Showing papers on "Absorption (logic) published in 1997"

Plasmon Resonance Shifts of Au-Coated Au 2 S Nanoshells: Insight into Multicomponent Nanoparticle Growth

Abstract: In this Letter we present a coherent description of the optical properties of a unique type of nanoparticle, metal-coated dielectric nanoparticles. These structures occur naturally in the form of Au-coated ${\mathrm{Au}}_{2}\mathrm{S}$ nanoparticles. During the course of nanoparticle growth, the plasmon-related absorption peak undergoes very large shifts in wavelength, from $\ensuremath{\sim}650$ to $\ensuremath{\sim}900\mathrm{nm}$. We show that this plasmon peak shift is purely classical in origin and is determined solely by the relative thickness of the Au shell and the ${\mathrm{Au}}_{2}\mathrm{S}$ core diameter. This understanding of the optical properties of these nanoparticles is used to elucidate the nanoparticle growth kinetics.

The Deuterium Abundance Towards Q1009+2956

Abstract: We present a measurement of the deuterium to hydrogen ratio (D/H) in a metal-poor absorption system at redshift $z=2.504$ towards the QSO 1009+2956. We apply the new method of Burles & Tytler (1997) to robustly determine D/H in high resolution \Lya forest spectra, and include a constraint on the neutral hydrogen column density determined from the Lyman continuum optical depth in low resolution spectra. We introduce six separate models to measure D/H and to assess the systematic dependence on the assumed underlying parameters. We find that the deuterium absorption feature contains a small amount of contamination from unrelated \ion{H}{1}. Including the effects of the contamination, we calculate the 67% confidence interval of D/H in this absorption system, log (D/H) $= -4.40 ^{+0.06}_{-0.08}$. This measurement agrees with the low measurement by Burles & Tytler (1997) towards Q1937--1009, and the combined value gives the best determination of primordial D/H, log (D/H)$_p = -4.47 ^{+0.030}_{-0.035}$ or D/H $= 3.39 \pm 0.25 \times 10^{-5}$. Predictions from standard big bang nucleosynthesis (SBBN) give the cosmological baryon to photon ratio, $\eta = 5.1 \pm 0.3 \times 10^{-10}$, and the baryon density in units of the critical density, $\Omega_b h^2 = 0.019 \pm 0.001$, where $H_0 = 100 h$ \kms Mpc$^{-1}$. The measured value of (D/H)$_p$ implies that the primordial abundances of both $^4$He and $^7$Li are high, and consistent with some recent studies. Our two low measurements of primordial D/H also place strong constraints on inhomogeneous models of big bang nucleosynthesis.

Interband Transitions in Sn x Ge 1-x Alloys

Abstract: Optical absorption measurements for diamond cubic ${\mathrm{Sn}}_{x}{\mathrm{Ge}}_{1\ensuremath{-}x}$ alloy films indicate strong interband transitions with a change in direct energy gap of $0.35l{E}_{g}l0.80\mathrm{eV}$ for $0.15gxg0$. The optical energy gap undergoes an indirect to direct transition in this composition range and decreases much faster with Sn content than predicted by tight binding and pseudopotential calculations in the virtual crystal approximation.

Spectroscopic properties of Er 3 + - and Yb 3 + -doped soda-lime silicate and aluminosilicate glasses

