scispace - formally typeset
Search or ask a question

Showing papers on "Absorption (electromagnetic radiation) published in 2004"


Journal ArticleDOI
TL;DR: In this paper, the wavelength dependence of light absorption by aerosols collected on filters is investigated throughout the near-ultraviolet to near-infrared spectral region using an optical transmission method.
Abstract: [1] The wavelength dependence of light absorption by aerosols collected on filters is investigated throughout the near-ultraviolet to near-infrared spectral region. Measurements were made using an optical transmission method. Aerosols produced by biomass combustion, including wood and savanna burning, and by motor vehicles, including diesel trucks, are included in the analysis. These aerosol types were distinguished by different wavelength (λ) dependences in light absorption. Light absorption by the motor vehicle aerosols exhibited relatively weak wavelength dependence; absorption varied approximately as λ−1, indicating that black carbon (BC) was the dominant absorbing aerosol component. By contrast, the biomass smoke aerosols had much stronger wavelength dependence, approximately λ−2. The stronger spectral dependence was the result of enhanced light absorption at wavelengths shorter than 600 nm and was largely reduced when much of the sample organic carbon (OC) was extracted by dissolution in acetone. This indicates that OC in addition to BC in the biomass smoke aerosols contributed significantly to measured light absorption in the ultraviolet and visible spectral regions and that OC in biomass burning aerosols may appreciably absorb solar radiation. Estimated absorption efficiencies and imaginary refractive indices are presented for the OC extracted from biomass burning samples and the BC in motor vehicle-dominated aerosol samples. The uncertainty of these constants is discussed. Overall, results of this investigation show that low-temperature, incomplete combustion processes, including biomass burning, can produce light-absorbing aerosols that exhibit much stronger spectral dependence than high-temperature combustion processes, such as diesel combustion.

1,283 citations


Journal ArticleDOI
TL;DR: In this article, an efficient bulk-heterojunction solar cells can be made using ZnO nanoparticles and a conjugated polymer, and the incident photon to current conversion efficiency (IPCE) closely resembles the absorption spectrum of the MDMO-PPV layer on glass, and reaches a value of 40 % at the absorption max.
Abstract: The authors show that efficient bulk-heterojunction solar cells can be made using ZnO nanoparticles and a conjugated polymer. These cells can be processed from soln. and exhibit an incident photon to current conversion efficiency up to 40 %. Nanocryst. ZnO (nc-ZnO) of approx. 5 nm diam. was synthesized and used. As the n-type semiconductor. Complexes with MDMO-PPV were used, with Aluminum and a PEDOT-PSS/ITO-coated glass hole-conducting electrode. The incident photon to current conversion efficiency (IPCE)closely resembles the absorption spectrum of the MDMO-PPV:nc-ZnO layer on glass, and reaches a value of 40 % at the absorption max. MDMO-PPV. Integration of this spectral response with the solar spectrum (AM1.5G, normalized 100 mW cm-2) affords an est. of the short-circuit c.d. of Jsc = 3.3 mA/cm2 under AM1.5 (1 sun) conditions. C.d.-voltage (J-V) measurements, carried out in the dark reveal excellent diode behavior, with electron current dominating the c.d. in forward bias. TEM micrographs showed intimate mixing of the ZnO and the MDMO-PPV, with the majority of the polymer domains smaller than a few tens of nanometers, which has been shown to be the exciton diffusion length in a similar PPV polymer.

920 citations


Journal ArticleDOI
TL;DR: In this paper, a co-doping effect between nitrogen and hydrogen is postulated to be responsible for the enhanced photoactivity of nitrogen-doped TiO2 materials in the range of visible light.
Abstract: TiO2(110) single crystals, doped with nitrogen via an NH3 treatment at 870 K, have been found to exhibit photoactivity at photon energies down to 2.4 eV, which is 0.6 eV below the band-gap energy for rutile TiO2. The active dopant state of the interstitial nitrogen that is responsible for this effect exhibits an N (1s) binding energy of 399.6 eV and is due to a form of nitrogen that is probably bound to hydrogen, which differs from the substitutional nitride state with an N (1s) binding energy of 396.7 eV. Optical absorption measurements also show enhanced absorption down to 2.4 eV for the NH3-treated TiO2(110). A co-doping effect between nitrogen and hydrogen is postulated to be responsible for the enhanced photoactivity of nitrogen-doped TiO2 materials in the range of visible light.

