Showing papers on "Absorption (electromagnetic radiation) published in 2003"

Fundamentals and applications

Abstract: Preface.PART I: LIGHT-MATTER INTERACTION: FUNDAMENTALS.Absorption and Emission of Radiation.Semiclassical Treatment of Absorption and Emission.The Optical Bloch Equations.Optical Bloch Equations of a Two-Level Atom.Quantized Fields and Dressed States.Forces from Atom-Light Interaction.The Laser.Elements of Optics.Index.

Effect of interchain interactions on the absorption and emission of poly(3-hexylthiophene)

Abstract: The absorption spectrum of polythiophene and its derivative poly(3-hexylthiophene) (P3HT) is usually described in terms of an intrachain exciton coupled to a single phonon mode. We show that this model is too simplistic for highly ordered, regioregular P3HT and that, analogous to the case of charged polarons in this material, interchain interactions must be taken into account to correctly describe the absorption spectrum. We show that the lowest energy feature in the $\ensuremath{\pi}\ensuremath{-}{\ensuremath{\pi}}^{*}$ region of the absorption spectrum is associated with an interchain absorption, the intensity of which is correlated with the degree of order in the polymer. Correspondingly, we show that the emission from P3HT also exhibits contributions from both interchain and intrachain states, in a manner similar to that recently shown for poly(phenylenevinylene). Having reinterpreted the physical origin of the features in the absorption and emission spectra of P3HT, we then model these spectra and show how they evolve as the degree of order in the polymer is changed by varying several physical parameters including temperature and regioregularity of the polymer.

Variations in the light absorption coefficients of phytoplankton, nonalgal particles, and dissolved organic matter in coastal waters around Europe

Abstract: [1] We measured the absorption properties of phytoplankton, nonalgal particles (NAP), and colored dissolved organic matter (CDOM) at about 350 stations in various coastal waters around Europe including the English Channel, Adriatic Sea, Baltic Sea, Mediterranean Sea, and North Sea. For comparison, we also collected data in the open ocean waters of North Atlantic. The exponential slope of the CDOM absorption spectrum varied within a narrow range around 0.0176 nm−1 (SD = 0.0020 nm−1). When data from all the regions were considered altogether, the relationship between phytoplankton absorption and chlorophyll concentration was generally similar to the one previously established for open oceanic waters. Our coastal data, however, show that significant departures from the general trend may occur due to peculiar pigment composition and cell size. In some coastal areas, high phaeopigment concentrations gave rise to especially high blue-to-red ratio of phytoplankton absorption. The NAP absorption covaried with the particle dry weight. Most absorption spectra of these particles were well described by an exponential function with a slope averaging 0.0123 nm−1 (SD = 0.0013 nm−1). In some highly turbid waters, the spectra exhibited a signature possibly associated with iron oxides. In the Baltic Sea, NAP absorption systematically showed lower values at wavelengths shorter than 440 nm than predicted from the fitted exponential function. Overall, the variability in the absorption properties of European coastal waters showed some consistent patterns despite the high diversity of the examined waters. Distinct features were identified in the phytoplankton and NAP components. An absorption budget is presented and parameterizations are proposed.

Measurements of molecular absorption spectra with the SCIAMACHY pre-flight model: instrument characterization and reference data for atmospheric remote-sensing in the 230–2380 nm region