Abstract: A spectroscopic investigation of an extensive series of ${\mathrm{Er}}^{3+}$-doped and ${\mathrm{Er}}^{3+}{,\mathrm{Y}\mathrm{b}}^{3+}$-codoped soda-lime-silicate (SL) and aluminosilicate (AS) glasses is presented. Compared to SL glasses, $4f$ transitions in AS glasses show higher oscillator strengths, larger inhomogeneous broadening, and smaller crystal-field splittings of the respective excited-state multiplets. The ${\mathrm{Er}}^{3+}$ excited-state relaxation dynamics is adequately described by a combination of the Judd-Ofelt model and the energy-gap law. With the exception of ${}^{4}{I}_{13/2},$ multiphonon relaxation is dominant for all excited states, making it possible to efficiently pump the 1.55 \ensuremath{\mu}m ${}^{4}{I}_{13/2}{\ensuremath{\rightarrow}}^{4}{I}_{15/2}$ emission by excitation of ${}^{4}{I}_{11/2}$ at around 980 nm. The absolute ${}^{4}{I}_{13/2}$ luminescence quantum yield, for low 980-nm excitation density $(\ensuremath{\sim}5{\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}),$ \ensuremath{\eta}, is $\ensuremath{\sim}0.9$ at 0.4 mol % ${\mathrm{Er}}_{2}{\mathrm{O}}_{3}$ and drops to about 0.65 upon increasing ${\mathrm{Er}}_{2}{\mathrm{O}}_{3}$ to 1.2 mol %, indicating the onset of energy-transfer processes. Samples with high ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$ impurity concentration suffer from significantly higher quenching of ${}^{4}{I}_{13/2}$ luminescence at higher ${\mathrm{Er}}^{3+}$ concentrations. Energy migration to the minority of ${\mathrm{Er}}^{3+}$ ions coordinated to ${\mathrm{OH}}^{\mathrm{\ensuremath{-}}}$, followed by efficient multiphonon relaxation accounts for this effect. At low excitation densities, the strong near-infrared absorption of ${\mathrm{Yb}}^{3+}$ in combination with efficient $\mathrm{Yb}\ensuremath{\rightarrow}\mathrm{Er}$ energy transfer increases the ${}^{4}{I}_{13/2}$ population density in ${\mathrm{Yb}}^{3+},{\mathrm{Er}}^{3+}$-codoped samples by up to 2 orders of magnitude compared to equivalent samples without ${\mathrm{Yb}}^{3+}$. The dependence of \ensuremath{\eta} on ${\mathrm{Yb}}^{3+}$ codotation of 0.4 mol % ${\mathrm{Er}}_{2}{\mathrm{O}}_{3}$-doped samples predicts that a minimum of $\ensuremath{\sim}0.8\mathrm{mol}%$ ${\mathrm{Yb}}_{2}{\mathrm{O}}_{3}$ is required to achieve efficient sensitization of ${\mathrm{Er}}^{3+}$ by ${\mathrm{Yb}}^{3+}$. The relative intensities of upconversion luminescence from ${}^{4}{S}_{3/2}$ and ${}^{2}{H}_{11/2}$ are used to analyze internal sample heating in detail. Due to the high absorption cross section of ${\mathrm{Yb}}^{3+}$, increasing the ${\mathrm{Yb}}^{3+}$ concentration in ${\mathrm{Yb}}^{3+},{\mathrm{Er}}^{3+}$-codoped samples of given length increases the absorbed power and subsequently the total density of multiphonon emission, leading to internal temperatures of up to 572 K in 0.4 mol % ${\mathrm{Er}}_{2}{\mathrm{O}}_{3}$ samples codoped with 4 mol % ${\mathrm{Yb}}_{2}{\mathrm{O}}_{3}$ and excited with $51{\mathrm{k}\mathrm{W}/\mathrm{c}\mathrm{m}}^{2}$. Multiphonon relaxation from ${}^{4}{I}_{13/2}$ is shown to be inefficient even at these high internal sample temperatures. From upconversion luminescence spectra of a series of glasses, the thermal conductivity is estimated to be between $3.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}$ and $7.7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}{\mathrm{W}\mathrm{}\mathrm{m}}^{\mathrm{\ensuremath{-}}1}{\mathrm{}\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$, in good agreement with the known value of $4.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}2}{\mathrm{W}\mathrm{}\mathrm{m}}^{\mathrm{\ensuremath{-}}1}{\mathrm{}\mathrm{K}}^{\mathrm{\ensuremath{-}}1}$ for soda-lime-silicate glass.

Cooperative Emission in π -Conjugated Polymer Thin Films

Abstract: Picosecond dynamics of exciton emission and absorption have been studied in neat thin films of a variety of poly (phenylene vinylene) derivatives. We found that the stimulated emission band of 120 nm width and $\ensuremath{\sim}1\mathrm{ns}$ duration, which is observed at low exciton density $n$, collapses at $ng{10}^{17}{\mathrm{cm}}^{\ensuremath{-}3}$ into a much narrower band of 7 nm width and lifetime $\ensuremath{\tau}\ensuremath{\ll}10\mathrm{ps}$. Based on its excitation intensity dependence, polarization, lifetime, illuminated area, and film thickness dependencies, we assign this narrow band to superfluorescence rather than to amplified spontaneous emission.

Characterization of iron oxides by x-ray absorption at the oxygen K edgeusing a full multiple-scattering approach

Abstract: \ensuremath{\alpha}-${\mathrm{Fe}}_{2}$${\mathrm{O}}_{3}$,${\mathrm{Fe}}_{3}$${\mathrm{O}}_{4}$, and FeO compounds are characterized by means of x-ray-absorption near-edge-structure spectroscopy at the oxygen K edge. Using increasing cluster sizes around the excited atom in the full multiple-scattering simulations, we are able to link the features present in the spectra of each iron oxide to its specific atomic arrangement and electronic structure. The prepeak structure is successfully reproduced and interpreted as transitions from the oxygen 1s core state to antibonding oxygen 2p states hybridized with Fe 3d orbitals. Their intensity and shape depends on the Fe site symmetry, the occupation number of the d levels, and the O-Fe bond length of each different iron oxide. Higher lying spectral features are shown to be related to scattering of the photoelectron by a particular oxygen shell and an extended x-ray absorption fine structure--like relation is established between their energy position and the distance of the corresponding shell from the photoabsorber.

A Lower Bound on the Cosmic Baryon Density

Abstract: We derive analytic lower bounds on the cosmic baryon density by requiring that the high-redshift IGM contain enough neutral hydrogen to produce the observed \lya absorption in quasar spectra. The key theoretical assumption is that absorbing structures are no more extended in redshift space than in real space. This assumption might not hold if \lya clouds are highly overdense and thermally broadened, but it is likely to hold in the gravitational instability picture for the \lya forest suggested by cosmological simulations, independently of the details of the cosmological model. The other ingredients in these bounds are an estimate of the UV background from quasars, a temperature $T~10^4K$ for the "warm" photoionized IGM that produces most of the \lya absorption, a value of the Hubble constant, and observational estimates of the mean \lya flux decrement $\Dbar$ or, for a more restrictive bound, the distribution function $P(\tau)$ of \lya optical depths. With plausible parameter values, the mean decrement bound implies a baryon density parameter $\Omb \ga 0.0125/h^2$. With conservative values, the bound weakens to $\Omb \ga 0.005/h^2$, but the required clustering of the IGM is then incompatible with other properties of quasar spectra. A recent observational determination of $P(\tau)$ implies $\Omb \ga 0.0125/h^2$ even for a conservative estimate of the UV background, and $\Omb \ga 0.018/h^2$ for a more reasonable estimate. These bounds are consistent with recent low estimates of the primordial deuterium-to-hydrogen ratio $\dtoh$, which imply $\Omb \approx 0.025/h^2$, but the $P(\tau)$ bound can only be reconciled with high $\dtoh$ estimates by abandoning standard big bang nucleosynthesis or the gravitational instability picture for the origin of the \lya forest. (Shortened abstract.)

