Showing papers on "Photon energy published in 2009"

The energy of charge-transfer states in electron donor-acceptor blends : insight into the energy losses in organic solar cells

Abstract: Here, a general experimental method to determine the energy ECT of intermolecular charge-transfer (CT) states in electron donor–acceptor (D–A) blends from ground state absorption and electrochemical measurements is proposed. This CT energy is calibrated against the photon energy of maximum CT luminescence from selected D–A blends to correct for a constant Coulombic term. It is shown that ECT correlates linearly with the open-circuit voltage (Voc) of photovoltaic devices in D–A blends via eVoc = ECT − 0.5 eV. Using the CT energy, it is found that photoinduced electron transfer (PET) from the lowest singlet excited state (S1 with energy Eg) in the blend to the CT state (S1 → CT) occurs when Eg − ECT > 0.1 eV. Additionally, it is shown that subsequent charge recombination from the CT state to the lowest triplet excited state (ET) of D or A (CT → T1) can occur when ECT − ET > 0.1 eV. From these relations, it is concluded that in D–A blends optimized for photovoltaic action: i) the maximum attainable Voc is ultimately set by the optical band gap (eVoc = Eg − 0.6 eV) and ii) the singlet–triplet energy gap should be ΔEST < 0.2 eV to prevent recombination to the triplet state. These favorable conditions have not yet been met in conjugated materials and set the stage for further developments in this area.

Direct Calculation of the Radiative Efficiency of an Accretion Disk Around a Black Hole

Abstract: Numerical simulation of magnetohydrodynamic (MHD) turbulence makes it possible to study accretion dynamics in detail. However, special effort is required to connect inflow dynamics (dependent largely on angular momentum transport) to radiation (dependent largely on thermodynamics and photon diffusion). To this end, we extend the flux-conservative, general relativistic MHD (GRMHD) code HARM from axisymmetry to full three dimensions. The use of an energy conserving algorithm allows the energy dissipated in the course of relativistic accretion to be captured as heat. The inclusion of a simple optically thin cooling function permits explicit control of the simulated disk's geometric thickness as well as a direct calculation of both the amplitude and location of the radiative cooling associated with the accretion stresses. Fully relativistic ray-tracing is used to compute the luminosity received by distant observers. For a disk with aspect ratio H/r 0.1 accreting onto a black hole with spin parameter a/M = 0.9, we find that there is significant dissipation beyond that predicted by the classical Novikov-Thorne model. However, much of it occurs deep in the potential, where photon capture and gravitational redshifting can strongly limit the net photon energy escaping to infinity. In addition, with these parameters and this radiation model, significant thermal and magnetic energy remains with the gas and is accreted by the black hole. In our model, the net luminosity reaching infinity is 6% greater than the Novikov-Thorne prediction. If the accreted thermal energy were wholly radiated, the total luminosity of the accretion flow would be 20% greater than the Novikov-Thorne value.

Vacuum ultraviolet beamline at the Swiss Light Source for chemical dynamics studies

Abstract: A bend-magnet vacuum ultraviolet (VUV) beamline, intended for chemical dynamics studies, was constructed and brought into operation at the Swiss Light Source (SLS) of the Paul Scherrer Institut. The beamline delivers synchrotron radiation in the 5–30 eV photon energy range with a photon flux of 10 11 photons/s at 10 eV and 10 12 photons/s at 20 eV with a resolving power of 2500. The resolving power increases to 10 4 at the cost of photon flux. An in-house designed rare gas filter is used to suppress higher harmonic radiation by a factor of >10 4 , yielding purely monochromatic light in the energy range of 5–21.6 eV. The filter is compact, easy to align, requires a total pumping power of less than 645 l/s and consumes only 3 normal l/h of filter gas. It is located at the end of the beamline, right in front of the experimental endstation. It is usually operated at a higher pressure than the endstation, which offers the additional benefit of protecting the beamline vacuum from sample contamination.

Multicomponent Polymeric Film for Red to Green Low Power Sensitized Up-Conversion

Abstract: The realization of red to green photon energy up-conversion in a multicomponent polymeric organic solid film with good photochemical stability is presented. Up-converted light is obtained by using an ultralow excitation power density in the range of 1 mW cm(-2), suitable to recover the low energy tail of the solar emission spectrum.