687 citations


Journal ArticleDOI
TL;DR: In this paper, a new method is presented which determines the aerosol light absorption from the simultaneous measurement of radiation passing through and scattered back from a particle-loaded /bre /lter.

520 citations


Journal ArticleDOI
TL;DR: Extinction, scattering, and absorption cross sections and efficiencies were experimentally measured for chemically clean silver nanoparticles in water for 16 different sizes ranging from 29 to 136 nm in this article.
Abstract: Extinction, scattering, and absorption cross sections and efficiencies were experimentally measured for chemically clean silver nanoparticles in water for 16 different sizes ranging from 29 to 136 nm. The measured efficiencies indicate that particles interact with light 4−10 times stronger than the geometric cross section suggests. Absorption and scattering components of the plasmon resonance were separately measured across the visible spectral range. A method, termed standard subtraction, is suggested for the simple and reliable determination of particle concentrations independent of their size, shape, and aggregation state.

455 citations


Journal ArticleDOI
TL;DR: Ultrafast x-ray techniques using diffraction and absorption are discussed with an emphasis on the absorption techniques, and sources and detectors for use in x-rays absorption spectroscopy are reviewed.
Abstract: A review. Ultrafast x-ray techniques using diffraction and absorption are discussed with an emphasis on the absorption techniques. Ultrafast x-ray sources and detectors for use in x-ray absorption spectroscopy are also reviewed. [on SciFinder (R)]

418 citations


Journal ArticleDOI
TL;DR: The far-infrared absorption and index of refraction of high resistivity, float-zone, crystalline silicon has been measured by terahertz time-domain spectroscopy.
Abstract: The far-infrared absorption and index of refraction of high-resistivity, float-zone, crystalline silicon has been measured by terahertz time-domain spectroscopy. The measured new upper limit for the absorption of this most transparent dielectric material in the far infrared shows unprecedented transparency over the range from 0.5 to 2.5 THz and a well-resolved absorption feature at 3.6 THz. The index of refraction shows remarkably little dispersion, changing by only 0.0001 over the range from 0.5 to 4.5 THz.

412 citations


Journal ArticleDOI
TL;DR: In this paper, microstructured silicon surfaces with femtosecond laser irradiation in the presence of SF6 were shown to display strong absorption of infrared radiation at energies below the band gap of crystalline silicon.
Abstract: We microstructured silicon surfaces with femtosecond laser irradiation in the presence of SF6. These surfaces display strong absorption of infrared radiation at energies below the band gap of crystalline silicon. We report the dependence of this below-band gap absorption on microstructuring conditions (laser fluence, number of laser pulses, and background pressure of SF6) along with structural and chemical characterization of the material. Significant amounts of sulfur are incorporated into the silicon over a wide range of microstructuring conditions; the sulfur is embedded in a disordered nanocrystalline layer less than 1 μm thick that covers the microstructures. The most likely mechanism for the below-band gap absorption is the formation of a band of sulfur impurity states overlapping the silicon band edge, reducing the band gap from 1.1 eV to approximately 0.4 eV.

286 citations


Journal ArticleDOI
TL;DR: The absorption of a single isolated metal cluster is directly measured using a novel far-field optical technique based on modulation of its position using optically detected single gold nanoparticles dispersed on a transparent substrate.
Abstract: The absorption of a single isolated metal cluster is directly measured using a novel far-field optical technique based on modulation of its position. Single gold nanoparticles with average diameters down to 5 nm, dispersed on a transparent substrate, are optically detected and their absolute absorption cross section determined.

