Showing papers on "Photon energy published in 2007"

Multiple Exciton Generation in Colloidal Silicon Nanocrystals

Abstract: Multiple exciton generation (MEG) is a process whereby multiple electron-hole pairs, or excitons, are produced upon absorption of a single photon in semiconductor nanocrystals (NCs) and represents a promising route to increased solar conversion efficiencies in single-junction photovoltaic cells. We report for the first time MEG yields in colloidal Si NCs using ultrafast transient absorption spectroscopy. We find the threshold photon energy for MEG in 9.5 nm diameter Si NCs (effective band gap E g ) 1.20 eV) to be 2.4 ± 0.1Eg and find an excitonproduction quantum yield of 2.6 ± 0.2 excitons per absorbed photon at 3.4Eg. While MEG has been previously reported in direct-gap semiconductor NCs of PbSe, PbS, PbTe, CdSe, and InAs, this represents the first report of MEG within indirect-gap semiconductor NCs. Furthermore, MEG is found in relatively large Si NCs (diameter equal to about twice the Bohr radius) such that the confinement energy is not large enough to produce a large blue-shift of the band gap (only 80 meV), but the Coulomb interaction is sufficiently enhanced to produce efficient MEG. Our findings are of particular importance because Si dominates the photovoltaic solar cell industry, presents no problems regarding abundance and accessibility within the Earth’s crust, and poses no significant environmental problems regarding toxicity.

Carrier multiplication in InAs nanocrystal quantum dots with an onset defined by the energy conservation limit

Abstract: Carrier multiplication (CM) is a process in which absorption of a single photon produces not just one but multiple electron-hole pairs (excitons). This effect is a potential enabler of next-generation, high-efficiency photovoltaic and photocatalytic systems. On the basis of energy conservation, the minimal photon energy required to activate CM is two energy gaps (2Eg). Here, we analyze CM onsets for nanocrystal quantum dots (NQDs) based upon combined requirements imposed by optical selection rules and energy conservation and conclude that materials with a significant difference between electron and hole effective masses such as III-V semiconductors should exhibit a CM threshold near the apparent 2Eg limit. Further, we discuss the possibility of achieving sub-2Eg CM thresholds through strong exciton-exciton attraction, which is feasible in NQDs. We report experimental studies of exciton dynamics (Auger recombination, intraband relaxation, radiative recombination, multiexciton generation, and biexciton shift) in InAs NQDs and show that they exhibit a CM threshold near 2Eg.

Solar Cells Based on Quantum Dots: Multiple Exciton Generation and Intermediate Bands

Abstract: Semiconductor quantum dots may be used in so-called third-generation solar cells that have the potential to greatly increase the photon conversion efficiency via two effects: (1) the production of multiple excitons from a single photon of sufficient energy and (2) the formation of intermediate bands in the bandgap that use sub-bandgap photons to form separable electron–hole pairs. This is possible because quantization of energy levels in quantum dots produces the following effects: enhanced Auger processes and Coulomb coupling between charge carriers; elimination of the requirement to conserve crystal momentum; slowed hot electron–hole pair (exciton) cooling; multiple exciton generation; and formation of minibands (delocalized electronic states) in quantum dot arrays. For exciton multiplication, very high quantum yields of 300–700% for exciton formation in PbSe, PbS, PbTe, and CdSe quantum dots have been reported at photon energies about 4–8 times the HOMO–LUMO transition energy (quantum dot bandgap), respectively, indicating the formation of 3–7 excitons/photon, depending upon the photon energy. For intermediate-band solar cells, quantum dots are used to create the intermediate bands from the con fined electron states in the conduction band. By means of the intermediate band, it is possible to absorb below-bandgap energy photons. This is predicted to produce solar cells with enhanced photocurrent without voltage degradation.

Bright Multi-keV Harmonic Generation from Relativistically Oscillating Plasma Surfaces

Abstract: The first evidence of x-ray harmonic radiation extending to 3.3 A, 3.8 keV (order n > 3200) from petawatt class laser-solid interactions is presented, exhibiting relativistic limit efficiency scaling (eta similar to n(-2.5)-n(-3)) at multi-keV energies. This scaling holds up to a maximum order, n(RO)similar to 8(1/2)gamma(3), where gamma is the relativistic Lorentz factor, above which the first evidence of an intensity dependent efficiency rollover is observed. The coherent nature of the generated harmonics is demonstrated by the highly directional beamed emission, which for photon energy h nu > 1 keV is found to be into a cone angle similar to 4 degrees, significantly less than that of the incident laser cone (20 degrees).