Abstract: Using the scanning imaging absorption spectrometer for atmospheric chartography (SCIAMACHY) pre-flight model satellite spectrometer, gas-phase absorption spectra of the most important atmospheric trace gases (O3, NO2, SO2, O2, OClO, H2CO, H2O, CO, CO2, CH4, and N2O) have been measured in the 230–2380 nm range at medium spectral resolution and at several temperatures between 203 and 293 K. The spectra show high signal-to-noise ratio (between 200 up to a few thousands), high baseline stability (better than 10−2) and an accurate wavelength calibration (better than 0.01 nm) and were scaled to absolute absorption cross-sections using previously published data. The results are important as reference data for atmospheric remote-sensing and physical chemistry. Amongst other results, the first measurements of the Wulf bands of O3 up to their origin above 1000 nm were made at five different temperatures between 203 and 293 K, the first UV-Vis absorption cross-sections of NO2 in gas-phase equilibrium at 203 K were recorded, and the ultraviolet absorption cross-sections of SO2 were measured at five different temperatures between 203 and 296 K. In addition, the molecular absorption spectra were used to improve the wavelength calibration of the SCIAMACHY spectrometer and to characterize the instrumental line shape (ILS) and straylight properties of the instrument. It is demonstrated that laboratory measurements of molecular trace gas absorption spectra prior to launch are important for satellite instrument characterization and to validate and improve the spectroscopic database.

A photovoltaic device structure based on internal electron emission

Abstract: There has been an active search for cost-effective photovoltaic devices since the development of the first solar cells in the 1950s (refs 1-3). In conventional solid-state solar cells, electron-hole pairs are created by light absorption in a semiconductor, with charge separation and collection accomplished under the influence of electric fields within the semiconductor. Here we report a multilayer photovoltaic device structure in which photon absorption instead occurs in photoreceptors deposited on the surface of an ultrathin metal-semiconductor junction Schottky diode. Photoexcited electrons are transferred to the metal and travel ballistically to--and over--the Schottky barrier, so providing the photocurrent output. Low-energy (approximately 1 eV) electrons have surprisingly long ballistic path lengths in noble metals, allowing a large fraction of the electrons to be collected. Unlike conventional cells, the semiconductor in this device serves only for majority charge transport and separation. Devices fabricated using a fluorescein photoreceptor on an Au/TiO2/Ti multilayer structure had typical open-circuit photovoltages of 600-800 mV and short-circuit photocurrents of 10-18 micro A cm(-2) under 100 mW cm(-2) visible band illumination: the internal quantum efficiency (electrons measured per photon absorbed) was 10 per cent. This alternative approach to photovoltaic energy conversion might provide the basis for durable low-cost solar cells using a variety of materials.

Large enhancement of four-wave mixing by suppression of photon absorption from electromagnetically induced transparency

Abstract: We analyze a four-wave-mixing (FWM) scheme in a five-level atomic system based on electromagnetically induced transparency (EIT). We show that EIT suppresses both two-photon and three-photon absorptions in the FWM scheme and enables the four-wave mixing to proceed through real, resonant intermediate states without absorption loss. The scheme results in a several orders of magnitude increase in the FWM efficiency in comparison with a recent scheme [Phys. Rev. Lett. 88, 143902 (2002)] and may be used for generating short-wavelength radiation at low pump intensities.

Biosensing based on light absorption of nanoscaled gold and silver particles

Abstract: The absorption spectrum of noble metal spherical nanoparticles is known to be strongly influenced by the dielectric constant of the surrounding material, and as such, these particles are well suited for biosensing applications. To perform biosensing using nanoparticles on a substrate, the metal particles are covalently attached onto quartz using an organic adhesion layer of mercaptosilanes. The particles in solution are characterized by UV-vis spectroscopy and transmission electron microscopy, while those attached to the quartz are characterized with UV-vis spectroscopy and atomic force microscopy. Antibodies are attached to the metal nanoparticles, and the antigen recognition is monitored via the change of light absorption when this binding event occurs. Not only is the absorbance originating from plasmon resonances of the particles influenced by the dielectric properties of molecules attached to the nanospheres but also the interband absorption of the particles changes, which will be demonstrated in this report. A light absorption change is detected when a molecular recognition occurs between the bioreceptor molecules attached to the nanoparticle and a biomolecular counterpart. This change in absorption can be very large when adhered molecules are at resonance (interband transitions). In addition, the presented type of biosensing can be a cost-effective and easy to use alternative to conventional biosensing techniques.

