Showing papers on "Absorption spectroscopy published in 2010"

The spectral signatures of Frenkel polarons in H- and J-aggregates.

Abstract: Electronic excitations in small aggregates, thin films, and crystals of conjugated organic molecules play a fundamental role in the operation of a wide array of organic-based devices including solar cells, transistors, and light-emitting diodes. Such excitations, or excitons, are generally spread out over several molecules: a balance between the delocalizing influence of resonant intermolecular coupling and the localizing influence of static and dynamic disorder determines the coherence range of the exciton. Because of the "soft" nature of organic materials, significant nuclear relaxation in the participating molecules also accompanies the electronic excitations. To properly understand energy or charge transport, one must treat intermolecular (excitonic) coupling, electron-vibrational coupling, and disorder on equal footing. In this Account, we review the key elements of a theoretical approach based on a multiparticle representation that describes electronic excitations in organic materials as vibronic excitations surrounded by a field of vibrational excitations. Such composite excitations are appropriately called Frenkel excitonic polarons. For many conjugated molecules, the bulk of the nuclear reorganization energy following electronic excitation arises from the elongation of a symmetric vinyl stretching mode with energy approximately 1400 cm(-1). To appreciate the impact of aggregation, we study how the vibronic progression of this mode, which dominates the isolated (solvated) molecule absorption and emission spectra, is distorted when molecules are close enough to interact with each other. As we demonstrate in this Account, the nature of the distortion provides a wealth of information about how the molecules are packed, the strength of the excitonic interactions between molecules, the number of molecules that are coherently coupled, and the nature of the disorder. We show that the aggregation-induced deviations from the Poissonian distribution of vibronic peak intensities take on two extremes identified with ideal H- and J-aggregates. The sign of the nearest neighbor electronic coupling, positive for H and negative for J, distinguishes the two basic aggregate forms. For several decades, researchers have known that H-aggregates exhibit blue-shifted absorption spectra and are subradiant while J-aggregates exhibit the opposite behavior (red-shifted absorption and superradiance). However, the exact inclusion of exciton-vibrational coupling reveals several more distinguishing traits between the two aggregate types: in H(J)-aggregates the ratio of the first two vibronic peak intensities in the absorption spectrum decreases (increases) with increasing excitonic coupling, while the ratio of the 0-0 to 0-1 emission intensities increases (decreases) with disorder and increases (decreases) with increasing temperature. These two extreme behaviors provide the framework for understanding absorption and emission in more complex morphologies, such as herringbone packing in oligo(phenylene vinylene)s, oligothiophenes and polyacene crystals, as well as the polymorphic packing arrangements observed in carotenoids.

Evidence for ultra-fast outflows in radio-quiet AGNs - I. Detection and statistical incidence of Fe K-shell absorption lines

Abstract: Context. Blue-shifted Fe K absorption lines have been detected in recent years between 7 and 10 keV in the X-ray spectra of several radio-quiet AGNs. The derived blue-shifted velocities of the lines can often reach mildly relativistic values, up to 0.2–0.4c. These findings are important because they suggest the presence of a previously unknown massive and highly ionized absorbing material outflowing from their nuclei, possibly connected with accretion disk winds/outflows.Aims. The scope of the present work is to statistically quantify the parameters and incidence of the blue-shifted Fe K absorption lines through a uniform analysis on a large sample of radio-quiet AGNs. This allows us to assess their global detection significance and to overcome any possible publication bias.Methods. We performed a blind search for narrow absorption features at energies greater than 6.4 keV in a sample of 42 radio-quiet AGNs observed with XMM-Newton . A simple uniform model composed by an absorbed power-law plus Gaussian emission and absorption lines provided a good fit for all the data sets. We derived the absorption lines parameters and calculated their detailed detection significance making use of the classical F-test and extensive Monte Carlo simulations.Results. We detect 36 narrow absorption lines on a total of 101 XMM-Newton EPIC pn observations. The number of absorption lines at rest-frame energies higher than 7 keV is 22. Their global probability to be generated by random fluctuations is very low, less than 3 × 10-8 , and their detection have been independently confirmed by a spectral analysis of the MOS data, with associated random probability 10-7 . We identify the lines as Fe XXV and Fe XXVI K-shell resonant absorption. They are systematically blue-shifted, with a velocity distribution ranging from zero up to ~0.3c, with a peak and mean value at ~0.1c. We detect variability of the lines on both EW s and blue-shifted velocities among different XMM-Newton observations even on time-scales as short as a few days, possibly suggesting somewhat compact absorbers. Moreover, we find no significant correlation between the cosmological red-shifts of the sources and the lines blue-shifted velocities, ruling out any systematic contamination by local absorption. If we define ultra-fast outflows (UFOs) those highly ionized absorbers with outflow velocities higher than 104 km s-1 , then the majority of the lines are consistent with being associated to UFOs and the fraction of objects with detected UFOs in the whole sample is at least ~35%. This fraction is similar for type 1 and type 2 sources. The global covering fraction of the absorbers is consequently estimated to be in the range C ∼ 0.4-0.6, thereby implying large opening angles.Conclusions. From our systematic X-ray spectral analysis on a large sample of radio-quiet AGNs we have been able to clearly assess the global veracity of the blue-shifted Fe K absorption lines at E > 7 keV and to overcome their publication bias. These lines indicate that UFOs are a rather common phenomenon observable in the central regions of these sources and they are probably the direct signature of AGN accretion disk winds/ejecta. The detailed photo-ionization modeling of these absorbers is presented in a companion paper.