Hydrogen bonds in liquid water studied by photoelectron spectroscopy

Abstract: The authors report on photoelectron emission spectroscopy measurements of the oxygen 1s orbital of liquid water, using a liquid microjet in ultrahigh vacuum. By suitably changing the soft x-ray photon energy, within 600-1200 eV, the electron probing depth can be considerably altered as to either predominantly access the surface or predominantly bulk water molecules. The absolute probing depth in liquid water was inferred from the evolution of the O1s signal and from comparison with aqueous salt solution. The presence of two distinctive components in the core-level photoelectron spectrum, with significantly different binding energies, is revealed. The dominant contribution, at a vertical binding energy of 538.1 eV, was found in bulk and surface sensitive spectra. A weaker component at 536.6 eV binding energy appears to be present only in bulk water. Hartree-Fock calculations of O1s binding energies in different geometric arrangements of the water network are presented to rationalize the experimental distribution of O1s electron binding energies.

Development of a Vacuum Ultra-Violet Laser-Based Angle-Resolved Photoemission System with a Super-High Energy Resolution Better Than 1 meV

Abstract: The design and performance of the first vacuum ultra-violet (VUV) laser-based angle-resolved photoemission (ARPES) system are described. The VUV laser with a photon energy of 6.994 eV and bandwidth of 0.26 meV is achieved from the second harmonic generation using a novel non-linear optical crystal KBe2BO3F2 (KBBF). The new VUV laser-based ARPES system exhibits superior performance, including super-high energy resolution better than 1 meV, high momentum resolution, super-high photon flux and much enhanced bulk sensitivity, which are demonstrated from measurements on a typical Bi2Sr2CaCu2O8 high temperature superconductor. Issues and further development related to the VUV laser-based photoemission technique are discussed.

Spectral and timing properties of a dissipative γ-ray burst photosphere

Abstract: We explore the observational appearance of the photosphere of an ultrarelativistic flow with internal dissipation of energy as predicted by the magnetic reconnection model. Previous study of the radiative transfer in the photospheric region has shown that gradual dissipation of energy results in a hot photosphere. There, inverse Compton scattering of the thermal radiation advected with the flow leads to powerful photospheric emission with spectral properties close to those of the observed prompt GRB emission. Here, we build on that study by calculating the spectra for a large range of the characteristics of the flow. An accurate fitting formula is given that provides the photospheric spectral energy distribution in the ∼10 keV to ∼10 MeV energy range (in the central engine frame) as a function of the basic physical parameters of the flow. It facilitates the direct comparison of the model predictions with observations, including the variability properties of the lightcurves. We verify that the model naturally accounts for the observed clustering in peak energies of the E f(E) spectrum. In this model, the Amati relation indicates a tendency for the most luminous bursts to have more energy per baryon. If this tendency also holds for individual GRB pulses, the model predicts the observed narrowing of the width of pulses with increasing photon energy.

X-ray-laser interaction with matter and the role of multiphoton ionization: Free-electron-laser studies on neon and helium

Abstract: At the soft-x-ray free-electron laser FLASH in Hamburg, we have studied multiphoton ionization on neon and helium by ion mass-to-charge spectroscopy. The experiments were performed in a focused beam at 42.8 and $38.4\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ photon energy and irradiance levels up to ${10}^{14}\phantom{\rule{0.3em}{0ex}}\mathrm{W}∕{\mathrm{cm}}^{2}$. Direct, sequential, and resonant two-, three-, and four-photon excitations were investigated by quantitative measurements as a function of the absolute photon intensity. The atomic and ionic photoionization cross sections derived indicate a clear dominance of sequential compared to direct multiphoton processes.

