Showing papers on "Absorption (logic) published in 2004"

Highly efficient four-wave mixing in double- Λ system in ultraslow propagation regime

Abstract: We perform a time-dependent analysis of four-wave mixing (FWM) in a double-$\ensuremath{\Lambda}$ system in an ultraslow-propagation regime and obtain the analytical expressions of pulsed probe laser, FWM-generated pulse, phase shifts and absorption coefficients, group velocities, and FWM efficiency. With these analytical expressions, we show that an efficiently generated FWM field can acquire the same ultraslow group velocity $({V}_{g}∕c\ensuremath{\sim}{10}^{\ensuremath{-}4}--{10}^{\ensuremath{-}5})$ and pulse shape of a probe pump and that the maximum FWM efficiency is greater than 25%, which is orders of magnitude larger than previous FWM schemes in the ultraslow-propagation regime.

Atomic Interferometer with Amplitude Gratings of Light and Its Applications to Atom Based Tests of the Equivalence Principle

Abstract: We have developed a matter wave interferometer based on the diffraction of atoms from effective absorption gratings of light. In a setup with cold rubidium atoms in an atomic fountain the interferometer has been used to carry out tests of the equivalence principle on an atomic basis. The gravitational acceleration of the two isotopes $^{85}\mathrm{R}\mathrm{b}$ and $^{87}\mathrm{R}\mathrm{b}$ was compared, yielding a difference $\ensuremath{\Delta}\mathbf{g}/\mathbf{g}=(1.2\ifmmode\pm\else\textpm\fi{}1.7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$. We also perform a differential free fall measurement of atoms in two different hyperfine states, and obtained a result of $\ensuremath{\Delta}\mathbf{g}/\mathbf{g}=(0.4\ifmmode\pm\else\textpm\fi{}1.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}7}$.

Mg II Absorption Systems in SDSS QSO Spectra

Abstract: We present the results of a MgII absorption-line survey using QSO spectra from the SDSS EDR. Over 1,300 doublets with rest equivalent widths greater than 0.3\AA and redshifts $0.366 \le z \le 2.269$ were identified and measured. We find that the $\lambda2796$ rest equivalent width ($W_0^{\lambda2796}$) distribution is described very well by an exponential function $\partial N/\partial W_0^{\lambda2796} = \frac{N^*}{W^*} e^{-\frac{W_0}{W^*}}$, with $N^*=1.187\pm0.052$ and $W^*=0.702\pm0.017$\AA. Previously reported power law fits drastically over-predict the number of strong lines. Extrapolating our exponential fit under-predicts the number of $W_0 \le 0.3$\AA systems, indicating a transition in $dN/dW_0$ near $W_0 \simeq 0.3$\AA. A combination of two exponentials reproduces the observed distribution well, suggesting that MgII absorbers are the superposition of at least two physically distinct populations of absorbing clouds. We also derive a new redshift parameterization for the number density of $W_0^{\lambda2796} \ge 0.3$\AA lines: $N^*=1.001\pm0.132(1+z)^{0.226\pm0.170}$ and $W^*=0.443\pm0.032(1+z)^{0.634\pm 0.097}$\AA. We find that the distribution steepens with decreasing redshift, with $W^*$ decreasing from $0.80\pm0.04$\AA at $z=1.6$ to $0.59\pm0.02$\AA at $z=0.7$. The incidence of moderately strong MgII $\lambda2796$ lines does not show evidence for evolution with redshift. However, lines stronger than $\approx 2$\AA show a decrease relative to the no-evolution prediction with decreasing redshift for $z \lesssim 1$. The evolution is stronger for increasingly stronger lines. Since $W_0$ in saturated absorption lines is an indicator of the velocity spread of the absorbing clouds, we interpret this as an evolution in the kinematic properties of galaxies from moderate to low z.

