhydride $[({\mathrm{SiH}}_{2}{)}_{n}]$ groups and structural inhomogeneities (due to the presence of voids) of an amorphous silicon network has been tested in hydrogenated amorphous silicon films prepared by rf magnetron sputtering (RF MS). Comparative infrared (IR) absorption, Raman spectroscopy and optical transmission measurements have been performed on samples deposited under the same plasma conditions at various substrate temperature (T). The results of the IR studies show that all samples exhibit the same hydrogen content incorporated as isolated monohydride (SiH) groups responsible for the absorption band at 2004 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. As a consequence, the gradual decrease in the total bonded hydrogen content as T increases is essentially due to the decrease in the amount of hydrogen bonded as polyhydride $[({\mathrm{SiH}}_{2}{)}_{n}]$ groups which absorb around 2086 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. A direct relationship is established between the changes in the IR spectra and the variation of structural inhomogeneities observed by Raman spectroscopy. The linewidth in the low-frequency side of the transversal acoustic (TA) -like band in the Raman spectra decreases monotonously as T increases. This decrease is attributed to a decrease in density of quasilocalized vibrational modes in structural inhomogeneities (disordered domains around the surface of voids) in the amorphous network. The good correlation between the IR and Raman results indicates that the $({\mathrm{SiH}}_{2}{)}_{n}$ groups are located in structural inhomogeneities due to the presence of voids in the amorphous network where the formation of these complexes is favorable. This interpretation is corroborated by the variation of both the static refractive index, which gives information about the density of material, and the dispersion energy, which measures the mean coordination number of Si atoms and the effective number of valence electron per Si atom in the amorphous network.

Charmonium suppression in lead-lead collisions: Is there a break in the J/ψ cross section?

Abstract: In the framework of a model based on nuclear absorption plus comover interaction, we compute the ${E}_{T}$ distribution of the $J/\ensuremath{\psi}$ in PbPb collisions at SPS and compare it with available NA50 data. Our analysis suggests that the existence of new physics (deconfinement phase transition) in the region ${E}_{T}\ensuremath{\lesssim}100$ GeV is unlikely and that signals of new physics should rather be searched in the region ${E}_{T}\ensuremath{\gtrsim}100$ GeV. The ${E}_{T}$ dependence of the $J/\ensuremath{\psi}$ transverse momentum has been computed. At large ${E}_{T}$ it turns out to be much flatter in the comover approach than in a phase transition framework. Estimates of the $J/\ensuremath{\psi}$ suppression at RHIC and LHC energies are also given.

First-principles calculations of electron-energy-loss near-edge structure and near-edge x-ray-absorption fine structure of BN polytypes using model clusters

Abstract: Effects of a core hole in the unoccupied density of states are systematically studied using model clusters of $h\ensuremath{-}\mathrm{BN},$ $c\ensuremath{-}\mathrm{BN},$ and $w\ensuremath{-}\mathrm{BN},$ in order to reproduce and interpret experimental electron-energy-loss near-edge structure /near-edge x-ray absorption fine structure at both B- and N-K edges. Wave functions are found to localize significantly near the core hole, thereby changing their energies as well as spatial distribution. They are very different from the Bloch states assumed in a band-structure calculation on the basis of a structural unit cell. When the presence of the core hole is ignored, in other words at the ground state, small clusters such as $h\ensuremath{-}({\mathrm{B}}_{7}{\mathrm{N}}_{12})$ exhibit better agreement with the experiment as compared with large clusters such as $h\ensuremath{-}({\mathrm{B}}_{91}{\mathrm{N}}_{108})$ because wave functions are made to localize in the small clusters. Features appeared in the experimental spectra are interpreted in terms of chemical bondings among atomic orbitals using overlap population diagrams. Absolute transition energies by Slater's transition state method agree with experimental values within an error of 1%.

X-Ray Evidence of an AGN in M82

Abstract: An X-ray spectrum of the famous starburst galaxy M82 consists of three components: soft, medium, and hard components (Tsuru et al. 1997). The spectrum of the hard component, which is spatially unresolved, is well represented by an absorbed thermal bremsstrahlung, or an absorbed power-law model. However the origin of the hard component was unclear. Thus, we made a monitoring observation with ASCA in 1996. Although the X-ray flux of the soft and medium components remained constant, a significant time variability of the hard component was found between $3\times10^{40}$ erg/s and $1\times10^{41}$ erg/s at various time scales from 10 ks to a month. The temperature or photon index of the hard component also changed. We proved that the spatial position of the hard component is the center of M82. The spectrum of the variable source obtained by subtracting the spectrum of the lowest state from the highest state suggests the strong absorption of $N_{\rm H} \sim 10^{22}$ cm$^2$, which means the variable source is embedded in the center of M82. All these suggest that a low-luminosity AGN exists in M82.

Comparison of wavelength dependence in cascade-, Λ-, and Vee-type schemes for electromagnetically induced transparency

Abstract: We present a theoretical study of the effects of mismatching wavelengths for the coupling and probe fields in Doppler-broadened media for the three basic energy level configurations commonly used to realize electromagnetically induced transparency (EIT). Three wavelength regimes are considered: mismatched wavelengths for which the coupling frequency is greater than the probe frequency, matched wavelengths for which the coupling and probe frequencies are equal, and mismatched wavelengths for which the probe frequency is greater than the coupling frequency. The transparency that may be induced in these regimes is compared for the cascade-, \ensuremath{\Lambda}-, and Vee-type systems. We show that in the first mismatched regime $({\ensuremath{\lambda}}_{c}l{\ensuremath{\lambda}}_{p})$ EIT is possible in all schemes and is in fact stronger than in the matched case. It is also demonstrated that for the second mismatched regime $({\ensuremath{\lambda}}_{c}g{\ensuremath{\lambda}}_{p})$ EIT can be realized most readily in the Vee-type configuration in the presence of Doppler broadening. These predictions are explained by considering the absorption as a function of both the probe field detuning and the atomic velocity.