Study on the ablation threshold induced by pulsed lasers at different wavelengths

Abstract: A study of the effects induced by pulsed laser ablation on different materials as a function of the laser wavelength is presented. In particular the ablation at low laser fluence, of the order of 10 8 –10 10 W/cm 2 with ns pulse width, is investigated experimentally on different metals, semiconductors and polymers. Two theoretical models, explain the experimental results about the fluence threshold value measurements, as depending on the laser wavelength are discussed. The photothermal process is valid for the estimation of the threshold fluence for IR and visible radiation, both inducing thermal heating in metals and semiconductors through the photon-free electron energy transfer. This model is not valid for polymers. The photochemical process is valid for the estimation of the threshold fluence for UV radiation, which photon energy is higher with respect to the chemical binding energy. This radiation induces chemical bond breaking in insulators and scission and cross linking effects can be produced. This last model is not valid for metals and semiconductors.

Time-dependent theory of double ionization of helium under XUV radiation

Abstract: We present non-perturbative time-dependent calculations of single and double ionization of helium, under XUV radiation of photon energy ranging from 40 to 45 eV, through the direct propagation of the time-dependent Schrodinger equation. The time-dependent wavefunction of the atom under the field is expanded in terms of correlated multichannel states normalized with incoming-wave boundary conditions. In addition to presenting a new non-perturbative approach to the three-body problem, in a fully correlated scheme, capable of providing in the same calculation photoelectron energy and angularly resolved spectra, as well as cross sections through the lowest non-vanishing order transition amplitude, we also present a detailed comparison of the values of certain key quantities that have been obtained through a variety of other methods. The degree of agreement we find, while lending credence to the approach and its versatility, also highlights the remaining open questions in this novel context of double ionization.

Commissioning and performance of the variable line spacing plane grating monochromator beamline at the Canadian Light Source

Abstract: The variable line spacing plane grating monochromator beamline at the Canadian Light Source (CLS) employs three grazing incidence variable line spacing gratings to cover a photon energy range of 5–250eV. It uses a 185mm period length planar permanent magnet insertion device as the photon source, sharing a straight section with another soft x-ray beamline at the CLS. The commissioning and performance of the beamline is reported. The high resolution photoabsorption spectra of Ar and PF5 gases are reported. A resolving power of over 40 000 for photons in the low energy region and >10000 for a wider energy range (8–200eV) can be achieved. A photon flux of up to 2×1012photons∕sper100mA with slit settings of 50μm has been measured.

Effects of Downscattering on the Continuum and Line Spectra in a Powerful Wind Environment: Monte Carlo Simulations, Analytical Results, and Data Analysis

Abstract: In an earlier paper, the general formulation and results for photon reprocessing (downscattering) that included recoil and Comptonization effects due to divergence of the flow were presented. In a second paper we showed the Monte Carlo (MC) simulated continuum and line spectra. We also provided an analytical description of the simulated continuum spectra using the diffusion approximation. We have simulated the propagation of monochromatic and continuum photons in a bulk outflow from a compact object. Electron scattering of the photons within the expanding flow leads to a decrease of their energy, which is of first order in V/c (where V is the outflow velocity). The downscattering effect of first order in V/c in the diverging flow is explained by semianalytical calculations and confirmed by MC simulations. We conclude that redshifted lines and downscattering bumps are intrinsic properties of the powerful outflows for which the Thomson optical depth is greater than 1. We fitted our model line profiles to the observations using four free parameters, β = V/c, the optical depth of the wind τ, the wind temperature kTe, and the original line photon energy E0. We show how the primary spectrum emitted close to the black hole is modified by reprocessing in the warm wind. In the framework of our wind model, the fluorescent iron line Kα is formed in the partly ionized wind as a result of illumination by central source continuum photons. The demonstrated application of our outflow model to the XMM-Newton observations of MCG -6-30-15 and to the ASCA observations of GRO J1655-40 points out a potential powerful spectral diagnostic for probes of the outflow-central object connection in Galactic and extragalactic black hole sources.