Interactions of hydrogen molecules with bond-centered interstitial oxygen and another defect center in silicon

Abstract: Infrared spectra obtained from Czochralski (CZ) silicon heated in the range $1100l~Tl~1300\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ in ${\mathrm{H}}_{2},$ ${\mathrm{D}}_{2},$ or mixtures of the two gases, show vibrational absorption at $1075{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ due to perturbed bond-centered interstitial oxygen atoms ${\mathrm{O}}_{i}.$ Deconvolutions of these absorption profiles imply that the observed perturbations to the ${\mathrm{O}}_{i}$ atom are due to adjacent defects incorporating two H atoms. This interpretation is confirmed by detection of modes ${\ensuremath{ u}}_{1}$ $({\mathrm{\ensuremath{ u}}}_{1\mathrm{H}\mathrm{H}}{,\mathrm{\ensuremath{ u}}}_{1\mathrm{H}\mathrm{D}}{,\mathrm{\ensuremath{ u}}}_{1\mathrm{D}\mathrm{D}})$ with ${\ensuremath{ u}}_{1\mathrm{H}\mathrm{H}}{=3789\mathrm{}\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and ${\ensuremath{ u}}_{2}$ $({\mathrm{\ensuremath{ u}}}_{2\mathrm{H}\mathrm{H}}{,\mathrm{\ensuremath{ u}}}_{2\mathrm{H}\mathrm{D}}{,\mathrm{\ensuremath{ u}}}_{2\mathrm{D}\mathrm{D}})$ with ${\ensuremath{ u}}_{2\mathrm{H}\mathrm{H}}{=3731\mathrm{}\mathrm{cm}}^{\mathrm{\ensuremath{-}}1},$ that both correlate in strength with that of the $1075{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ absorption. A third set of modes ${\ensuremath{ u}}_{3}$ $({\mathrm{\ensuremath{ u}}}_{3\mathrm{H}\mathrm{H}}{,\mathrm{\ensuremath{ u}}}_{3\mathrm{H}\mathrm{D}}{,\mathrm{\ensuremath{ u}}}_{3\mathrm{D}\mathrm{D}}),$ with ${\ensuremath{ u}}_{3\mathrm{H}\mathrm{H}}{=3618\mathrm{}\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ detected in heat-treated CZ and float zone silicon and not related to the absorption at $1075{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1},$ must be due to H-H pair defects trapped at an unknown impurity or lattice defect. The frequencies of the ${\ensuremath{ u}}_{1\mathrm{H}\mathrm{D}},$ ${\ensuremath{ u}}_{2\mathrm{H}\mathrm{D}},$ and ${\ensuremath{ u}}_{3\mathrm{H}\mathrm{D}}$ modes imply that the three defects are ${\mathrm{H}}_{2}$ molecules with weakened bonds and small dipole moments resulting from interactions with either adjacent oxygen atoms $({\ensuremath{ u}}_{1},{\ensuremath{ u}}_{2})$ or a second type of trap $({\ensuremath{ u}}_{3}).$ Annealing treatments imply that isolated ${\mathrm{O}}_{i}\ensuremath{-}{\mathrm{H}}_{2}$ complexes can dissociate for $Tg~70\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ allowing interstitial ${\mathrm{H}}_{2}$ molecules to diffuse away but they can be retrapped during a subsequent anneal at $Tl~50\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$

Structure and magnetism of Pd in Pd/Fe multilayers studied by x-ray magnetic circular dichroism at the Pd L 2,3 sedges

Abstract: We have measured the ${\mathrm{L}}_{2,3}$ absorption edges of Pd in Pd/Fe multilayers with different Pd interlayer thicknesses. The spectral features observed after the ${\mathrm{L}}_{3}$ and ${\mathrm{L}}_{2}$ white lines indicate that in the atomic layer adjacent to Fe the Pd atoms have a local structure different from fcc. X-ray magnetic circular dichroism (XMCD) at these edges shows that the atoms directly at the interface are strongly polarized with a total moment of about 0.4${\mathrm{\ensuremath{\mu}}}_{\mathrm{B}}$ /atom. The orbital moments are found to be small (0.0--0.04${\mathrm{\ensuremath{\mu}}}_{\mathrm{B}}$ ). The thickness dependence of the XMCD shows that Pd atoms carry a magnetic moment up to four layers from the interface. The magnetic interface appears larger than the crystallographic one.