285 citations


Journal ArticleDOI
18 Nov 2004-Nature
TL;DR: It is shown how current directly injected into a freely suspended individual single-wall carbon nanotube can be used to excite, detect and control a specific vibrational mode of the molecule.
Abstract: The interplay between discrete vibrational and electronic degrees of freedom directly influences the chemical and physical properties of molecular systems. This coupling is typically studied through optical methods such as fluorescence, absorption and Raman spectroscopy. Molecular electronic devices provide new opportunities for exploring vibration-electronic interactions at the single molecule level. For example, electrons injected from a scanning tunnelling microscope tip into a metal can excite vibrational excitations of a molecule situated in the gap between tip and metal. Here we show how current directly injected into a freely suspended individual single-wall carbon nanotube can be used to excite, detect and control a specific vibrational mode of the molecule. Electrons tunnelling inelastically into the nanotube cause a non-equilibrium occupation of the radial breathing mode, leading to both stimulated emission and absorption of phonons by successive electron tunnelling events. We exploit this effect to measure a phonon lifetime of the order of 10 ns, corresponding to a quality factor of well over 10,000 for this nanomechanical oscillator.

284 citations


Journal ArticleDOI
TL;DR: Multidistance time-resolved diffuse reflectance spectroscopy of the head of a healthy adult after intravenous administration of a bolus of indocyanine green is reported on.
Abstract: We report on multidistance time-resolved diffuse reflectance spectroscopy of the head of a healthy adult after intravenous administration of a bolus of indocyanine green. Intracerebral and extracerebral changes in absorption are deduced from moments (integral, mean time of flight, and variance) of the distributions of times of flight of photons (DTOFs), recorded simultaneously at four different source-detector separations. We calculate the sensitivity factors converting depth-dependent changes in absorption into changes of moments of DTOFs by Monte Carlo simulations by using a layered model of the head. We validate our method by analyzing moments of DTOFs simulated for the assumed changes in absorption in different layers of the head model.

Journal ArticleDOI
TL;DR: In this article, a multi-axis Differential Optical Absorption Spectroscopy (MAX-DOAS) method for the determination of atmospheric aerosol properties is presented.
Abstract: [1] Multi AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) observations of the oxygen dimer O4 which can serve as a new method for the determination of atmospheric aerosol properties are presented. Like established methods, e.g., Sun radiometer and LIDAR measurements, MAX-DOAS O4 observations determine optical properties of aerosol under atmospheric conditions (not dried). However, the novel technique has two major advantages: It utilizes differential O4 absorption structures and thus does not require absolute radiometric calibration. In addition, O4 observations using this method provide a new kind of information: since the atmospheric O4 profile depends strongly on altitude, they can yield information on the atmospheric light path distribution and in particular on the atmospheric aerosol profile. From O4 observations during clear days and from atmospheric radiative transfer modeling, we conclude that our new method is especially sensitive to the aerosol extinction close to the ground. In addition, O4 observations using this method yield information on the penetration depth of the incident direct solar radiation. O4 observations at different azimuth angles can also provide information on the aerosol scattering phase function. We found that MAX-DOAS O4 observations are a very sensitive method: even aerosol extinction below 0.001 could be detected. In addition to the O4 absorptions we also investigated the magnitude of the Ring effect and the (relative) intensity. Both quantities yield valuable further information on atmospheric aerosols. From the simultaneous analysis of the observed O4 absorption and the measured intensity, in particular, information on the absorbing properties of the aerosols might be derived. The aerosol information derived from MAX-DOAS observations can be used for the quantitative analysis of various trace gases also analyzed from the measured spectra.

Journal ArticleDOI
TL;DR: In this paper, the key SESAM parameters such as saturation fluence, modulation depth, and nonsaturable losses are measured with a high accuracy to obtain stable pulse generation for a given laser.
Abstract: Semiconductor saturable absorber mirror (SESAM) devices have become a key component of ultrafast passive mode-locked laser sources Here we describe in more detail how the key SESAM parameters such as saturation fluence, modulation depth, and nonsaturable losses are measured with a high accuracy These parameters need to be known and controlled to obtain stable pulse generation for a given laser A high-precision, wide dynamic range setup is required to measure this nonlinear reflectivity of saturable absorbers The challenge to measure a low modulation depth and key measures necessary to obtain an accurate calibration are described in detail The model function for the nonlinear reflectivity is based on a simple two-level travelling wave system We include spatial beam profiles, nonsaturable losses and higher-order absorption, such as two-photon absorption and other induced absorption Guidelines to extract the key parameters from the measured data are given