Dose dependence of radiation damage for protein crystals studied at various X-ray energies

Abstract: Radiation damage to protein crystals is the most serious problem in obtaining accurate structures from protein crystallography. In order to examine the photon energy dependence of radiation damage, 12 to 15 data sets from each of nine tetragonal lysozyme crystals were collected at nine different X-ray energies (6.5, 7.1, 8.3, 9.9, 12.4, 16.5, 20.0, 24.8 and 33.0 keV) using beamline BL41XU at SPring-8. All results were compared on the basis of absorbed dose, expressed in Gray (Gy). Crystallographic statistics, such as the values of lattice constants, Rmerge and I/σ(I), for each data set degraded at all nine energies as the exposure time for each crystal increased. In all data sets, radiation damage was observed after the absorbed dose exceeded 106 Gy. However, from the point of view of crystallographic statistics normalized to the absorbed dose, no clear dependence on photon energy was observed in these results. Structural refinement showed that the average B-factor for the last data set was larger than that for the first data set at all energies tested. However, no energy dependence of radiation damage on B-factor was found. Furthermore, disruption of disulfide bonds due to radiation damage was observed in electron density maps even at the highest photon energy (33 keV) used in this study. Therefore, these results suggest that radiation damage in the energy range investigated could be evaluated based on absorbed dose without energy dependence, and that it is important to minimize the absorbed dose in a crystal sample for obtaining an accurate protein structure.

Effective atomic numbers for photon energy absorption of essential amino acids in the energy range 1 keV to 20 MeV

Abstract: Effective atomic numbers for photon energy-absorption (ZPEAeff) of essential amino acids histidine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine have been calculated by a direct method in the energy region of 1 keV to 20 MeV. The ZPEAeff values have been found to change with energy and composition of the amino acids. The variations of mass energy-absorption coefficient, effective atomic number for photon interaction (ZPIeff) and ZPEAeff with energy are shown graphically. Significant differences exist between ZPIeff and the ZPEAeff in the energy region of 8–100 keV for histidine and threonine; 6–100 keV for leucine, lysine, tryptophan, phenylalanine and valine; 15–400 keV for methionine. The effect of absorption edge on effective atomic numbers and the possibility of defining two set values of these parameters at the K-absorption edge of high-Z element present in the amino acids are discussed. The reasons for using ZPEAeff rather than the commonly used ZPIeff in medical radiation dosimetry for the calculation of absorbed dose in radiation therapy are also discussed.

Resonant soft x-ray reflectivity of organic thin films

Abstract: At photon energies close to absorption edges in the soft x-ray range, the complex index of refraction, n=1−δ−iβ, of organic materials varies rapidly as a function of photon energy in a manner that strongly depends on the chemical moieties and functionalities present in the material. The authors present details of how these molecular structure specific variations in the complex index of refraction can be utilized to enhance and tune the contrast in reflectivity experiments of organic films. This near edge contrast enhancement mimics the specific contrast achieved through deuterium labeling in neutron reflectivity (NR). This relatively new x-ray approach, resonant soft x-ray reflectivity (RSoXR), thus combines aspects of NR and conventional x-ray reflectivity (XR), yet does not require special chemical procedures. The capabilities of RSoXR are exemplified using a number of polymeric bi- and multilayers. Furthermore, a direct comparison of RSoXR to conventional x-ray reflectivity and NR for polystyrene and p...

Energy dependence of effective atomic numbers for photon energy absorption and photon interaction: studies of some biological molecules in the energy range 1 keV-20 MeV.

Abstract: Effective atomic numbers for photon energy absorption, ZPEA,eff, and for photon interaction, ZPI,eff, have been calculated by a direct method in the photon-energy region from 1 keV to 20 MeV for biological molecules, such as fatty acids (lauric, myristic, palmitic, stearic, oleic, linoleic, linolenic, arachidonic, and arachidic acids), nucleotide bases (adenine, guanine, cytosine, uracil, and thymine), and carbohydrates (glucose, sucrose, raffinose, and starch). The ZPEA,eff and ZPI,eff values have been found to change with energy and composition of the biological molecules. The energy dependence of the mass attenuation coefficient, ZPEA,eff, and the mass energy-absorption coefficient, ZPI,eff, is shown graphically and in tabular form. Significant differences of 17%−38% between ZPI,eff and ZPEA,eff occur in the energy region 5–100 keV. The reasons for these differences, and for using ZPEA,eff rather than ZPI,eff in calculations of the absorbed dose, are discussed.