Electronic structure of nanostructured ZnO from x-ray absorption and emission spectroscopy and the local density approximation

Abstract: $\mathrm{O}\phantom{\rule{0.3em}{0ex}}1s$ absorption spectroscopy (XAS) and $\mathrm{O}\phantom{\rule{0.3em}{0ex}}K\ensuremath{\alpha}$ emission spectroscopy (XES) were performed to study the electronic structure of nanostructured ZnO. The band gap is determined by the combined absorption-emission spectrum. Resonantly excited XES spectra showing an energy dependence in the spectral shape reveal the selected excitations to the different $\mathrm{Zn}\phantom{\rule{0.3em}{0ex}}3d$, $4s$, and $4p$ states in hybridization with $\mathrm{O}\phantom{\rule{0.3em}{0ex}}2p$ states. The partial density of state obtained from local density approximation (LDA) and $\mathrm{LDA}+U$ calculations are compared with the experimental results. The $\mathrm{LDA}+U$ approach is suitable to correct LDA self-interaction error of the cation $d$ states. The atomic eigenstates of $3d$ in zinc and $2p$ in oxygen are energetically close, which induces the strong interaction between $\mathrm{Zn}\phantom{\rule{0.3em}{0ex}}3d$ and $\mathrm{O}\phantom{\rule{0.3em}{0ex}}2p$ states. This anomalous valence band cation-$d$--anion-$p$ hybridization is verified by taking into account the strong localization of the $\mathrm{Zn}\phantom{\rule{0.3em}{0ex}}3d$ states.

Electronic structure of titania aerogels from soft x-ray absorption spectroscopy

Abstract: The electronic structure of ${\mathrm{TiO}}_{2}$ aerogels is studied by soft x-ray absorption near-edge structure (XANES) spectroscopy. High-resolution O K-edge and Ti ${L}_{2,3}$-edge XANES spectra of aerogels are compared with those of rutile, anatase, and unrelaxed amorphous phases of full-density ${\mathrm{TiO}}_{2}.$ Results show that all the main spectroscopic features of aerogels, reflecting the element-specific partial density of empty electronic states and correlation effects, can be attributed to the absence of long-range order in stoichiometric amorphous ${\mathrm{TiO}}_{2}.$ Based on these results, we discuss the effects of short- and long-range order on the electronic structure of ${\mathrm{TiO}}_{2}.$

New nucleosynthesis constraint on the variation of G.

Abstract: Big bang nucleosynthesis can provide, via constraints on the expansion rate at that time, limits on possible variations in Newton's constant, $G$. The original analyses were performed before an independent measurement of the baryon-to-photon ratio from the cosmic microwave background was available. Combining this with recent measurements of the primordial deuterium abundance in quasar absorption systems now allows one to derive a new tighter constraint on $G$ without recourse to considerations of helium or lithium abundances. We find that, compared to today's value, ${G}_{0}$, ${G}_{\mathrm{BBN}}/{G}_{0}={1.01}_{\ensuremath{-}0.16}^{+0.20}$ at the $68%$ confidence level.

DUST AND DIFFUSE INTERSTELLAR BANDS IN THE z~a = 0.524 ABSORPTION SYSTEM TOWARD AO 0235+164

Abstract: The authors present new HST STIS NUV-MAMA and STIS CCD observations of the BL Lac object AO 0235+164 and the intervening damped Ly {alpha} (DLA) line at z{sub {alpha}} = 0.524. The line profile gives N(HI) = 5 {+-} 1 x 10{sup 21} cm{sup -2} and, combined with the H I 21 cm absorption data leads to a spin temperature of Ts = 220 K {+-} 60 K. Those spectra also show a strong, broad feature at the expected position of the 2175 {angstrom} graphitic dust feature at z{sup {alpha}} = 0.524. Assuming a Galactic type dust extinction curve at z{sub {alpha}} = 0.524 gives a dust-to-gas ratio of 0.19 Galactic, but the fit, assuming the underlying, un-reddened spectrum is a single power-law, is poor in the far-UV. A dust-to-gas ratio of 0.19 Galactic is similar to the LMC, but the AO 0235+164 spectrum does not fit the LMC extinction curve, or the SMC extinction curve (which has practically no 2175 {angstrom} feature). A possible interpretation includes dust similar to Galactic, but with less of the small particles that produce the far-UV extinction. The metallicity of the z{sub {alpha}} = 0.524 absorber, estimated from the observed N(HI) and excess X-raymore » absorption (beyond Galactic) derived from contemporaneous and archival ASCA and ROSAT.« less

Limits on variations of the quark masses, QCD scale, and fine structure constant

Abstract: We perform calculations of the dependence of nuclear magnetic moments on quark masses and obtain limits on the variation of the fine structure constant $\ensuremath{\alpha}$ and ${(m}_{q}/{\ensuremath{\Lambda}}_{\mathrm{QCD}})$ from recent measurements of hydrogen hyperfine (21 cm) and molecular rotational transitions in quasar absorption systems, atomic clock experiments with hyperfine transitions in H, Rb, Cs, ${\mathrm{Yb}}^{+},$ ${\mathrm{Hg}}^{+},$ and optical transition in ${\mathrm{Hg}}^{+}.$ Experiments with ${\mathrm{Cd}}^{+},$ deuterium/hydrogen, molecular ${\mathrm{SF}}_{6},$ and Zeeman transitions in ${}^{3}\mathrm{H}\mathrm{e}/\mathrm{X}\mathrm{e}$ are also discussed.