Understanding and predicting the temporal response of laser-induced incandescence from carbonaceous particles

Abstract: This paper describes a model for analyzing and predicting the temporal behavior of laser-induced incandescence (LII) from combustion-generated soot, carbon black, and other carbonaceous particles on a nanosecond time scale. The model accounts for particle heating by absorption of light from a pulsed laser and cooling by sublimation, conduction, and radiation. The model also includes mechanisms for oxidation, melting, and annealing of the particles and nonthermal photodesorption of carbon clusters from the particle surface. At fluences above 0.1 J/cm2, particle temperatures during the laser pulse are determined by the balance between absorption and sublimation, whereas at lower fluences particle temperatures do not reach the sublimation temperature, and temperatures are predominantly controlled by absorption and conduction. After the laser pulse, temperatures are predominantly controlled by conductive cooling rates. Oxidative heating may compete with conductive cooling on these time scales. Annealing of the particles to a more ordered phase of carbon is predicted to occur at fluences as low as 0.02 J/cm2. Annealing may strongly influence sublimation rates, and changes in emissivity during annealing are predicted to increase signal decay rates. Supersonic expansion of the carbon clusters sublimed from the surface is calculated to occur at fluences above 0.12 J/cm2. When compared with LII measurements recorded in a flame at atmospheric pressure, the model reproduces the shapes and relative magnitudes of LII temporal profiles over a wide range of laser fluences. Comparisons between model predictions and experimental observations suggest that the particles do not melt at laser fluences that lead to melting of bulk graphite. These comparisons also indicate that the energy released during particle annealing is much smaller than that released during annealing of neutron- or electron-irradiated graphite. Despite good agreement between model and experimental results, large uncertainties exist for input parameters used to calculate annealing rates and rates of oxidation, conduction, absorption, emission, and photolytic desorption of carbon clusters for both the initial and annealed particles.

Simultaneous in situ measurement of CO, H2O, and gas temperatures in a full-sized coal-fired power plant by near-infrared diode lasers.

Abstract: We present what is to our knowledge the first near-infrared diode-laser-based absorption spectrometer that is suitable for simultaneous in situ measurement of carbon monoxide, water vapor, and temperature in the combustion chamber (20-m diameter, 13-m path length) of a 600-MW lignite-fired power plant. A fiber-coupled distributed-feedback diode-laser module at 1.56 microm served for CO detection, and a Fabry-Perot diode laser at 813 nm was used to determine H2O concentrations and temperature from multiline water spectra. Despite severe light losses (transmission, <10(-8)) and strong background radiation we achieved a resolution of 1.9 x 10(-4) (1sigma) fractional absorption, equivalent to 200 parts in 10(6) by volume of CO (at 1450 K, 10(5) Pa) with 30-s averaging time.

Absorption and drug development

Abstract: Absorption and drug development , Absorption and drug development , کتابخانه مرکزی دانشگاه علوم پزشکی تهران

Inhomogeneous optical absorption around the K point in graphite and carbon nanotubes

Abstract: The optical absorption spectra of \ensuremath{\pi} electrons are calculated for graphite and carbon nanotubes. Particular attention is paid to the processes contributing to the optical absorption as a function of the electron wave vector k and light polarization direction. The optical absorption amplitude around the K point in the Brillouin zone has a node in the two-dimensional Brillouin zone of graphite. The formula for the absorption scattering matrix around the K point is given analytically by expanding the matrix element into a Taylor series. The chirality dependence of the absorption matrix element of a single-wall carbon nanotube is presented.

In vivo absorption and scattering spectroscopy of biological tissues

Abstract: Different approaches for absorption and scattering spectroscopy of living tissues are discussed. In particular, a unique system for time-resolved reflectance and transmittance spectroscopy is presented, capable of acquiring in vivo absorption and scattering spectra of diffusive media between 600 and 1000 nm. A review of typical spectra obtained from a variety of tissue structures is shown, including female breast, forearm, abdomen, and forehead. A second-level analysis of the measured spectra permits an estimation of the concentrations of the key tissue absorbers, as well as of the Mie-equivalent scattering radii. Further, absorption and scattering spectra can be used to estimate the penetration depth of light in tissues as a function of wavelength, which is a crucial parameter in view of the possible application of optical in vivo molecular imaging in clinical diagnosis. Finally, an example of the applicability of the methodology to other biological media such as fruits and vegetables is shown.