Modelling the response function of energy dispersive X-ray spectrometers with silicon detectors

Abstract: A new, analytical description of the physical processes determining the spectral response of an energy dispersive X-ray spectrometer with a silicon detector (Si(Li) or silicon drift detector (SDD)) is presented. The model considers the detector statistical noise, the electronic noise, the incomplete charge collection (ICC) that gives rise to the peak tailing, the escape effect, the fluorescence of the front contact or the dead layer and hot photoelectrons that cause the shelf. Only five free parameters are necessary to model the response function: the electronic noise, three parameters describing the shape of the charge collection efficiency beneath the front contact and the thickness of the detector front layer. Once the five parameters are adjusted to have agreement between a measured and a calculated response function, the response function can be calculated for any other photon energy in the range from 0.1 keV to 30 keV. The algorithm is implemented in IDL and MATLAB and is available also as MATLAB stand-alone program. It enables the determination of the optimum parameter set by fitting a calculated response function to a measured one for monochromatic radiation. A (m,n)-type matrix can be calculated whereby m represents the number of channels for the response function and n the number of photon energies in the selected range. The matrix can be used to convolute a calculated spectrum for comparison with a measured one. The calculated response functions are in agreement with the pulse height distributions measured with monochromatic synchrotron radiation in the energy range from 0.1 keV to 10 keV for three spectrometers with detector crystals different in construction. It is shown that the improved description of the detector response enables the detection of minor components of characteristic lines in fluorescence spectra, which have been attributed earlier to the detector.

Measurement of inclusive radiative B-meson decays with a photon energy threshold of 1.7 GeV

Abstract: Using 605 fb(-1) of data collected at the Upsilon(4S) resonance we present a measurement of the inclusive radiative B-meson decay channel, B -> X-s gamma. For the lower photon energy thresholds of 1.7, 1.8, 1.9, and 2.0 GeV, as defined in the rest frame of the B meson, we measure the partial branching fraction and the mean and variance of the photon energy spectrum. At the 1.7 GeV threshold we obtain the partial branching fraction BF(B -> X-s gamma)=(3.45 +/- 0.15 +/- 0.40)x10(-4), where the errors are statistical and systematic.

X-ray two-photon photoelectron spectroscopy: a theoretical study of inner-shell spectra of the organic para-aminophenol molecule.

Abstract: The inner-shell single and double ionization spectra of the organic molecule para-aminophenol are calculated using many-body Green's function methods. The inner-shell double ionization spectrum displays more pronounced sensitivity to the chemical environment and to electronic many-body effects than does the inner-shell single ionization spectrum. A kinetic model is employed to determine the probability of inner-shell double hole formation in para-aminophenol exposed to an intense, 1 fs x-ray pulse. The resulting photoelectron spectrum at a photon energy of 1 keV is calculated. This work suggests that x-ray two-photon photoelectron spectroscopy using x-ray free-electron lasers will provide access to electronic-structure information not currently available.

On the lattice parameters of silicon carbide

Abstract: The thermal expansion coefficients of the hexagonal SiC polytypes 4H and 6H and with Al and N dopants have been determined for temperatures between 300 and 1770 K. Further, a set of the room temperature lattice parameters in dependence on doping with N, Al, and B has been obtained. Data for the thermal expansion were taken on a triple axis diffractometer for high energy x rays with a photon energy of 60 keV, which allows the use of large single crystals with a volume of at least 6×6×6 mm3 without the need to consider absorption. The room temperature measurements for samples with different dopants have been performed on a four-circle diffractometer. The thermal expansion coefficients along the a- and c-directions, α11 and α33, increase from 3×10−6 K−1 at 300 K to 6×10−6 K−1 at 1750 K. It is found that α11 and α33 are isotropic within 107 K−1. At high temperatures both coefficients for doped samples are ∼0.2×10−6 and 0.3×10−6 K−1 lower than for the undoped material.

Photoconductivity of Bulk‐Film‐Based Graphene Sheets

Abstract: Time-resolved photoconductivity measurements are carried out on graphene films prepared by using soluble graphene oxide. High photocurrent generation efficiency is observed for these graphene-based films, and the relationships between their photoconductivity and different preparation methods, incident light intensity, external electric field, and photon energies are investigated. Higher photoconductivity is observed with higher photon energy at same incident light intensity. By fitting the experimental data to the Onsager model, the primary quantum yields for charge separation to generate bound electron‐hole pairs and the initial ion-pair thermalization separation distance are calculated.