Surface and bulk electronic structure of ScN ( 001 ) investigated by scanning tunneling microscopy/spectroscopy and optical absorption spectroscopy

Abstract: $\mathrm{ScN}(001)$ $1\ifmmode\times\else\texttimes\fi{}1$ surfaces have been prepared by growing $\mathrm{ScN}$ on $\mathrm{MgO}(001)$ using radio frequency molecular beam epitaxy. In situ ultrahigh vacuum scanning tunneling spectroscopy indicates that the Fermi level at the surface lies slightly above the $\mathrm{Sc}\phantom{\rule{0.3em}{0ex}}3d$ conduction band edge, which is attributed to a downward band bending at the surface. In situ scanning tunneling microscopy is used to image the $\mathrm{Sc}$ and $\mathrm{N}$ atom sublattices. While only one atom $(\mathrm{Sc})$ appears at small negative bias, both atoms ($\mathrm{Sc}$ and $\mathrm{N}$) appear at certain positive sample biases due to the partially ionic nature of the bonding. Charge accumulation near ionized subsurface donors is evident from the long-range topographic distortions at the surface. The combination of tunneling spectroscopy and optical absorption results show that $\mathrm{ScN}$ has an indirect bandgap of $0.9\ifmmode\pm\else\textpm\fi{}0.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and a direct transition at $2.15\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$.

Temperature-dependent photoluminescence from single-walled carbon nanotubes

Abstract: Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy of pillar-suspended single-walled carbon nanotubes has been measured for temperatures between 300 and $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The atmospheric environment strongly affects the low-temperature luminescence. The PL intensity is quenched at temperatures below $\ensuremath{\sim}40\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ for nanotubes in high vacuum, while nanotubes in helium ambient remain luminescent. The PL peak emission energy is only very weakly dependent on temperature, with a species-dependent blueshift upon cooling corresponding to a relative shift in bandgap of $\ensuremath{-}3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}\phantom{\rule{0.3em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ or less. The integrated peak intensities change by only a factor of 2, with linewidths showing a moderate temperature dependence. In PLE, the second absorption peak energy $({E}_{22})$ is also only weakly temperature dependent, with no significant shift and a limited reduction in linewidth upon cooling to $20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In addition to the previously assigned nanotube PL peaks seen at room temperature, at least two distinct new classes of PL peaks are observed at cryogenic temperatures.

Spectroscopy in an extremely thin vapor cell: Comparing the cell-length dependence in fluorescence and in absorption techniques

Abstract: We compare the behavior of absorption and of resonance fluorescence spectra in an extremely thin Rb vapor cell as a function of the ratio of $L∕\ensuremath{\lambda}$, with $L$ the cell thickness $(L\ensuremath{\sim}150--1800\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ and $\ensuremath{\lambda}$ the wavelength of the Rb ${D}_{2}$ line $(\ensuremath{\lambda}=780\phantom{\rule{0.3em}{0ex}}\mathrm{mn})$. The Dicke-type coherent narrowing [G. Dutier et al., Europhys. Lett. 63, 35 (2003)] is observed only in transmission measurements, in the linear regime, with its typical collapse and revival, which reaches a maximum for $L=(2n+1)\ensuremath{\lambda}∕2$ ($n$ integer). It is shown not to appear in fluorescence, whose behavior-amplitude, and spectral width, is more monotonic with $L$. Conversely, at high-intensity, the sub-Doppler saturation effects are shown to be the most visible in transmission around $L=n\ensuremath{\lambda}$.