Charge-transfer range for photoexcitations in conjugated polymer/fullerene bilayers and blends

Abstract: Time-resolved photoinduced absorption was measured on bilayers of poly-[${2\ensuremath{-}\mathrm{m}\mathrm{e}\mathrm{t}\mathrm{h}\mathrm{o}\mathrm{x}\mathrm{y},5\ensuremath{-}(2}^{\ensuremath{'}}$-ethylhexoxy)-1,4-phenylenevinylene] (MEH-PPV) and fullerene $({\mathrm{C}}_{60}),$ and on ${\mathrm{M}\mathrm{E}\mathrm{H}\ensuremath{-}\mathrm{P}\mathrm{P}\mathrm{V}/\mathrm{C}}_{60}$ composite films of various concentrations. We find that even in the picosecond regime, charge transfer from the conjugated polymer to ${\mathrm{C}}_{60}$ slows down the decay dynamics relative to the decay in the pure samples. The fact that charge transfer occurs in the picosecond time scale in bilayer structure (thickness\ensuremath{\approx}200 \AA{}) implies that diffusion of localized excitations to the interface is not the dominant mechanism; the charge-transfer range is a significant fraction of the film thickness. From an analysis of the excited-state decay curves, we estimate the charge-transfer range to be 80 \AA{} and interpret that range as resulting from quantum delocalization of the photoexcitations.

Optical characterization of Pr 3 + -doped yttria-stabilized zirconia single crystals

Abstract: The optical absorption and fluorescence of Pr${}^{3+}$ ions in yttria-stabilized zirconia single crystals are investigated. Fluorescence emissions from the ${}^{1}{D}_{2}$ level are clearly dominant and low intensity emission lines from the ${}^{3}{P}_{0}$ and ${}^{1}{G}_{4}$ states are also observed. Analysis with the Judd-Ofelt theory of the absorption intensities has been made assuming that only $\ensuremath{\sim}40$% of the praseodymium ions contribute to the optical absorption bands. Quantum efficiency values of $\ensuremath{\eta}{(}^{3}{P}_{0})\ensuremath{\sim}0.2$ and $\ensuremath{\eta}{(}^{1}{D}_{2})\ensuremath{\sim}$ 1 are obtained at room temperature. ${}^{1}{D}_{2}$ fluorescence quenching has been observed in heavily-doped samples due to cross relaxation processes among neighboring Pr${}^{3+}$ ions. Analysis using the Inokuti-Hirayama model shows that electric dipole-dipole interactions are mainly responsible for the quenching effect. Pr${}^{3+}$ ions are present in seven and sixfold configurations with a statistical distribution. The energy position of the $4f5d$ configuration is very different for each center. The fluorescence dynamics is explained by a mechanism involving thermally assisted population of the ${}^{3}{P}_{1,2}{+}^{1}{I}_{6}$ upper levels and fast relaxation to the ${}^{1}{D}_{2}$ level via states of the excited $4f5d$ configuration.

Experimental determination of excitonic structure in polythiophene

Abstract: The clearly resolved ${}^{1}{B}_{u}$ $(\ensuremath{ u}=1)$ exciton and its vibronic structures show up in ordinary absorption and photoluminescence spectra of films of highly ordered poly(3-octylthiophene). The electroabsorption and the two-photon absorption spectra have unraveled the charge-transfer type ${}^{1}{A}_{g}$ $(\ensuremath{ u}=2)$ exciton state lying 0.5 eV above the ${}^{1}{B}_{u}$ $(\ensuremath{ u}=1)$ exciton state, while the electroluminescence spectrum shows the emission from the triplet exciton ${(}^{3}{B}_{u})$ lying 0.45 eV below the corresponding ${}^{1}{B}_{u}$ exciton. On the basis of the excitonic level diagram as well as the FeCl${}_{4}^{\ensuremath{-}}$-doping induced spectral change, the distinct photoinduced band observed at 1.0 eV is assigned to the internal transition between the triplet-exciton levels, ${}^{3}{B}_{u}$ $(\ensuremath{ u}=1)$ $\ensuremath{\rightarrow}$ ${}^{3}{A}_{g}$ $(\ensuremath{ u}=2)$.

Intensity-dependent relaxation dynamics and the nature of the excited-state species in solid-state conducting polymers

Abstract: Femtosecond transient-absorption dynamics are presented for an oligomer and a number of polymers from the poly(arylene vinylene) family for excitation densities in the range from 10{sup 18}{endash}10{sup 21} cm{sup {minus}3}. We report careful studies of the wavelength and pump-intensity dependence of the photoinduced absorption in a model five-ring oligomer, 2-methoxy-5-(2{sup {prime}}-ethylhexyloxy)-distyryl benzene. We show that a single photoexcited species, the intrachain exciton, is responsible for all observable spectral features at low excitation densities. At higher excitation densities, nonlinear relaxation processes are observed in both polymers and the oligomer, consistent with exciton-exciton annihilation as a nonlinear-decay mechanism. {copyright} {ital 1997} {ital The American Physical Society}

The Effects of an Early Galactic Wind on the Evolution of D,3He, and Z

Abstract: The predictions of the abundances of D and {sup 3}He from big bang nucleosynthesis (BBN) and recent observations of these two isotopes suggest the need to develop new chemical evolution models. In particular, we examine the role of an early episode of massive star formation that would induce a strong destruction of D and a galactic wind. We discuss the ability of these models to match the observed local properties of the solar neighborhood such as the gas mass fraction, oxygen abundance, the age-metallicity relation, and the present-day mass function. We also examine in detail the ability of the chemical evolution models discussed to reproduce the apparent lack of low-mass, low-metallicity stars in the solar neighborhood, namely the G-dwarf distribution. Indeed, we find models which satisfy the above constraints while at the same time allowing for a large primordial D/H ratio as is reportedly measured in some quasar absorption systems at high z, without the overproduction of heavy elements. The latter constraint is achieved by employing a simple dynamical model for a galactic wind. {copyright} {ital 1997} {ital The American Astronomical Society}