First results of XPAD3, a new photon counting chip for X-ray CT-scanner with energy discrimination

Abstract: XVAD3 is a single photon counting chip based on hybrid pixel counters, with low noise, high dynamics and high speed readout. Its features have been improved to provide an high counting rate capability, a very low threshold, an energy discrimination and a fast image readout. The chip is designed in 0.25 mum IBM technology, and contains 9600 pixels (130 mum times 130 mum) distributed into 80 columns of 120 elements each. An innovative architecture has been designed in order to prevent the digital circuits from disturbing the very sensitive analogue cells. XPAD3 was realized into two versions. The XPAD3-S version accepts positive input charges and offers an energy range from 4 keV to 40 keV with a single threshold. The XPAD3-C version accepts negative input charge and has an energy range from 6 keV to 60 keV with a windowed energy selection set by two independent thresholds. This last feature will be quite useful for experimentations where high contrast resolution is needed. The XPAD3 circuits can be bump-bonded with Si, CdTe or GaAs sensors to match detection efficiency with increasing photon energy. The aim of this development is to combine several XPAD3 circuits to build an 7.5 cm x 12 cm sensitive area for the development of a small animal micro-CT scanner, PIXSCAN and synchrotron X-ray scattering experiments. First prototypes of single chip detectors bump- bonded with a Si sensor and double chip detectors bump-bonded with a CdTe sensor have been produced and tested and preliminary results are presented.

``ro´ POT.0´ photoproduction in ultra-peripheral relativistic heavy ion collisions with STAR

Abstract: Photoproduction reactions occur when the electromagnetic field of a relativistic heavy ion interacts with another heavy ion. The STAR collaboration presents a measurement of rho^0 and direct pi^+pi^- photoproduction in ultra-peripheral relativistic heavy ion collisions at sqrt(s_{NN})=200 GeV. We observe both exclusive photoproduction and photoproduction accompanied by mutual Coulomb excitation. We find a coherent cross-section of sigma(AuAu) -> Au^*Au^*rho^0 = 530 pm 19 (stat.) pm 57 (syst.) mb, in accord with theoretical calculations based on a Glauber approach, but considerably below the predictions of a color dipole model. The rho^0 transverse momentum spectrum (p_{T}^2) is fit by a double exponential curve including both coherent and incoherent coupling to the target nucleus; we find sigma_{inc}/sigma_{coh} = 0.29 pm 0.03 (stat.) pm 0.08 (syst.). The ratio of direct pi^+pi^- to rho^0 production is comparable to that observed in gamma p collisions at HERA, and appears to be independent of photon energy. Finally, the measured rho^0 spin helicity matrix elements agree within errors with the expected s-channel helicity conservation.

Thin film photodetector, method and system

Abstract: A photodetector, comprises a first section comprising at least one p-n junction that converts photon energy into a separate charge carrier and hole carrier; and another section of semiconductors of opposing conductivity type connected electrically in series and thermally in parallel in a heat dissipating and electric generating relationship to the cell to augment generation of electric energy of the first section.

Systematic investigation of resonance-induced single-harmonic enhancement in the extreme-ultraviolet range

Abstract: We demonstrate the intensity enhancement of single harmonics in high-order harmonic generation from laser plasma. We identified several targets (In, Sn, Sb, Cr, and Mn) that demonstrate resonance-induced enhancement of single harmonic, that are spectrally close to ionic transitions with strong oscillator strengths. We optimized and obtained enhancements of the 13th, 17th, 21st, 29th, and 33rd harmonics from the above targets, by varying the chirp of the 800 nm wavelength femtosecond laser. We also observe harmonic enhancement by using frequency-doubled pump laser (400 nm wavelength). For Mn plasma pumped by the 400 nm wavelength laser, the maximum order of the enhanced harmonic observed was the 17th order ({lambda}=23.5 nm), which corresponds to the highest photon energy (52.9 eV) reported for an enhanced single harmonic.

Effects of system geometry and other physical factors on photon sensitivity of high-resolution positron emission tomography.