Narrow Escape, Part I

Abstract: A Brownian particle with diffusion coefficient $D$ is confined to a bounded domain of volume $V$ in $\rR^3$ by a reflecting boundary, except for a small absorbing window. The mean time to absorption diverges as the window shrinks, thus rendering the calculation of the mean escape time a singular perturbation problem. We construct an asymptotic approximation for the case of an elliptical window of large semi axis $a\ll V^{1/3}$ and show that the mean escape time is $E\tau\sim\ds{\frac{V}{2\pi Da}} K(e)$, where $e$ is the eccentricity of the ellipse; and $K(\cdot)$ is the complete elliptic integral of the first kind. In the special case of a circular hole the result reduces to Lord Rayleigh's formula $E\tau\sim\ds{\frac{V}{4aD}}$, which was derived by heuristic considerations. For the special case of a spherical domain, we obtain the asymptotic expansion $E\tau=\ds{\frac{V}{4aD}} [1+\frac{a}{R} \log \frac{R}{a} + O(\frac{a}{R}) ]$. This problem is important in understanding the flow of ions in and out of narrow valves that control a wide range of biological and technological function.

XMM-Newton observation of the relaxed cluster A478: gas and dark matter distribution from 0.01 R_200 to 0.5 R_200

Abstract: We present an \xmm mosaic observation of the hot ($kT\sim6.5$ keV) and nearby ($z=0.0881$) relaxed cluster of galaxies A478. We derive precise gas density, gas temperature, gas mass and total mass profiles up to $12\arcmin$ (about half of the virial radius $R_{200}$). The gas density profile is highly peaked towards the center and the surface brightness profile is well fitted by a sum of three $\beta$--models. The derived gas density profile is in excellent agreement, both in shape and in normalization, with the published Chandra density profile (measured within $5\arcmin$ of the center). Projection and PSF effects on the temperature profile determination are thoroughly investigated. The derived radial temperature structure is as expected for a cluster hosting a cooling core, with a strong negative gradient at the cluster center. The temperature rises from $\sim2$ keV up to a plateau of $\sim6.5$ keV beyond 2' (i.e. $r>208\rm{kpc}=0.1 R_{200}$, $R_{200}=2.08$ Mpc being the virial radius). From the temperature profile and the density profile and under the hypothesis of hydrostatic equilibrium, we derived the total mass profile of A478 down to 0.01 and up to 0.5 the virial radius. We tested different dark matter models against the observed mass profile. The Navarro, Frenk & White (\cite{navarro97}) model is significantly preferred to other models. It leads to a total mass of $M_{200}=1.1\times 10^{15}$ M$_\odot$ for a concentration parameter of $c=4.2\pm0.4$. The gas mass fraction slightly increases with radius. The gas mass fraction at a density contrast of $\delta=2500$ is $\fgas=0.13\pm0.02$, consistent with previous results on similar hot and massive clusters. We confirm the excess of absorption in the direction of A478.[abridged]

Interaction of argon clusters with intense VUV-laser radiation: the role of electronic structure in the energy-deposition process.

Abstract: The response of Ar clusters to intense vacuum-ultraviolet pulses is investigated with photoion spectroscopy. By varying the laser wavelength, the initial excitation was either tuned to absorption bands of surface or bulk atoms of clusters. Multiple ionization is observed, which leads to Coulomb explosion. The efficiency of resonant 2-photon ionization for initial bulk and surface excitation is compared with that of the nonresonant process at different laser intensities. The specific electronic structure of clusters plays almost no role in the explosion dynamics at a peak intensity larger than $1.8\ifmmode\times\else\texttimes\fi{}{10}^{12}\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$. The inner ionization of atoms for resonant and nonresonant excitation is then saturated and the energy deposition is mainly controlled by the plasma heating rate. Molecular dynamics simulations indicate that standard collisional heating cannot fully account for the strong energy absorption.

Cubic TiO 2 as a potential light absorber in solar-energy conversion

Abstract: Materials are currently sought for use in the photo-induced decomposition of water on crystalline electrodes. Titanium dioxide is valuable in this respect. The electronic structural properties of cubic ${\mathrm{TiO}}_{2}$ polymorphs were investigated by means of first-principles methods. We demonstrate that both fluorite- and pyrite-type ${\mathrm{TiO}}_{2}$ have important optical absorptive transitions in the region of the visible light. A cubic ${\mathrm{TiO}}_{2}$ phase that can efficiently absorb the sunlight would be an important candidate material for the development of the solar cells. Also, we present results on the $\mathrm{Ti}$ $L$ edges for the two different titania forms. We predict that a qualitative spectroscopic discrimination of the cubic polymorphs can be achieved by following the $\mathrm{Ti}$ $2p\ensuremath{\rightarrow}3d$ x-ray transitions.