Molecular Hydrogen Absorption in the z= 1.97 Damped Lyman alpha Absorption system toward QSO 0013-004

Abstract: We present a new ultra-violet spectrum of the QSO 0013-004 with 0.9 \AA resolution obtained with the MMT Blue spectrograph. The \upsilon = 0 - 0, 1 - 0, 2 - 0 and 3 - 0 Lyman bands of H_2 associated with the z = 1.9731 damped Ly alpah absorption line system have been detected. The H_2 column density is N(H_2) = 6.9 (\pm 1.6)\times 10^{19} cm^{-2}, and the Doppler parameter b = 15\pm 2 km/s. The populations of different rotational levels are measured and used to derive the excitation temperatures. The estimated kinetic temperature T_K\sim 70 K, and the total particle number density n(H) \sim 300 cm^{-3}. The UV photoabsorption rate $\beta_0 \sim 6.7\times 10^{-9}$ s^{-1}, about a factor of few times greater than that in a typical diffuse Milky Way interstellar cloud. The total hydrogen column density is $N(H) = 6.4(\pm 0.5)\times 10^{20} cm^{-2}$. The fractional H_2 abundance f = 2N(H_2)/(2N(H_2) + N(H I)) \sim 0.22 \pm 0.05 is the highest among all observed damped Ly\al absorbers. The high fractional H_2 abundance is consistent with the inferred presence of dust and strong C I absorption in this absorber.

Upconversion mechanisms in Er 3 + -doped Ba 2 YCl 7

Abstract: Efficient near infrared to visible upconversion luminescence is reported in the low phonon-energy host material ${\mathrm{Ba}}_{2}{\mathrm{YCl}}_{7}:x%$ ${\mathrm{Er}}^{3+}$ ($x=1,$ 10, 100). The upconversion mechanisms upon ${}^{4}{I}_{9/2}$ excitation $(\ensuremath{\approx}800\mathrm{nm})$ are investigated by means of excitation and time-resolved luminescence spectroscopy. ${}^{2}{H}_{9/2}$ is found to be populated by energy transfer upconversion and excited-state absorption, depending on the excitation energy. Characteristic excited-state absorption peaks in the excitation spectra are used as a fingerprint to determine the population mechanisms for the other states with high luminescence intensities: ${}^{4}{S}_{3/2}{/}^{2}{H}_{11/2},$ ${}^{4}{F}_{9/2},$ and ${}^{4}{G}_{11/2}.$ The assignment is confirmed by temperature, concentration, power, and time-dependent measurements. The relative intensities are accounted for by a Judd-Ofelt analysis.

Theoretical analysis of x-ray-absorption near-edge fine structure at the O and metal K edges of LaFeO3 and LaCoO3

Abstract: We present experimental x-ray-absorption spectra at the oxygen and $3d$ transition-metal $K$ edges of ${\mathrm{LaFeO}}_{3}$ and ${\mathrm{LaCoO}}_{3}$. We interpret the experimental results in terms of detailed theoretical calculations based on multiple-scattering theory. Along with providing an understanding of the origin of various experimental features, we investigate the effects of structural distortions and the core-hole potential in determining the experimental spectral shape. The results indicate that the core-hole potential as well as many-body effects within the valence electrons do not have any strong effect on the spectra suggesting that the spectral features can be directly interpreted in terms of the electronic structure of such compounds.

Magnetic properties of (m-MPYNN + )[Mn 12 O 12 (O 2 CPh) 16 (H 2 O) 4 ] - : Enhancement of magnetic relaxation in the Mn 12 cluster caused by the organic radical

Abstract: We report magnetic properties of the organic/inorganic hybrid salt, $(m\ensuremath{-}{\mathrm{MPYNN}}^{+})[{\mathrm{Mn}}_{12}{\mathrm{O}}_{12}({\mathrm{O}}_{2}\mathrm{CPh}{)}_{16}({\mathrm{H}}_{2}\mathrm{O}{)}_{4}{]}^{\ensuremath{-}}$ (2), where $m\ensuremath{-}{\mathrm{MPYNN}}^{+}$ $(=m\ensuremath{-}N\ensuremath{-}$methylpyridinium nitronylnitroxide) is a stable organic radical cation with $S=\frac{1}{2}$ and $[{\mathrm{Mn}}_{12}{\mathrm{O}}_{12}({\mathrm{O}}_{2}\mathrm{CPh}{)}_{16}({\mathrm{H}}_{2}\mathrm{O}{)}_{4}{]}^{\ensuremath{-}}$ is a 12-nuclei Mn cluster anion with $S=\frac{19}{2}$, together with the simple salt, [${\mathrm{PPh}}_{4}^{+}$(=tetraphenylphosphonium)] $[{\mathrm{Mn}}_{12}{\mathrm{O}}_{12}({\mathrm{O}}_{2}\mathrm{CPh}{)}_{16}({\mathrm{H}}_{2}\mathrm{O}{)}_{4}{]}^{\ensuremath{-}}\ensuremath{\cdot}2{\mathrm{H}}_{2}\mathrm{O}$ ( 1). The EPR spectra of 2 are composed of the separate absorption signals of the ${\mathrm{Mn}}_{12}$ cluster and $m\ensuremath{-}{\mathrm{MPYNN}}^{+}$, indicating a negligible exchange interaction between them. Below 65 K, however, an overlap between the absorption tails is clearly observed, suggesting an energy transfer between the resonances. While the temperature dependences of the paramagnetic susceptibilities of 1 and 2 are similar above $\ensuremath{\sim}4 \mathrm{K}$, the two salts show quite different spin dynamics below it. The magnetization curve and ac susceptibility measurements reveal that the organic radical enhances the relaxation of the magnetization of the ${\mathrm{Mn}}_{12}$ cluster and prevents the magnetization freezing.