Abstract: We are studying two new detector technologies that directly measure the three-dimensional coordinates of 511 keV photon interactions for high-resolution positron emission tomography (PET) systems designed for small animal and breast imaging. These detectors are based on (1) lutetium oxyorthosilicate (LSO) scintillation crystal arrays coupled to position-sensitive avalanche photodiodes (PSAPD) and (2) cadmium zinc telluride (CZT). The detectors have excellent measured 511 keV photon energy resolutions ( 8% photon sensitivity for the LSO-PSAPD box configuration and >15% for CZT box geometry, using a 350-650 keV energy window setting. These simulation results compare well with analytical estimations. The trend is different for a clinical whole-body PET system that uses conventional LSO-PMT block detectors with larger crystal elements. Simulations predict roughly the same sensitivity for both box and cylindrical detector configurations. This results from the fact that a large system diameter (>80 cm) results in relatively small inter-module gaps in clinical whole-body PET. In addition, the relatively large block detectors (typically >5 x 5 cm(2) cross-sectional area) and large crystals (>4 x 4 x 20 mm(3)) enable a higher fraction of detector scatter photons to be absorbed compared to a small animal system. However, if the four detector sides (panels) of a box-shaped system geometry are configured to move with respect to each other, to better fit the transaxial FOV to the actual size of the object to be imaged, a significant increase in photon sensitivity is possible. Simulation results predict a 60-100% relative increase of photon sensitivity for the proposed small animal PET box configurations and >60% increase for a clinical whole-body system geometry. Thus, simulation results indicate that for a PET system built from rectangular-shaped detector modules, arranging them into a box-shaped system geometry may help us to significantly boost photon sensitivity for both small animal and clinical PET systems.

The Energy-dependent X-Ray Timing Characteristics of the Narrow-Line Seyfert 1 Mrk 766

Abstract: We present the energy-dependent power spectral density (PSD) and cross-spectral properties of Mkn 766, obtained from combining data obtained during an XMM-Newton observation spanning six revolutions in 2005 with data obtained from an XMM-Newton long-look in 2001. The PSD shapes and rms-flux relations are found to be consistent between the 2001 and 2005 observations, suggesting the 2005 observation is simply a low-flux extension of the 2001 observation and permitting us to combine the two data sets. The resulting PSD has the highest temporal frequency resolution for any AGN PSD measured to date. Applying a broken power-law model yields break frequencies which increase in temporal frequency with photon energy. Obtaining a good fit when assuming energy-independent break frequencies requires the presence of a Lorentzian at 4.6 +/- 0.4 x 10(exp -4)Hz whose strength increases with photon energy, a behavior seen in black hole X-ray binaries. The cross-spectral properties are measured; temporal frequency-dependent soft-to-hard time lags are detected in this object for the first time. Cross-spectral results are consistent with those for other accreting black hole systems. The results are discussed in the context of several variability models, including those based on inwardly-propagating viscosity variations in the accretion disk.

Momentum and energy dependence of the anomalous high-energy dispersion in the electronic structure of high temperature superconductors.

Abstract: Using high-resolution angle-resolved photoemission spectroscopy we have studied the momentum and photon energy dependence of the anomalous high-energy dispersion, termed waterfalls, between the Fermi level and 1 eV binding energy in several high-${T}_{c}$ superconductors. We observe strong changes of the dispersion between different Brillouin zones and a strong dependence on the photon energy around 75 eV, which we associate with the resonant photoemission at the $\mathrm{Cu}3p\ensuremath{\rightarrow}3{d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ edge. We conclude that the high-energy ``waterfall'' dispersion results from a strong suppression of the photoemission intensity at the center of the Brillouin zone due to matrix element effects and is, therefore, not an intrinsic feature of the spectral function. This indicates that the new high-energy scale in the electronic structure of cuprates derived from the waterfall-like dispersion may be incorrect.

Two-photon double ionization of helium in the region of photon energies 42-50 eV

Abstract: We report the total integrated cross section (TICS) of two-photon double ionization of helium in the photon energy range from $42\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}50\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. Our computational procedure relies on a numerical solution of the time-dependent Schr\"odinger equation on a square-integrable basis and subsequent projection of this solution on a set of final field-free states describing correlation in the two-electron continuum. Our results suggest that the TICS grows monotonically as a function of photon energy in the region of $42--50\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, possibly reaching a maximum in the vicinity of $50\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. We also present fully resolved triple-differential cross sections for selected photon energies.