Excited-State Relaxation of Ag 8 Clusters Embedded in Helium Droplets

Abstract: Neutral silver clusters ${\mathrm{Ag}}_{N}$ are grown in ultracold helium nanodroplets. By exploiting a strong absorption resonance recently found for ${\mathrm{Ag}}_{8}$, first photoelectron spectra of this neutral species are recorded. Variation of the laser photon energy reveals that direct vertical two-photon ionization is hindered by rapid relaxation into the lower edge of a long-living excited state manifold. The analysis of the dynamics gives a precise value of $(6.89\ifmmode\pm\else\textpm\fi{}0.09)\text{ }\mathrm{eV}$ for the vertical ionization potential of ${\mathrm{Ag}}_{8}$. The influence of the helium matrix on photoemission is discussed.

Electronic structure of MS (M = Ca, Mg, Fe, Mn): X-ray absorption analysis

Abstract: Experimental x-ray absorption spectra were measured near the sulfur K and ${L}_{2,3}$ edges of MgS, CaS, MnS, and FeS. An agreement between full multiple scattering theoretical simulations and experiment is reached for comparatively large clusters (consisting of 9--13 shells) of sulfides with the NaCl-type structure (MgS,CaS,MnS), while a small cluster of four shells appears sufficient for the NiAs-type structure of FeS. It was shown that in the ${\mathrm{Mg}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x}\mathrm{S}$ solid solution, iron sulfide changes its structure from NiAs-type $(B8)$ to NaCl-type $(B1)$ under the influence of the MgS matrix. The partial electronic densities of unoccupied states have been calculated for MgS and CaS. It is found that the bottom of the conduction band of MgS is formed by Mg s and S s states and of CaS by Ca d states and S d, s, and p states. A special kind of hybridization has been found: it is shown that the sulfur p states are ``squeezed out'' of the energy region of the iron d states.

Optical properties of copper clusters embedded in alumina: An experimental and theoretical study of size dependence

Abstract: Optical properties of copper clusters of $3\phantom{\rule{0.5em}{0ex}}\text{to}\phantom{\rule{0.5em}{0ex}}5\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in diameter, produced by laser vaporization and embedded in alumina, are investigated and compared to a semi-quantal model [based on time-dependent local-density approximation (TDLDA) and density functional theory (DFT)], including the absorption and screening properties of the ionic core background and the surrounding matrix. To begin with, the experiments show that the alumina trapped clusters are oxidized if codeposited at room temperature whereas they do remain metallic by holding the substrate temperature at $400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$. On the other hand, reducing under ${\mathrm{H}}_{2}\text{\ensuremath{-}}{\mathrm{N}}_{2}$ atmosphere of samples made at room temperature is an alternative to elaborate embedded metallic copper clusters. Consequently, the alumina matrix evaporated on a substrate at $400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ is optically characterized and the oxidization level of the copper clusters is carefully investigated through x-ray photoelectron spectroscopy (XPS). Concerning the optical properties of the copper clusters, a strong damping and a broadening of the surface plasmon resonance (SPR) with decreasing size is observed, in good agreement with theoretical calculations. This damping, much more important than in gold clusters, is correlated to a strong coupling of the collective excitation with interband transitions. These results allow us to validate the semiquantal model for the three systems Au, Ag, and Cu, in which the size effects are ruled by the electronic spillout phenomenon, the surface of ineffective ionic core polarization and the local porosity at the metal/matrix interface. Lastly, the inhomogeneous effects such as size dispersion are shown to be of less importance compared to intrinsic size effects in the optical properties.

Triplet exciton state and related phenomena in the β -phase of poly(9,9-dioctyl)fluorene