Journal ArticleDOI
TL;DR: In this paper, the performance of carbon dioxide (CO2) absorption into aqueous solutions of single and blended alkanolamines was evaluated experimentally in a bench-scale absorber packed with high-efficiency packings.
Abstract: The performance of carbon dioxide (CO2) absorption into aqueous solutions of single and blended alkanolamines was evaluated experimentally in a bench-scale absorber packed with high-efficiency packings. The absorption experiments were conducted under atmospheric pressure, using a feed gas mixture containing 10% CO2 and 90% nitrogen. Monoethanolamine (MEA), diethanolamine (DEA), diisopropanolamine (DIPA), methyldiethanolamine (MDEA), 2-amino-2-methyl-1-propanol (AMP), and their mixtures including MEA−MDEA, DEA−MDEA, MEA−AMP, and DEA−AMP were tested in this work. The absorption performance was presented in terms of the CO2 removal efficiency, absorber height requirement, effective interfacial area for mass transfer, and overall mass-transfer coefficient (KGae). Comparison of the absorption performance between the tested alkanolamines was made over ranges of operating conditions to establish the correlation between single- and blended-alkanolamine systems.

Journal ArticleDOI
TL;DR: In this paper, the authors describe new algorithms, not previously available, for predicting atmospheric absorption of sound at high altitudes, and a basis for estimating atmospheric absorption up to 160 km is described.
Abstract: This paper describes new algorithms, not previously available, for predicting atmospheric absorption of sound at high altitudes. A basis for estimating atmospheric absorption up to 160 km is described. The estimated values at altitudes above 90 km must be considered as only approximate due to uncertainties about the composition of the atmosphere above 90 km and simplifying assumptions. At high altitudes, classical and rotational relaxation absorption are dominant, as opposed to absorption by molecular vibrational relaxation that is the principle atmospheric absorption loss mechanism for primary sonic booms propagating downward from a cruising supersonic aircraft. Classical and rotational relaxation absorption varies inversely with atmospheric pressure, thus increasing in magnitude at high altitudes as atmospheric pressure falls. However, classical and rotational losses also relax at the high values of frequency/pressure reached at high altitudes and thus, for audio and infrasonic frequencies, begin to dec...

Journal ArticleDOI
TL;DR: In this paper, the interaction of Lyα photons produced by the first stars in the universe with intergalactic hydrogen prior to reionization was investigated, and accurate values of the heating and scattering rates and the spin-kinetic temperature coupling coefficient were presented.
Abstract: We investigate the interaction of Lyα photons produced by the first stars in the universe with intergalactic hydrogen prior to reionization. The background Lyα spectral profile is obtained by solving a Fokker-Planck equation. We present accurate values of the heating and scattering rates and the spin-kinetic temperature coupling coefficient. We show that the heating rate induced by the Lyα scattering is much lower than found previously and is basically negligible. The dominant heating source is most likely X-rays from the first ionizing sources, which are able to penetrate into the atomic medium. The scattering of Lyα photons couples the hydrogen spin temperature to the kinetic temperature. If the first ionizing sources in the universe did not emit significant X-rays, the spin temperature would be rapidly brought down to the very low gas kinetic temperature, and a 21 cm absorption signal against the cosmic microwave background (CMB) larger than 100 mK would be predicted. However, we argue that sufficient X-rays are likely to have been emitted by the first stellar population, implying that the gas kinetic temperature should rapidly increase, turning a reduced and brief absorption signal into emission, with a smaller amplitude of ~20 mK. The detection of the 21 cm absorption and emission features would be a hallmark in unravelling the history of the "dark age" before reionization.

Journal ArticleDOI
TL;DR: In this paper, free carriers generated by two-photon-absorption in silicon-on-insulator (SOI) waveguides can introduce large losses which limit the usable pump power for Raman amplification at telecommunication wavelengths.
Abstract: We show experimentally that free carriers generated by two-photon-absorption in silicon-on-insulator (SOI) waveguides can introduce large losses which limit the usable pump power for Raman amplification at telecommunication wavelengths. The measured pump loss agreed with a theoretical model of the free-carrier absorption arising from two-photon-induced free carrier generation inside the waveguide.