Toward a Self-Consistent Model of the Ionized Absorber in NGC 3783

Abstract: We present a detailed model for the ionized absorbing gas evident in the 900 ks Chandra HETGS spectrum of NGC 3783. The analysis was carried out with PHASE, a new tool designed to model X-ray and UV absorption features in ionized plasmas. The 0.5-10 keV intrinsic continuum of the source is well represented by a single power law (Γ = 1.53) and a soft blackbody component (kT ~ 0.1 keV). The spectrum contains over 100 features, which are well fitted by PHASE with just six free parameters. The model consists of a simple two-phase absorber with a difference of ≈35 in the ionization parameter and a difference of ≈4 in the column density of the phases. The two absorption components turned out to be in pressure equilibrium and are consistent with a single outflow (≈750 km s-1), a single turbulent velocity (300 km s-1), and solar elemental abundances. The main features of the low-ionization phase are an Fe M-shell unresolved transition array (UTA) and the O VII lines. The O VII features, usually identified with the O VIII and a warm absorber, are instead produced in a cooler medium that also produces O VI lines. The UTA sets tight constraints on the ionization degree of the absorbers, making the model more reliable. The high-ionization phase is required by the O VIII and the Fe L-shell lines, and there is evidence for an even more ionized component in the spectrum. A continuous range of ionization parameters is disfavored by the fits, particularly to the UTA. Our model indicates a severe blending of the absorption and emission lines, as well as strong saturation of the most intense O absorption lines. This is in agreement with the O VII (τλ = 0.33) and O VIII (τλ = 0.13) absorption edges required to fit the spectrum. The low-ionization phase can be decomposed into three subcomponents on the basis of the outflow velocity, FWHM, and H column densities found for three of the four UV absorbers detected in NGC 3783. However, the ionization parameters are systematically smaller in our model than those derived from UV data, indicating a lower degree of ionization. Finally, our model predicts a Ca XVI line for the feature observed at around 21.6 A (a feature formerly identified as O VII), constraining the contribution from a zero-redshift absorber.

Oxygen vacancies in ZnO

Abstract: We present our investigations on the absorption and emission properties of the neutral oxygen vacancy in ZnO. Temperature-dependent photoluminescence (PL) experiments allow to determine the zero-phonon-energy of the emission and its phonon-coupling parameters. The absorption to the singlet excited state is observed in PL-excitation experiments at energies above 3.1 eV. Relaxation drives the system from the singlet, diamagnetic, excited state to the emissive, paramagnetic triplet state. Applying the Mollwo–Ivey relation for the energetical position of the F-centre absorption as a function of the anion–cation distance, we find that the oxygen vacancies in ZnO fit perfectly in the line given by MgO, CaO, SrO and BaO.

The magnetic field of an isolated neutron star from X-ray cyclotron absorption lines

Abstract: Isolated neutron stars are highly magnetized, fast-rotating objects that form as an end point of stellar evolution. They are directly observable in X-ray emission, because of their high surface temperatures. Features in their X-ray spectra could in principle reveal the presence of atmospheres, or be used to estimate the strength of their magnetic fields through the cyclotron process, as is done for X-ray binaries. Almost all isolated neutron star spectra observed so far appear as featureless thermal continua. The only exception is 1E1207.4-5209 (refs 7-9), where two deep absorption features have been detected, but with insufficient definition to permit unambiguous interpretation. Here we report a long X-ray observation of the same object in which the star's spectrum shows three distinct features, regularly spaced at 0.7, 1.4 and 2.1 keV, plus a fourth feature of lower significance, at 2.8 keV. These features vary in phase with the star's rotation. The logical interpretation is that they are features from resonant cyclotron absorption, which allows us to calculate a magnetic field strength of 8 x 10(10) G, assuming the absorption arises from electrons.