Abstract: Using both time-resolved emission and cw photoinduced absorption spectroscopy as a function of temperature, the aggregation phenomena ($\ensuremath{\beta}$-phase formation) observed in poly(9,9-dioctyl)fluorene is studied. All spectra of the $\ensuremath{\beta}$ phase, including absorption, prompt and delayed fluorescence, phosphorescence, and photoinduced triplet absorption feature very narrow linewidths, which are unique within the class of conjugated polymers. From the comparison of the latter data with amorphous polyfluorene, poly(9,9-diethylhexyl)fluorene, as well as with the fully planar ladder-type poly(paraphenylene), we conclude that the origin of the $\ensuremath{\beta}$ phase cannot simply be an extended intrachain conjugation, but interchain interactions are involved. Furthermore, the $\ensuremath{\beta}$ phase acts as an energetic trap for both singlet and triplet excitons initially created on amorphous chain segments. The delayed fluorescence kinetics of the $\ensuremath{\beta}$ phase were measured at different temperatures. From the analysis of these decays within the framework of dispersive triplet migration in a Gaussian density of states distribution, further evidence is provided that the delayed fluorescence originates from triplet-triplet annihilation. At room temperature, it is clear that triplet excitons migrate over large distances, exceeding that of singlet excitons. Also, the segregation time between dispersive triplet migration and classical thermally activated hopping, is in the case of $\ensuremath{\beta}$-phase containing samples, dependent on the separation of the $\ensuremath{\beta}$-phase domains.

Influence of carrier-carrier interaction on time-dependent intersubband absorption in a semiconductor quantum well

Abstract: Using ultrafast terahertz spectroscopy, we measure the temporal evolution of the intersubband absorption spectrum of a $\mathrm{GaAs}∕{\mathrm{Al}}_{03}{\mathrm{Ga}}_{07}\mathrm{As}$ double-quantum-well structure with an energy spacing between the first two subbands smaller than the longitudinal optical phonon energy We show that the interaction between the photoexcited carriers has a considerable influence on the time-dependent absorption When varying the photoexcited sheet carrier density between $1\ifmmode\times\else\texttimes\fi{}{10}^{10}\phantom{\rule{03em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$ and more than $1\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{03em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$, we find (i) a strong dependence of the intersubband scattering rate on the density of optically generated carriers, and (ii) a temporal shift of the intersubband resonance as the population in the second subband decays, ie, as the photoexcited carriers relax into the quantum-well ground state

Metal-dimer atomic reconstruction leading to deep donor states of the anion vacancy in II-VI and chalcopyrite semiconductors.

Abstract: First-principles total-energy calculations reveal a novel local atomic reconstruction mode around anion vacancies in II-VI and chalcopyrite compounds resulting from the formation of metal dimers. As a consequence, the neutral Se vacancy has an unexpected low symmetry in ZnSe and becomes a deep donor in both ZnSe and $\mathrm{C}\mathrm{u}\mathrm{G}\mathrm{a}\mathrm{S}{\mathrm{e}}_{2}$, contrary to the common belief regarding chalcopyrites. The calculated optical transition energies explain the hitherto puzzling absorption bands observed in the classic experiments of the color center in ZnS.

Electronic structures of B 2p and C 2p levels in boron-doped diamond films studied using soft x-ray absorption and emission spectroscopy

Abstract: X-ray absorption (XAS) and emission (XES) spectroscopy near B K and C K edges have been performed on metallic ($\ensuremath{\sim}01\phantom{\rule{03em}{0ex}}\mathrm{at}\phantom{\rule{02em}{0ex}}%$ B, B-diamond) and semiconducting ($\ensuremath{\sim}003\phantom{\rule{03em}{0ex}}\mathrm{at}\phantom{\rule{02em}{0ex}}%$ B and N, BN-diamond) doped diamond films Both B K XAS and XES spectra show a metallic partial density of states (PDOS) with the Fermi energy of $1853\phantom{\rule{03em}{0ex}}\mathrm{eV}$, and there is no apparent boron-concentration dependence in contrast to the different electric property In C K XAS spectrum of B-diamond, the impurity state ascribed to boron is clearly observed near the Fermi level The Fermi energy is found to be almost same with the top of the valence band of nondoped diamond: ${E}_{V}=2839\phantom{\rule{03em}{0ex}}\mathrm{eV}$ C K XAS of BN-diamond shows both the B-induced shallow level and N-induced deep and broad levels as the in-gap states, in which the shallow level is in good agreement with the activation energy $({E}_{a}=037\phantom{\rule{03em}{0ex}}\mathrm{eV})$ estimated from the temperature dependence of the conductivity; namely, the change in $\mathrm{C}\phantom{\rule{02em}{0ex}}2p$ PDOS of impurity-induced metallization is directly observed The electric property of this diamond is ascribed mainly to the electronic structure of $\mathrm{C}\phantom{\rule{02em}{0ex}}2p$ near the Fermi level The observed XES spectra are compared with the discrete variational $\mathrm{X}\ensuremath{\alpha}$ ($\mathrm{DVX}\ensuremath{\alpha}$) cluster calculation The $\mathrm{DVX}\ensuremath{\alpha}$ result supports the strong hybridization between $\mathrm{B}\phantom{\rule{02em}{0ex}}2p$ and $\mathrm{C}\phantom{\rule{02em}{0ex}}2p$ observed in XAS and XES spectra, and suggests that the small amount of boron $(\ensuremath{\leqslant}01\phantom{\rule{03em}{0ex}}\mathrm{at}\phantom{\rule{02em}{0ex}}%)$ in diamond occupies the substitutional site rather than interstitial site