Journal ArticleDOI
TL;DR: In this paper, a suite of 27 common, Fe-bearing reference compounds, including sulfides, carbonates, phosphates, oxides, oxyhydroxides, and phyllosilicates, was analyzed to empirically assess the utility of X-ray absorption near-edge structure (XANES) and extended Xray absorption fine structure (EXAFS) for identifying a particular Fe mineral (or class of minerals) in a soil or sediment mixture.
Abstract: Synchrotron X-ray absorption spectroscopy (XAS) is becoming an increasingly popular tool for the analysis of element speciation in complex natural mixtures such as soils and sediments. Identification of a particular mineral or amorphous solid in a heterogeneous mixture by XAS depends on the spectral uniqueness of the element in the bonding environment associated with a component, and on absorption effects from the components and the matrix. A suite of 27 common, Fe-bearing reference compounds, including sulfides, carbonates, phosphates, oxides, oxyhydroxides, and phyllosilicates, was analyzed to empirically assess the utility of X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) for identifying a particular Fe mineral (or class of minerals) in a soil or sediment mixture. We show that XANES spectral features are useful for distinguishing qualitatively among major mineral classes, but not necessarily for identifying minerals within classes. A practical detection limit (based on empirical mixtures) for most mineral classes is on the order of 5% of the total atomic Fe absorption, but detection limits vary depending on the spectral uniqueness of the components, the number of components, and the matrix. Calibration curves for Fe sulfide and non-sulfide (phyllosilicate ± oxide) component mixtures were made independently from the analyses of XANES and EXAFS fluorescence spectra of binary and ternary mineral mixtures (at 5% total Fe) in a quartz matrix to mimic natural sediments. Because of differences in sample and matrix absorption and fluorescence among sulfide and phyllosilicate minerals, apparent component fractions of pyrite derived from fits deviated significantly from linear binary mixtures. We show that corrections for non-linear fluorescence (as much as 20%) are particularly important for binary mineral mixtures with different densities and absorption characteristics (such as sulfides and phyllosilicates), and for mixtures with similar component abundances (i.e., far from one end-member). An application of the fluorescence calibration for XANES and EXAFS is shown for natural samples. This study points out the importance of a high-quality, experimentally consistent reference compound library, and the need for calibration of fluorescence spectra, in order to quantify accurately the component fractions of complex mixtures using XAS.

OtherDOI
28 Jan 2004
TL;DR: The Stillwater and East Boulder mines in south-central Montana are the only primary platinum-group metals (PGMs) mines in the United States and were owned by one company.
Abstract: Domestic Production and Use: The Stillwater and East Boulder Mines in south-central Montana are the only primary platinum-group metals (PGMs) mines in the United States and were owned by one company. Small quantities of PGMs were also recovered as byproducts of copper refining by companies in Texas and Utah. Catalysts for airpollution abatement continued to be the leading demand sector for PGMs. Catalysts were also used in other airpollution-abatement processes to remove organic vapors, odors, and carbon monoxide. Chemical uses include catalysts for organic synthesis, production of nitric acid, and fabrication of laboratory equipment. Platinum alloys, in cast or wrought form, are commonly used for jewelry. Platinum, palladium, and a variety of complex gold-silvercopper alloys are used as dental restorative materials.


Journal ArticleDOI
TL;DR: High nonlinearity ranging from 2000 to 27,000 times that of fused silica, depending on the incident optical intensity is revealed in Ag(x)(As2Se3) chalcogenide glasses.
Abstract: We prepared Ag(x)(As0.4Se0.6)(100-x) chalcogenide glasses by a melt-quenching method and measured their linear and nonlinear optical properties to evaluate their potential applications to all-optical ultrafast switching devices. Their nonlinear refraction and absorption were measured by the Z-scan method at 1.05 microm. The addition of Ag to As2Se3 glass led to an increase in the nonlinear refractive index without introducing an increase in the nonlinear absorption coefficient. The glass with a Ag content of x = 20 at. % revealed high nonlinearity ranging from 2000 to 27,000 times that of fused silica, depending on the incident optical intensity.