Properties of CeO2 and Ce1-xZrxO2 Nanoparticles: X-ray Absorption Near-Edge Spectroscopy, Density Functional, and Time-Resolved X-ray Diffraction Studies

Abstract: In this article the structural and electronic properties of CeO2 and Ce1-xZrxO2 nanoparticles are investigated using time-resolved X-ray diffraction, X-ray absorption near-edge spectroscopy (XANES)...

Photoacoustic and filter-based ambient aerosol light absorption measurements : Instrument comparisons and the role of relative humidity

Abstract: [1] Ambient measurements are reported of aerosol light absorption from photoacoustic and filter-based instruments (aethalometer and a particle soot absorption photometer (PSAP)) to provide insight on the measurement science. Measurements were obtained during the Big Bend Regional Aerosol and Visibility Observational Study at the Big Bend National Park in South Texas. The aethalometer measurements of black carbon concentration at this site correlate reasonably well with photoacoustic measurements of aerosol light absorption, with a slope of 8.1 m2/g and a small offset. Light absorption at this site never exceeded 2.1 Mm−1 during the month of collocated measurements. Measurements were also obtained, as a function of controlled relative humidity between 40% and 90%, during the Photoacoustic IOP in 2000 at the Department of Energy Southern Great Plains Cloud and Radiation Testbed site (SGP). PSAP measurements of aerosol light absorption correlated very well with photoacoustic measurements, but the slope of the correlation indicated the PSAP values were larger by a factor of 1.61. The photoacoustic measurements of light absorption exhibited a systematic decrease when the RH increased beyond 70%. This apparent decrease in light absorption with RH may be due to the contribution of mass transfer to the photoacoustic signal. Model results for the limiting case of full water saturation are used to evaluate this hypothesis. A second PSAP measured the light absorption for the same humidified samples, and indicated very erratic response as the RH changed, suggesting caution when interpreting PSAP data under conditions of rapid relative humidity change.

Influence of optical properties on two-photon fluorescence imaging in turbid samples

Abstract: A numerical model was developed to simulate the effects of tissue optical properties, objective numerical aperture (N.A.), and instrument performance on two-photon-excited fluorescence imaging of turbid samples. Model data are compared with measurements of fluorescent microspheres in a tissuelike scattering phantom. Our results show that the measured two-photon-excited signal decays exponentially with increasing focal depth. The overall decay constant is a function of absorption and scattering parameters at both excitation and emission wavelengths. The generation of two-photon fluorescence is shown to be independent of the scattering anisotropy, g, except for g > 0.95. The N.A. for which the maximum signal is collected varies with depth, although this effect is not seen until the focal plane is greater than two scattering mean free paths into the sample. Overall, measurements and model results indicate that resolution in two-photon microscopy is dependent solely on the ability to deliver sufficient ballistic photon density to the focal volume. As a result we show that lateral resolution in two-photon microscopy is largely unaffected by tissue optical properties in the range typically encountered in soft tissues, although the maximum imaging depth is strongly dependent on absorption and scattering coefficients, scattering anisotropy, and objective N.A..

Electron and hole injection in PbSe quantum dot films.

Abstract: Electrochemical methods are used to inject charge into films of colloidal semiconductor nanocrystals of PbSe. The injection of electrons and holes into quantum confined states is confirmed by monitoring changes in the IR absorption spectrum. Holes are injected into the 1Sh state, causing a bleach of the 1Sh-1Se and 1Sh-1Pe interband transitions and inducing a 1Sh-1Ph intraband absorption. Electrons can be sequentially injected into the 1Se and 1Pe states, first bleaching the 1Sh-1Se and 1Ph-1Se interband transitions and inducing a 1Se-1Pe intraband absorption, and then bleaching the 1Sh-1Pe transition and inducing a 1Pe-1De intraband absorption.