Threshold anomaly with weakly bound projectiles: Elastic scattering of Be 9 + Al 27

Abstract: Elastic scattering of the weakly bound $^{9}\mathrm{Be}$ on $^{27}\mathrm{Al}$ was measured at near barrier energies. The optical model data analysis with the real and imaginary parts of a global double-folding potential does not show strong evidence of the usual threshold anomaly. The same result was obtained by using a Woods-Saxon shape optical potential and calculating the potential strengths at the strong absorption radius. The reason for this behavior may be explained by the presence of break-up and∕or transfer channels at low energies.

Charge-transfer transitions between wires and spacers in an inorganic-organic quasi-one-dimensional crystal methylviologen lead iodide

Abstract: We have studied the optical property and electronic structure of an inorganic-organic quasi-one-dimensional crystal, methylviologen lead iodide. Two absorption bands were observed in the near ultraviolet and visible regions. The one is an absorption band at $3.1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ with a large oscillator strength, which is assigned to one-dimensional excitons confined in the inorganic chains. The other is a broad band observed between 2.0 and $2.8\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ and attributed to charge-transfer transitions from the lead iodide chains to the methylviologen molecules. From the luminescence excitation spectra, it is found that there are two kinds of charge-transfer transitions from the valence band and the nonbonding state of the inorganic chains to the lowest unoccupied molecular orbital of methylviologen.

Vibrationally Resolved K-shell Photoionization of CO with Circularly Polarized Light

Abstract: Diffraction of a low energy ($l4\text{ }\text{ }\mathrm{e}\mathrm{V}$) carbon-$K$-photoelectron wave that is created inside a CO molecule by absorption of a circularly polarized photon is investigated. The measurements resolve the vibrational states of the $K$-shell ionized $\mathrm{C}{\mathrm{O}}^{\mathrm{+}}$ molecule and display the photoelectron diffraction patterns in the molecular frame. These show significant variation for the different vibrational states. This effect is stronger than predicted by state-of-the-art theory. As this study is performed close to C-$K$-threshold and, therefore, far below the molecule's $\ensuremath{\sigma}$-shape resonance, this surprisingly strong effect is not related to that resonance phenomenon.

Optical two-photon absorption in GaAs measured by optical-pump terahertz-probe spectroscopy

Abstract: Time-resolved terahertz experiments in a novel setup were used for a direct observation of the competition between the single- and two-photon absorption in highly excited GaAs. The experiments were carried out near $800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ where the single photon absorption usually dominates. The crystal surface was excited by femtosecond laser pulses with fluences up to $4.2\phantom{\rule{0.3em}{0ex}}\mathrm{mJ}∕{\mathrm{cm}}^{2}$, for which the single-photon absorption saturates and the two-photon absorption becomes the leading absorption mechanism. The two-photon absorption coefficient $\ensuremath{\beta}$, its anisotropy and the depth profile of the photoexcited carriers were determined.

Effect of H insertion on the magnetic, electronic, and structural properties of CeCoSi

Abstract: Contrary to other cerium equiatomic intermetallic systems ($\mathrm{Ce}TX$ with $T=\text{transition}$ metal, $X=p$ metal) which become magnetically ordered upon hydrogen insertion, magnetic measurements characterize an antiferromagnet for CeCoSi and a spin fluctuation system for its hydride CeCoSiH The presented results of electrical and magnetic properties of the silicide CeCoSi demonstrate the existence of an antiferromagnetic transition at ${T}_{N}=88(2)\phantom{\rule{03em}{0ex}}\mathrm{K}$ For the purpose of further assessing the experimental findings, the electronic and magnetic sructures of CeCoSi and CeCoSiH are self-consistently calculated within the local spin density functional LSDF theory The analysis of the electronic structures and of the chemical bonding properties lead to suggest that the chemical effect of hydrogen prevails over the cell expansion effect which would enhance the magnetization otherwise Spin orbit coupling effects on cerium are examined A potential antiferromagnetic structure with a propagation along the $c$ axis is proposed for CeCoSi ground state