Effective Atomic Numbers for Fe?Mn Alloy Using Transmission Experiment

Abstract: The effective atomic numbers (Zeff) and effective electron density (Ne) for Fe–Mn alloy with different Mn contents are calculated using the mass attenuation coefficients μ/ρ obtained via XCOM in the photon energy range of 1 keV–1 GeV. The results are compared with the values measured at the photon energy of 662, 1170 and 1332 keV.

Commissioning and performance of X-ray absorption spectroscopy beamline at the Siam Photon Laboratory

Abstract: We report commissioning results and performance of X-ray absorption spectroscopy (XAS) beamline, BL-8, at the Siam Photon Laboratory. BL-8 has been opened for users since the year 2006. It is tunable by a fixed-exit double crystal monochromator equipped with InSb(1 1 1), Si(1 1 1), and Ge(2 2 0) crystals covering photon energy from 1830 to 9000 eV. Thus elemental absorption K-edges of silicon up to copper can be investigated. Other heavier elements may be studied via their L or M edges. The front end is windowless and the beamline is terminated with a Kapton window followed by the XAS station equipped with ionization chambers for transmission-mode measurements. The measured photon flux at sample is approximately 10 8 –10 10 photons/s/100 mA for the 1 mm×10 mm beam size. The commissioning XANES spectra of sulfur standards and EXAFS spectra of copper are presented.

Single photon ionization of hydrogen bonded clusters with a soft x-ray laser: (HCOOH)x and (HCOOH)y(H2O)z.

Abstract: Pure, neutral formic acid (HCOOH)n+1 clusters and mixed (HCOOH)∕(H2O) clusters are investigated employing time of flight mass spectroscopy and single photon ionization at 26.5eV using a very compact, capillary discharge, soft x-ray laser. During the ionization process, neutral clusters suffer little fragmentation because almost all excess energy above the vertical ionization energy is taken away by the photoelectron, leaving only a small part of the photon energy deposited into the (HCOOH)n+1+ cluster. The vertical ionization energy minus the adiabatic ionization energy is enough excess energy in the clusters to surmount the proton transfer energy barrier and induce the reaction (HCOOH)n+1+→(HCOOH)nH++HCOO making the protonated (HCOOH)nH+ series dominant in all data obtained. The distribution of pure (HCOOH)nH+ clusters is dependent on experimental conditions. Under certain conditions, a magic number is found at n=5. Metastable dissociation rate constants of (HCOOH)nH+ are measured in the range (0.1–0.8)×...

Computational study of AgCl and AgBr semiconductors

Abstract: We present the results of a theoretical study of the structural and optoelectronic properties of silver halides AgCl and AgBr, using the full-potential linearized augmented plane wave (FP-LAPW). In this approach, the local density approximation (LDA) and generalized gradient approximation (GGA) are used for the exchange correlation potential. Moreover, the alternative form of GGA proposed by Engel and Vosko [Phys. Rev. B 47 (1993) 13164] (EVGGA) is also used for the optoelectronic properties. The calculated total energy allowed us to investigate several structural properties in particular the lattice constant, bulk modulus, pressure derivative of bulk modulus and cohesive energy. A numerical first-principles calculation of the elastic constants was used to calculate C11, C12 and C44. Band structure, density of states, band gap pressure coefficients, charge density and effective masses are also given. On the other hand, an accurate calculation of linear optical functions (refraction index and the dielectric function) is performed in the photon energy range up to 20 eV. The results obtained are compared with other calculations and experimental measurements.

Polaron effects on the optical absorptions in asymmetrical semi-parabolic quantum wells

Abstract: The linear and nonlinear optical absorptions considering the weak-coupling electron-LO-phonon interaction in asymmetrical semiparabolic quantum wells are theoretically investigated. The numerical results for the typical GaAs/AlxGa1-xAs material show that the factors of Al content x, the relaxation time and the photon energy have great influence on the optical absorption coefficients. Moreover, the theoretical values of the optical absorptions are more than a factor of 2-3 higher than the one in the structure without considering the electron-LO-phonon interaction by calculating. (C) 2007 Elsevier B.V. All rights reserved.