Charge ordering in La 1.8-x Eu 0.2 Sr x CuO 4 studied by resonant soft x-ray diffraction

Abstract: Resonant soft X-ray scattering with photon energies near the O K and the Cu L3 edges was used to study charge ordering in the system La_{1.8-x}Eu_{0.2}Sr_xCuO_4 as a function of temperature for x = 0.125 and 0.15. From the superstructure diffraction intensities a charge ordering with a doping dependent wave vector is derived which is in this system well below the transition temperature of the low-temperature tetragonal phase but well above the onset of spin ordering. This indicates that charge ordering is the primary driving force for the formation of stripe-like phases in two-dimensional doped cuprates. Analysis of the lineshape of the scattered intensity as a function of photon energy yields evidence for a high hole concentration in the stripes.

BaD ElPh: A 4 m normal-incidence monochromator beamline at Elettra

Abstract: At the Elettra synchrotron light laboratory, a new undulator-based beamline with photon energy ranging from about 46 to 40 eV has been installed and opened to users The beamline, based on a 4 m normal-incidence monochromator (NIM) with spherical gratings and movable exit slit, serves an end station to perform primarily high-resolution angle-resolved photoemission spectroscopy experiments in the low photon energy regime The salient features of the instrument and some of the commission data are reported in this paper

Blazar halos as probe for extragalactic magnetic fields and maximal acceleration energy

Abstract: High energy photons from blazars interact within tens of kpc with the extragalactic photon background, initiating electromagnetic pair cascades. The charged component of such cascades is deflected by extragalactic magnetic fields (EGMF), leading to halos even around initially point-like sources. We calculate the intensity profile of the resulting secondary high-energy photons for different assumptions on the initial source spectrum and the strength of the EGMF, employing also fields found earlier in a constrained simulation of structure formation including MHD processes. We find that the observation of halos around blazars like Mrk~180 probes an interesting range of EGMF strengths and acceleration models: In particular, blazar halos test if the photon energy spectrum at the source extends beyond \sim 100 TeV and how anisotropic this high energy component is emitted.

Projection x-ray imaging with photon energy weighting: experimental evaluation with a prototype detector.

Abstract: The signal-to-noise ratio (SNR) in x-ray imaging can be increased using a photon counting detector which could allow for rejecting electronics noise and for weighting x-ray photons according to their energies This approach, however, was not feasible for a long time because photon counting x-ray detectors with very high count rates, good energy resolution and a large number of small pixels were required These problems have been addressed with the advent of new detector materials, fast readout electronics and powerful computers In this work, we report on the experimental evaluation of projection x-ray imaging with a photon counting cadmium?zinc?telluride (CZT) detector with energy resolving capabilities The detector included two rows of pixels with 128 pixels per row with 09 ? 09 mm2 pixel size, and a 2 Mcount pixel?1 s?1 count rate The x-ray tube operated at 120 kVp tube voltage with 2 mm Al-equivalent inherent filtration The x-ray spectrum was split into five regions, and five independent x-ray images were acquired at a time These five quasi-monochromatic x-ray images were used for x-ray energy weighting and material decomposition A tissue-equivalent phantom was used including contrast elements simulating adipose, calcifications, iodine and air X-ray energy weighting improved the SNR of calcifications and iodine by a factor of 132 and 136, respectively, as compared to charge integrating Material decomposition was performed by dual energy subtraction The low- and high-energy images were generated in the energy ranges of 25?60 keV and 60?120 keV, respectively, by combining five monochromatic image data into two X-ray energy weighting was applied to low- and high-energy images prior to subtraction, and this improved the SNR of calcifications and iodine in dual energy subtracted images by a factor of 134 and 125, respectively, as compared to charge integrating The detector energy resolution, spatial resolution, linearity, count rate, noise and image uniformity were investigated The limitations of this technology were emphasized and possible solutions were discussed

Final State Distributions of O2 Photodesorbed from TiO2(110)

Abstract: The UV photodesorption of molecular oxygen from a reduced TiO2(110) single-crystal surface was investigated as a function of photon excitation energy, substrate temperature, and preannealing conditions. A pump-delayed-probe method using pulsed lasers for UV excitation (pump) and VUV ionization (probe) were used in conjunction with time-of-flight mass spectrometry to measure velocity distributions of the desorbing O2 molecules. The measured velocity distributions exhibit three distinct features, two of which are attributed to prompt desorption resulting in “fast” velocity distributions and one “slow” channel whose average kinetic energy tracks the surface temperature. The latter is assigned to trapping-desorption of photoexcited O2* which are trapped in the physisorption well prior to thermal desorption. The velocity distributions show no dependence on photon energy over the range studied (3.45−4.16 eV), consistent with a substrate-mediated, hole-capture desorption mechanism. The observed prompt desorption...