Raman scattering and absorption study of the high-pressure wurtzite to rocksalt phase transition of GaN

Abstract: The high-pressure wurtzite to rocksalt phase transition of high quality epitaxial GaN has been studied using Raman scattering and optical absorption spectroscopy. The results are compared with ab initio calculations performed within the framework of density functional theory. The Raman scattering due to the phonon modes of the wurtzite phase was followed up to $69\phantom{\rule{0.3em}{0ex}}\text{GPa}$; the phase transition was marked by the appearance of new features which we assign to disorder activated, symmetry forbidden, scattering from the rocksalt phase. There is no evidence for the recently proposed metallic $\text{MgO}\text{\ensuremath{-}}h$ intermediate phase. Absorption measurements show an absorption feature at $1.7\phantom{\rule{0.3em}{0ex}}\text{eV}$ for the rocksalt phase at $69\phantom{\rule{0.3em}{0ex}}\text{GPa}$.

Structural properties of the geometrically frustrated pyrochlore Tb 2 Ti 2 O 7

Abstract: Although materials that exhibit nearest-neighbor-only antiferromagnetic interactions and geometrical frustration theoretically should not magnetically order in the absence of disorder, few such systems have been observed experimentally. One such system appears to be the pyrochlore ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}.$ However, previous structural studies indicated that ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ is an imperfect pyrochlore. To clarify the situation, we performed neutron powder-diffraction (NPD) and x-ray absorption fine-structure (XAFS) measurements on samples that were prepared identically to those that show no magnetic order. The NPD measurements show that the long-range structure of ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ is well ordered with no structural transitions between 4.5 and 600 K. In particular, mean-squared displacements ${u}^{2}\mathrm{'}\mathrm{s}$ for each site follow a Debye model with no offsets. No evidence for Tb/Ti site interchange was observed within an upper limit of 2%. Likewise, no excess or deficiency in the oxygen stoichiometry was observed, within an upper limit of 2% of the nominal pyrochlore value. Tb ${L}_{\mathrm{III}}$- and Ti K-edge XAFS measurements from 20 K to 300 K similarly indicate a well-ordered local structure. Other aspects of the structure are considered. We conclude that ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ has, within experimental error, an ideal, disorder-free pyrochlore lattice, thereby allowing the system to remain in a dynamic, frustrated spin state to the lowest observed temperatures.

Concentration-dependent short-range order in the relaxor ferroelectric (1-x)pb(sc,ta)o3-xpbtio3

Abstract: Using the x-ray diffraction and x-ray absorption fine structure techniques, we have probed $\mathrm{Ti}$, $\mathrm{Ta}$, and $\mathrm{Sc}$ local environments in the solid solution system $(1\ensuremath{-}x{)\mathrm{Pb}(\mathrm{Sc},\mathrm{Ta})\mathrm{O}}_{3}\text{\ensuremath{-}}x{\mathrm{PbTiO}}_{3}$. This system is known to display a variety of ferroelectric behaviors ranging from variable order-disorder, to relaxor, to a mixed phase region, and then finally to normal ferroelectric, as the value of $x$ is increased. We find, in agreement with neutron diffraction studies, no detectable displacements of $\mathrm{Ta}$ or $\mathrm{Sc}$ atoms from their oxygen cage centers in any of these systems. Surprisingly, we find that the $\mathrm{Ti}$ atom is displaced along (111) from its inversion symmetry center for $x=0.05$. However, this average local $\mathrm{Ti}$ displacement gradually changes from (111) to (001) as $x$ increases, whereas the global crystal structure abruptly changes from rhombohedral to tetragonal at $x=0.45$. Our experimental results and theoretical modeling of others together suggest that this system consists of mixed regions, some characterized by a (111) $\mathrm{Ti}$ displacement and others characterized by a (001) $\mathrm{Ti}$ displacement. The displacement averaged over all regions becomes more weighted toward (001) as $x$ increases. Another significant result is that all our samples (with $x$ ranging from 0 to 0.5) have a high degree of local ordering of the $\mathrm{B}$ sites with alternate occupation of $\mathrm{Ta}$ and $\mathrm{Sc}$ atoms.