Journal ArticleDOI
TL;DR: In this article, the optical absorption edge covers a wide energy range from the intrinsic band gap of InN of about 0.7 to about 1.7 eV which is close to the previously accepted band gap.
Abstract: InN films with free electron concentrations ranging from mid-1017 to mid-1020 cm−3 have been studied using optical absorption, Hall effect, and secondary ion mass spectrometry. The optical absorption edge covers a wide energy range from the intrinsic band gap of InN of about 0.7 to about 1.7 eV which is close to the previously accepted band gap of InN. The electron concentration dependence of the optical absorption edge energy is fully accounted for by the Burstein–Moss shift. Results of secondary ion mass spectrometry measurements indicate that O and H impurities cannot fully account for the free electron concentration in the films.

Journal ArticleDOI
TL;DR: In this article, the microwave response properties of the ZnO nanowire-polyester composites fabricated into a planar plate with the area of 180×180 mm2 and the thickness of about 1 mm.
Abstract: We report on the microwave response properties of the ZnO nanowire-polyester composites fabricated into a planar plate with the area of 180×180 mm2 and the thickness of about 1 mm. Strong microwave absorption has been observed in X band and the maximum absorption is enhanced as the concentration of the nanowires increases in the composites. Both the low complex permittivity and the low dissipation of the pure nanowires demonstrate the pure nanowires are low-loss materials for microwave absorption in X band. The strong absorption is related to interfacial multipoles at the interface between the polyester and the ZnO nanowires, a high surface-to-volume ratio and a similar shape of the nanowires to antenna.

Journal ArticleDOI
TL;DR: Waveguide terahertz time-domain spectroscopy is demonstrated to have the sensitivity to characterize nanometer-thick water layers on the surfaces of a parallel-plate metal waveguide.
Abstract: Waveguide terahertz time-domain spectroscopy is demonstrated to have the sensitivity to characterize nanometer-thick water layers on the surfaces of a parallel-plate metal waveguide. The measured far-infrared absorption and index of refraction of the 20-nm water layers are in reasonable self-consistent agreement with those of bulk water.

Journal ArticleDOI
TL;DR: In this paper, a two-component pump-probe spectroscopy of micelle-suspended single-walled carbon nanotubes reveals two components dynamics: the slow component (5-20 ps), which has not been observed previously, is resonantly enhanced whenever the pump photon energy coincides with an absorption peak and is attributed to interband carrier recombination, whereas the always-present fast component (0.3-1.2 ps) is interpreted as intraband carrier relaxation in nonresonantly excited carbon nano-ubes.
Abstract: Wavelength-dependent pump-probe spectroscopy of micelle-suspended single-walled carbon nanotubes reveals two-component dynamics. The slow component (5-20 ps), which has not been observed previously, is resonantly enhanced whenever the pump photon energy coincides with an absorption peak and we attribute it to interband carrier recombination, whereas we interpret the always-present fast component (0.3-1.2 ps) as intraband carrier relaxation in nonresonantly excited nanotubes. The slow component decreases drastically with decreasing pH (or increasing H+ doping), especially in large-diameter tubes. This can be explained as a consequence of the disappearance of absorption peaks at high doping due to the entrance of the Fermi energy into the valence band, i.e., a 1D manifestation of the Burstein-Moss effect.

Journal ArticleDOI
TL;DR: In this article, femtosecond laser pulses were used to fabricate straight and bent through-channels, which had diameters on the order of tens of microns, high aspect ratios, and good wall-surface quality.
Abstract: Ultra-short-pulse lasers have proved to be effective tools for micromachining a wide range of materials. When the ultra-short laser pulse is focused inside the bulk of a transparent medium, nonlinear absorption occurs only near the focal volume that is subjected to high intensity. Three-dimensional structures can be fabricated inside transparent materials by taking advantage of this volumetric absorption. In this paper, femtosecond laser pulses were used to fabricate straight and bent through-channels. Drilling was initiated from the rear surface to preserve consistent absorption of the laser pulse. When the debris was not removed efficiently, variation of the channel diameter and occasional termination of the drilling process were observed. Machining in the presence of a liquid and additional use of ultrasonic wave agitation facilitated the debris ejection. The machined channels had diameters on the order of tens of microns, high aspect ratios, and good wall-surface quality.