Shape effects in scattering and absorption by randomly oriented particles small compared to the wavelength

Abstract: We study the effects of particle shape on the absorption and scattering cross sections of randomly oriented particles in the Rayleigh domain, i.e. particles that are very small compared to the wavelength of radiation both inside and outside the particle. In particular, we investigate the validity of the so-called statistical approach. In this approach it is assumed that the scattering and absorption properties of irregularly shaped particles can be simulated by the average properties of a distribution of simple shapes. We depart from the assumption of homogeneous spherical particles in two ways: 1) by using various distributions of elongated and flattened homogeneous ellipsoids and spheroids, 2) by using a distribution of hollow spherical particles. We derive explicit formulas for the shape averaged scattering and absorption cross sections as functions of the refractive index for various distributions of particle shapes in the Rayleigh limit. We compare the absorption cross sections as functions of the refractive index of the various distributions with each other and with those of homogeneous spheres. We also study the effects of the distributions on the shapes of absorption spectra. We find that there is a strong similarity between the absorption spectra of distributions of various non-spherical homogeneous particles and a distribution of hollow spheres. We show that the positions of features in the shape averaged mass absorption coefficient as a function of wavelength of small ellipsoidal, spheroidal or hollow spherical crystalline forsterite particles coincide with those deduced from the spectral energy distributions of various astronomical sources and with measurements of the mass absorption coefficients of small particles. This is not the case when we use homogeneous spherical particles.

Changing look: from Compton-thick to Compton-thin, or the re-birth of fossil AGN

Abstract: We discuss the properties of a small sample of Seyfert 2 galaxies whose X-ray spectrum changed appearance on time scales of years, becoming reflection-dominated from Compton-thin, or viceversa. A reflection--dominated spectrum is usually taken as evidence of Compton-thick absorption, but we instead argue that such a spectrum is due to a temporary switching--off of the nuclear radiation. The observations discussed here may help explaining mismatches between optical and X-ray classifications, and provide new strong and direct evidence of the presence of more than one cold circumnuclear region in Seyfert 2 galaxies.

Strong two-photon absorption of self-assembled butadiyne-linked bisporphyrin

Abstract: Two-photon absorption (2PA) properties of self-assembled porphyrins were investigated. The butadiyne-linked porphyrin array exhibited a 20 times larger 2PA cross section than the meso-meso-linked self-assembled array due to the expansion of pi-conjugation. Higher-order nonlinear absorption was also observed in the former porphyrin.

Size dependence of lifetime and absorption cross section of Si nanocrystals embedded in SiO2

Abstract: Photoluminescence lifetimes and optical absorption cross sections of Si nanocrystals embedded in SiO 2 have been studied as a function of their average size and emission energy. The lifetimes span from 20 μs for the smallest sizes (2.5 nm) to more than 200 μs for the largest ones (7 nm). The passivation of nonradiative interface states by hydrogenation increases the lifetime for a given size. In contrast with porous Si, the cross section per nanocrystal shows a nonmonotonic behavior with emission energy. In fact, although the density of states above the gap increases for larger nanocrystals, this trend is compensated by a stronger reduction of the oscillator strength, providing an overall reduction of the absorption cross section per nanocrystal for increasing size.

A critical review of the absorption cross-sections of O3 and NO2 in the ultraviolet and visible

Abstract: The available laboratory measurements of UV-Vis (240–790 nm) absorption cross-sections of O3 and NO2 are critically reviewed taking into account the variation of the cross-sections with temperature (in the 200–300 K range) and with total pressure (

Saturable and reverse saturable absorption of Rhodamine B in methanol and water

Abstract: We have investigated nonlinear absorption of Rhodamine B dye in methanol and water near resonance (532 nm) on the higher-energy (435 nm) and lower-energy (600 nm) sides of the absorption band, using an open-aperture Z-scan technique with nanosecond pulses. We observed reverse saturable absorption (RSA) at 435 nm in both of the solvents, and a transition from saturable absorption (SA) to RSA with an increase in either intensity or concentration at 600 nm in methanol. A transition from RSA to SA with an increase in concentration at 600 nm was observed with water as the solvent. We used theoretical analysis based on rate equations to determine the two-photon and excited-state absorption coefficients from the experimental results.