The complex broadband X-ray spectrum of the starburst galaxy M82

Abstract: The broadband X-ray spectrum of the prototypical starburst galaxy M82 is very complex. At least three spectral components are required to fit the combined {ital ROSAT} and {ital ASCA} spectrum in the 0.1{endash}10 keV range. The observed X-ray flux in this band is dominated by a hard {Gamma}=1.7, heavily absorbed power-law component which originates in the nucleus and near-nuclear disk of the galaxy. Among the candidates for the origin of this hard X-ray emission, the most plausible appears to be inverse-Compton scattered emission from the interaction of the copious infrared photon flux of M82 with supernova-generated relativistic electrons. The measured intrinsic luminosity of the power-law component agrees closely with calculations of the expected inverse-Compton luminosity. Moreover, the radio and X-ray emission in the nucleus of M82 have the same spectral slope, which should be the case if both types of emission are nonthermal and are associated with a common population of electrons. The other two spectral components, thermal plasmas with characteristic temperatures kT{approx}0.6 and 0.3 keV, are associated with the star formation and starburst-driven wind in M82. The warmer thermal component is heavily absorbed as well and must also originate in the central region of the galaxy. The softer thermalmore » component, however, is not absorbed, and is likely to represent the X-ray emission that extends along the minor axis of M82. The amount of absorption required in the three-component model suggests that the intrinsic luminosity of M82 in the 0.1{endash}10 keV band is about four times greater than its observed luminosity of 4{times}10{sup 40}ergss{sup {minus}1}. {copyright} {ital 1997} {ital The American Astronomical Society}« less

Bond-Length Distortions in Strained Semiconductor Alloys

Abstract: Extended x-ray absorption fine structure measurements performed at $\mathrm{In}\ensuremath{-}K$ edge have resolved the outstanding issue of bond-length strain in semiconductor-alloy heterostructures. We determine theIn-As bond length to be $2.581\ifmmode\pm\else\textpm\fi{}0.004\AA{}$ in a buried, 213 \AA{} thick ${\mathrm{Ga}}_{0.78}{\mathrm{In}}_{0.22}\mathrm{As}$ layer grown coherently on GaAs(001). This bond length corresponds to a strain-induced contraction of $0.015\ifmmode\pm\else\textpm\fi{}0.004\AA{}$ relative to the In-As bond length in bulk ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{In}}_{x}\mathrm{As}$ of the same composition; it is consistent with a simple model which assumes a uniform bond-length distortion in the epilayer despite the inequivalent In-As and Ga-As bond lengths.

Longitudinal Josephson-plasma excitation in Bi2Sr2CaCu2O8+δ: Direct observation of the Nambu-Goldstone mode in a superconductor

Abstract: Electromagnetic resonant absorption phenomena in a microwave frequency range have recently been studied in the single-crystalline high-temperature superconductor ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ in magnetic fields. Using a rectangular microwave cavity resonator technique with ${\mathrm{TE}}_{102}$ mode this absorption is unambiguously identified to be the collective longitudinal Josephson plasma excitation. Since the superconducting state is a state with a broken phase symmetry, the ordered state should be accompanied by the collective excitation (Nambu-Goldstone mode). This excitation has long been thought not to be observable, because of the formation of the large Coulomb gap (Anderson-Higgs-Kibble mechanism), above which strong damping mechanisms of excited plasma are presumably present. However, this Coulomb gap can be very small in the case of anisotropic layered system such as ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}},$ and the plasma mode may lie in a microwave frequency region. Using characteristic dispersion relations, which enables us to separate out the longitudinal mode from the transverse one, the microwave absorption observed in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ is unambiguously attributed to the longitudinal excitations. We believe that this is the first and the direct experimental evidence of the Nambu-Goldstone mode in a superconductor and provides a direct proof that the Anderson-Higgs-Kibble (AHK) mechanism within the concept of spontaneously breaking symmetry is indeed valid in the case of superconducting phase transition. Since the finite plasma frequency observed here signifies formation of the finite mass of the plasma (phason) due to the AHK mechanism in the relativistic sense, the above-mentioned scenario in a superconductor corresponds to a direct mapping from the unified gauge-field theory of weak interaction and electromagnetic interaction shown by Weinberg and Salam.