Blazar halos as probe for extragalactic magnetic fields and maximal acceleration energy

Abstract: High-energy photons from blazars interact within tens of kpc with the extragalactic photon background, initiating electromagnetic pair cascades. The charged component of such cascades is deflected by extragalactic magnetic fields (EGMFs), leading to halos even around initially point-like sources. We calculate the intensity profile of the resulting secondary high-energy photons for different assumptions on the initial source spectrum and the strength of the EGMF, employing also fields found earlier in a constrained simulation of structure formation including magnetohydrodynamics processes. We find that the observation of halos around blazars like Mrk 180 probes an interesting range of EGMF strengths and acceleration models: in particular, blazar halos test if the photon energy spectrum at the source extends beyond ~100 TeV and how anisotropic this high-energy component is emitted.

Double photoionization of CO2 molecules in the 34-50 eV energy range.

Abstract: The double photoionization of CO2 molecules has been studied in the 34−50 eV photon energy range, by the use of synchrotron radiation and detecting electron−ion and electron−ion−ion coincidences. T...

Nonlinear optical rectification of a hydrogenic impurity in a disc-like quantum dot

Abstract: An investigation of the nonlinear optical rectification of a hydrogenic impurity, which is in a two-dimensional disc-like quantum dot (QD) with parabolic confinement potential, has been performed by using the perturbation method in the effective mass approximation. Both the electric field and the confinement effects on the energy are investigated in detail. The results are presented as a function of the incident photon energy for the different values of the confinement strength and the electric field. It is found that the nonlinear optical properties of hydrogenic impurity states in a disc-like QD are strongly affected by the confinement strength and the electric field.

Absorption spectra of a donor impurity in a quantum ring

Abstract: The linear and the third-order nonlinear optical absorption spectra of a donor impurity confined by a quantum ring are studied using the matrix diagonalization method within the effective-mass approximation. The linear and the third-order nonlinear optical absorption coefficients between the ground (L = 0) and the first excited state (L = 1) have been examined based on the computed energies and wavefunctions. The results are presented as a function of the incident photon energy for the different values of the confinement strength, the incident optical density and the ring radius. We found the confinement strength, the incident optical density and the ring radius have a great effect on the linear, the third-order nonlinear and total absorption spectra. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhanced high harmonic generation from multiply ionized argon above 500 eV through laser pulse self-compression.

Abstract: By combining laser pulse self-compression and high harmonic generation within a single waveguide, we demonstrate high harmonic emission from multiply charged ions for the first time. This approach enhances the laser intensity and counteracts ionization-induced defocusing, extending the cutoff photon energy in argon above 500 eV for the first time, with higher spectral intensity and cutoff energy than He for the same input laser parameters. This Letter demonstrates a pathway for extending high harmonic emission to very high photon energies using large, multiply charged, ions with high ionization potentials.

Electronic transitions in disk-shaped quantum dots induced by twisted light

Abstract: We theoretically investigate the absorption and emission of light carrying orbital angular momentum (twisted light) by quasi-two-dimensional (disk-shaped) quantum dots in the presence of a static magnetic field. We calculate the transition matrix element for the light-matter interaction and use it to explore different scenarios, depending on the initial and final states of the electron undergoing the optically induced transition. We make explicit the selection rule for the conservation of the $z$ projection of the orbital angular momentum. For a realistic set of parameters (quantum dot size, beam waist, photon energy, etc.) the strength of the transition induced by twisted light is 10% of that induced by plane waves. Finally, our analysis indicates that it may be possible to select precisely the electronic level one wishes to populate using the appropriate combination of light-beam parameters suggesting technological applications to the quantum control of electronic states in quantum dots.

Interaction of Stern layer and domain structure on photochemistry of lead-zirconate-titanate

Abstract: Perovskite ferroelectric materials such as PZT have long been known to have wideband semiconducting properties. It has also been found that they have interesting spatially controllable surface photochemical effects that are not seen in 'normal' semiconductors. This has led to their being studied as possible tools in areas such as metal salt reduction and oxidation for nanoparticle growth. This paper discusses the effects of incident photon energy on the reduction of Ag0 onto PZT(30/70) surfaces with particular emphasis on the part played by energy band bending and the Stern layer. It was found that for increasing photon energy between 4.4 and 5.0?eV both the [1?1?1] and the [1?0?0] orientations of PZT followed a similar trend in that the average Ag0 cluster cross-sectional area increased by a ratio of ca 1.6 to 1. This increase was put down to the higher energy photons exciting more electrons from deeper in the density of states for the material allowing a greater reduction rate of Ag+ at the surface.