Journal ArticleDOI
TL;DR: In this paper, the steady-state current of a two-level system with a two level system was derived by solving the coupled rate equations for electron transfer in the system under illumination as a function of bias applied to the contacts.
Abstract: ~Received 5 July 2003; revised manuscript received 17 September 2003; published 30 January 2004! We present a simple model of a molecular photovoltaic device consisting of a two-level system, connected to external contacts by chains of one or more charge transporting orbitals. Electrons may be promoted in the two-level system by photon absorption, and charge transported to the external circuit by electron transfer between neighboring orbitals. Photon absorption and emission are described by a generalized Planck equation and electron transfer is described by nonadiabatic Marcus theory. We find the steady-state current by solving the set of coupled rate equations for electron transfer in the system under illumination as a function of bias applied to the contacts. We calculate monochromatic current-voltage characteristics and power conversion efficiency as a function of the system size, orbital energy levels, and electron transfer rates, and compare with the monochromatic detailed balance limit. Using realistic values of the energy levels and charge-transfer rates, we are able to reproduce a number of commonly observed features in the current-voltage characteristics. These include a ‘‘kink’’ in the current-voltage curve close to open circuit when large interfacial energy steps are present or mobilities are low, and a reduction of the open-circuit voltage and crossing of the light and dark current curves when interfacial recombination is strong. We show that open-circuit voltage is dominated by the acceptor-donor energy gap when recombination is important, and by the optical gap when recombination is low. We confirm previous reports that photovoltaic energy conversion can be achieved by interfacial asymmetry alone and that a potential difference between the electrodes is unnecessary. Improved photovoltaic efficiency of molecular heterojunctions requires ohmic contacts, improved charge-carrier mobilities, and tuning of the electron-transfer rates at the heterojunction. Maximizing the rate of charge separation does not necessarily lead to maximum efficiency.

Journal ArticleDOI
TL;DR: In this paper, the authors examined the behavior of the properties of line emission and absorption as a function of the ionization parameter, density and column density of model constant density clouds, and showed that the effects of many strongly damped resonances below the K ionization thresholds conspire to smear the edge, thereby potentially affecting the astrophysical interpretation of absorption features in the 7-9 keV energy band.
Abstract: We calculate the efficiency of iron K line emission and iron K absorption in photoionized models using a new set of atomic data. These data are more comprehensive than those previously applied to the modeling of iron K lines from photoionized gases, and allow us to systematically examine the behavior of the properties of line emission and absorption as a function of the ionization parameter, density and column density of model constant density clouds. We show that, for example, the net fluorescence yield for the highly charged ions is sensitive to the level population distribution produced by photoionization, and these yields are generally smaller than those predicted assuming the population is according to statistical weight. We demonstrate that the effects of the many strongly damped resonances below the K ionization thresholds conspire to smear the edge, thereby potentially affecting the astrophysical interpretation of absorption features in the 7-9 keV energy band. We show that the centroid of the ensemble of K(alpha) lines, the K(beta) energy, and the ratio of the K(alpha(sub 1)) to K(alpha(sub 2)) components are all diagnostics of the ionization parameter of our model slabs.

Book ChapterDOI
20 Jan 2004

Journal ArticleDOI
TL;DR: In this paper, single-walled carbon nanotubes (SWNTs) were compared with spectra from ensembles, showing that the peak shape and peak width are almost the same for a single nanotube as they are for the corresponding species in a large ensemble.
Abstract: Photoluminescence (PL) and photoluminescence excitation (PLE) spectra are obtained from individual single-walled carbon nanotubes (SWNTs). Individual SWNT spectra are compared with spectra from ensembles. The PL spectrum of an individual SWNT in air at room temperature has a single asymmetric peak of width typically 10 to 15 meV, with no detected background. Both absorption and emission are strongly polarized along the tube axis. Photoluminescence excitation spectroscopy on single SWNTs clearly confirms the unique, one-to-one association of optical absorption resonances with individual emission peaks. Resonances in the PLE spectra are typically $\ensuremath{\approx}30\mathrm{meV}$ wide, with the PL intensity enhanced tenfold over nonresonant excitation. Whether for emission or absorption, the peak shape and peak width are almost the same for a single nanotube as they are for the corresponding species in a large ensemble. That is, there is no significant inhomogeneous broadening. While ensemble measurements are complicated by the superposition of many PL peaks from many different species, single nanotube spectra clearly isolate a single peak and are thus simpler to interpret.