Atomic coherence effects in four-level systems: Doppler-free absorption within an electromagnetically-induced-transparency window

Abstract: We report here an effect in a four-level ladderlike system, which is in contrast to the usual quantum interference effects such as electromagnetically induced transperency (EIT) or coherent population trapping: we predict the occurrence of a narrow absorption peak within the EIT window when an EIT atomic system interacts with an additional driving rf field. The Doppler-free-central absorption appears when the three-photon resonance condition is satisfied. In the limit of the rf field strength ${\mathrm{\ensuremath{\Omega}}}_{\mathrm{rf}}$\ensuremath{\rightarrow}0, the usual EIT profile is recovered.

Quantum confinement effect in thin quantum wires

Abstract: The electronic states and optical transition properties of three semiconductor wires Si, GaAs, and ZnSe are studied by the empirical pseudopotential homojunction model. The energy levels, wave functions, optical transition matrix elements, and lifetimes are obtained for wires of square cross section with width from 2 to 5 ($\sqrt{2}$a/2), where a is the lattice constant. It is found that these three kinds of wires have different quantum confinement properties. For Si wires, the energy gap is pseudodirect, and the wave function of the electronic ground state consists mainly of four bulk \ensuremath{\Delta} states. The optical transition matrix elements are much smaller than that of a direct transition, and increase with decreasing wire width. Where the width of wire is 7.7 \AA{}, the Si wire changes from an indirect energy gap to a direct energy gap due to mixing of the bulk ${\mathrm{\ensuremath{\Gamma}}}_{15}$ state. For GaAs wires, the energy gap is also pseudodirect in the width range considered, but the optical transition matrix elements are larger than those of Si wires by two orders of magnitude for the same width. However, there is no transfer to a direct energy gap as the wire width decreases. For ZnSe wires, the energy gap is always direct, and the optical transition matrix elements are comparable to those of the direct energy gap bulk semiconductors. They decrease with decreasing wire width due to mixing of the bulk ${\mathrm{\ensuremath{\Gamma}}}_{1}$ state with other states. All quantum confinement properties are discussed and explained by our theoretical model and the semiconductor energy band structures derived. The calculated lifetimes of the Si wire, and the positions of photoluminescence peaks, are in good agreement with experimental results.

Dynamic properties of interstitial carbon and carbon-carbon pair defects in silicon

Abstract: Interstitial carbon, ${\mathrm{C}}_{\mathrm{i}}$, defects in Si exhibit a number of unexplained features. The ${\mathrm{C}}_{\mathrm{i}}$ defect in the neutral charge state gives rise to two almost degenerate vibrational modes at 920 and 931 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ whose 2:1 absorption intensity ratio naturally suggests a trigonal defect in conflict with uniaxial stress measurements. The dicarbon, ${\mathrm{C}}_{\mathrm{s}}$-${\mathrm{C}}_{\mathrm{i}}$, defect is bistable, and the energy difference between its A and B forms is surprisingly small even though the bonding is very different. In the B form appropriate to the neutral charge state, a silicon interstitial is believed to be located near a bond-centered site between two ${\mathrm{C}}_{\mathrm{s}}$ atoms. This must give rise to vibrational modes which involve the motion of both C atoms in apparent conflict with the results of photoluminescence experiments. We use an ab initio local density functional cluster method, AIMPRO, to calculate the structure and vibrational modes of these defects and find that the ratio of the absorption intensities of the local modes of ${\mathrm{C}}_{\mathrm{i}}$ is in reasonable agreement with experiment even though the structure of the defect is not trigonal. We also show that modes in the vicinity of those detected by photoluminescence for the B form of the dicarbon center involve independent movements of the two C atoms. Finally, the trends in the relative energies of the A and B forms in three charge states are investigated.

Electrical, magneto-, and optical conductivity of quasicrystals in the Al-Re-Pd system

Abstract: Measurements of the electrical conductivity \ensuremath{\sigma}(T) and magnetoconductivity \ensuremath{\Delta}\ensuremath{\sigma}(H) of icosahedral ${\mathrm{Al}}_{70}$${\mathrm{Re}}_{10}$${\mathrm{Pd}}_{20}$ and ${\mathrm{Al}}_{70}$${\mathrm{Re}}_{8.6}$${\mathrm{Pd}}_{21.4}$ quasicrystals at low temperatures and in high magnetic fields H are reported. Below 0.2nK, \ensuremath{\sigma} of ${\mathrm{Al}}_{70}$${\mathrm{Re}}_{10}$${\mathrm{Pd}}_{20}$ varies as \ensuremath{\sigma}(T)=${\mathrm{\ensuremath{\sigma}}}_{0}$+${\mathrm{aT}}^{1\mathrm{/}2}$ with ${\mathrm{\ensuremath{\sigma}}}_{0}$=30 ${\mathrm{\ensuremath{\Omega}}}^{\mathrm{\ensuremath{-}}1}$${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ and a0. The conductivity decreases with increasing magnetic field. For ${\mathrm{Al}}_{70}$${\mathrm{Re}}_{8.6}$${\mathrm{Pd}}_{21.4}$ \ensuremath{\sigma}(T) decreases with decreasing T and saturates at the level ${\mathrm{\ensuremath{\sigma}}}_{0}$=1.7n${\mathrm{\ensuremath{\Omega}}}^{\mathrm{\ensuremath{-}}1}$${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ below 0.1nK. In magnetic fields up to 40 kOe, \ensuremath{\Delta}\ensuremath{\sigma}(H) is negative below 1 K and \ensuremath{\sigma}(T,H) remains temperature-independent at the lowest temperatures. The optical conductivity \ensuremath{\sigma}(\ensuremath{\omega}) is obtained from reflectivity data in the frequency range between 15 and ${10}^{5}$${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. The main contributions to \ensuremath{\sigma}(\ensuremath{\omega}) are a broad absorption signal centered at about 0.1neV and a sizable peak with its maximum at 2.6neV.

Luminescence properties of ZrO 2 − C a O eutectic crystals with ordered lamellar microstructure activated with Er 3 + ions

Abstract: ${\mathrm{Er}}^{3+}$-doped ${\mathrm{ZrO}}_{2}\ensuremath{-}\mathrm{C}\mathrm{a}\mathrm{O}$ eutectic crystals with ordered microstructure have been grown by the laser floating zone method. The microstructure consists of alternating 2 \ensuremath{\mu}m thick lamellae of calcia-stabilized zirconia (CaSZ) and ${\mathrm{CaZrO}}_{3}$ crystals. ${\mathrm{Er}}^{3+}$ enters both phases but at a higher concentration in CaSZ. Due to its higher refractive index, the latter phase acts as a light guiding medium. The optical properties of ${\mathrm{Er}}^{3+}$-doped eutectic, as well as its components, CaSZ and ${\mathrm{CaZrO}}_{3}$ crystals, are analyzed. Absorption and emission spectra of ${\mathrm{CaZrO}}_{3}$ consist of narrow bands. For this crystal most of the ${\mathrm{Er}}^{3+}$ Stark level positions are given. In contrast, the bands for CaSZ are broad, as it corresponds to a defective material. Absorption oscillator strengths, radiative-transition probabilities and emission lifetimes have been measured at different temperatures, between 600 and 4 K. Absorption oscillator strengths are 1.5 times higher for ${\mathrm{Er}}^{3+}$ in ${\mathrm{CaZrO}}_{3}$ than in CaSZ, while multiphonon deexcitation probabilities are one order of magnitude smaller, with the exception of the lowest energy ${}^{4}{I}_{13/2}{\ensuremath{\rightarrow}}^{4}{I}_{15/2}$ emission, which presents the maximum quantum efficiency in both phases.

Measurements of inverse faraday effect and absorption of circularly polarized laser light in plasmas

Abstract: Measurements of the inverse Faraday effect and of the absorption of circularly polarized laser light in plasmas are reported. The experiments were performed with a Nd:YAG laser system. For the laser irradiance range studied here, $9\ifmmode\times\else\texttimes\fi{}{10}^{13}\char21{}2.5\ifmmode\times\else\texttimes\fi{}{10}^{14}\mathrm{W}/{\mathrm{cm}}^{2}$, the absorption of circularly polarized light was higher by 14% relative to the absorption of linear polarized light. It is suggested that the above increase in the laser absorption is related to the axial magnetic field in the plasma created by the circularly polarized laser light. Axial magnetic fields of tens of kilogauss were measured at irradiances in the range of 1${0}^{12}$\char21{}1${0}^{13}$ W/c${\mathrm{m}}^{2}$ using the Faraday rotation diagnostic.

Weibel instability due to inverse bremsstrahlung absorption

Abstract: A new Weibel source due to the inverse bremsstrahlung absorption is presented. It has been shown that in homogeneous plasmas, this mechanism may drive strong collisionless Weibel modes with growth rates of order of \ensuremath{\gamma}\ensuremath{\sim}${10}^{11}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ and negligible group velocities. In the laser-produced plasmas, for short laser wavelengths (${\ensuremath{\lambda}}_{\mathrm{L}}$1 \ensuremath{\mu}m) and high laser fluxes (Ig${10}^{14}$ W/${\mathrm{cm}}^{2}$), this Weibel source is most efficient as the ones due to the heat flux and the plasma expansion. The useful scaling law of the convective e-foldings, with respect to the laser and the plasma parameters, is also derived.

Dramatic changes in the 3s autoionization process at the beginning of the ar i sequence

Abstract: The 3s{r_arrow}np resonances were observed to change dramatically in appearance with increasing ionization along the ArI sequence, and within the 3s{r_arrow}np channel of Ca{sup 2+}. By applying the Dyson equation method to positive ions for the first time, newly investigated double-electron processes were shown to play a crucial role in the interpretation of the resonance structure. The changes within the resonances result from their position relative to the Cooper minima in the 3{ital s} and 3{ital p} photoabsorption. {copyright} {ital 1997} {ital The American Physical Society}

2p resonant photoemission study of TiO 2 s

Abstract: Resonant photoemission measurements of ${\mathrm{TiO}}_{2}$ (100) at the Ti ${\mathrm{L}}_{2,3}$ edge are reported. For transitions from 2p to 3d states of ${\mathrm{e}}_{\mathrm{g}}$ symmetry there is a resonance enhancement of the intensity of the band-gap defect states by a factor of 6. The valence band shows resonance enhancements for transitions to states of both ${\mathrm{e}}_{\mathrm{g}}$ and ${\mathrm{t}}_{2\mathrm{g}}$ symmetry. The maximum enhancement of 3 for the high binding energy indicates that the whole of the O 2p derived valence band is hybridized with Ti 3d states, with the higher binding-energy part being more strongly mixed. The defect-state resonances are broader than the absorption resonances and have a width of about 4 eV due to the multiplet structure.