Structure and valency of a cobalt-phosphate water oxidation catalyst determined by in situ X-ray spectroscopy.

Abstract: A water oxidation catalyst generated via electrodeposition from aqueous solutions containing phosphate and Co2+ (Co−Pi) has been studied by in situ X-ray absorption spectroscopy. Spectra were obtained for Co−Pi films of two different thicknesses at an applied potential supporting water oxidation catalysis and at open circuit. Extended X-ray absorption fine structure (EXAFS) spectra indicate the presence of bis-oxo/hydroxo-bridged Co subunits incorporated into higher nuclearity clusters in Co−Pi. The average cluster nuclearity is greater in a relatively thick film (∼40−50 nmol Co ions/cm2) deposited at 1.25 V vs NHE than in an extremely thin film (∼3 nmol Co ions/cm2) deposited at 1.1 V. X-ray absorption near edge structure (XANES) spectra and electrochemical data support a Co valency greater than 3 for both Co−Pi samples when catalyzing water oxidation at 1.25 V. Upon switching to open circuit, Co−Pi undergoes a continuous reduction due to residual water oxidation catalysis, as indicated by the negative s...

Light absorption by organic carbon from wood combustion

Abstract: . Carbonaceous aerosols affect the radiative balance of the Earth by absorbing and scattering light. While black carbon (BC) is highly absorbing, some organic carbon (OC) also has significant absorption, especially at near-ultraviolet and blue wavelengths. To the extent that OC absorbs visible light, it may be a non-negligible contributor to positive direct aerosol radiative forcing. Quantification of that absorption is necessary so that radiative-transfer models can evaluate the net radiative effect of OC. In this work, we examine absorption by primary OC emitted from solid fuel pyrolysis. We provide absorption spectra of this material, which can be related to the imaginary refractive index. This material has polar character but is not fully water-soluble: more than 92% was extractable by methanol or acetone, compared with 73% for water and 52% for hexane. Water-soluble OC contributes to light absorption at both ultraviolet and visible wavelengths. However, a larger portion of the absorption comes from OC that is extractable only by methanol. Absorption spectra of water-soluble OC are similar to literature reports. We compare spectra for material generated with different wood type, wood size and pyrolysis temperature. Higher wood temperature is the main factor creating OC with higher absorption; changing wood temperature from a devolatilizing state of 210 °C to a near-flaming state of 360 °C causes about a factor of four increase in mass-normalized absorption at visible wavelengths. A clear-sky radiative transfer model suggests that, despite the absorption, both high-temperature and low-temperature OC result in negative top-of-atmosphere radiative forcing over a surface with an albedo of 0.19 and positive radiative forcing over bright surfaces. Unless absorption by real ambient aerosol is higher than that measured here, it probably affects global average clear-sky forcing very little, but could be important in energy balances over bright surfaces.

Plasmon-Enhanced Charge Carrier Generation in Organic Photovoltaic Films Using Silver Nanoprisms

Abstract: We use photoinduced absorption spectroscopy to measure long-lived photogenerated charge carriers in optically thin donor/acceptor conjugated polymer blend films near plasmon-resonant silver nanoprisms. We measure up to 3 times more charge generation, as judged by the magnitude of the polaron absorption signal, in 35 nm thin blend films of poly(3-hexylthiophene)/phenyl-C61-butyric acid methyl ester on top of films of silver nanoprisms (∼40−100 nm edge length). We find that the polaron yields increase linearly with the total sample extinction. These excitation enhancements could in principle be used to increase photocurrents in thin organic solar cells.

Luminescent Ag7 and Ag8 Clusters by Interfacial Synthesis

Abstract: Molecular quantum clusters of noble metals are a fascinating area of contemporary interest in nanomaterials. While Au11, [1] Au13, [2] and Au55 [3] have been known for a few decades, several new clusters were discovered recently. These include Au8, [4] Au18, [5] Au25, [6] Au38, [7] and so on. Au11 has also been the subject of recent research. In view of their luminescence, several of these clusters are expected to be important in biolabeling and fluorescence resonance energy transfer as well as for creating luminescent patterns. There are many examples of template-assisted synthesis of water-soluble luminescent silver clusters with cores ranging from Ag2 to Ag8, having characteristic electronic transitions between 400–600 nm. However, unlike the case of gold, there are only limited examples of monolayer-protected silver analogues. Silver clusters protected with aryl, aliphatic, and chiral thiols have been reported, some of which have characteristic optical and mass spectrometric signatures. There is also a family of well-characterized metal-rich silver chalcogenide clusters. Besides single-crystal diffraction, mass spectrometry has also been used for detailed understanding of these clusters. Ag clusters with and without luminescence have also been reported. Herein we present gram-scale syntheses of two luminescent silver clusters, protected by small molecules containing thiol groups, with well-defined molecular formulas, by interfacial synthesis. This new synthetic approach has become promising in several other areas including semiconductor nanoparticles, two-dimensional superlattices, and 3D structures. A crude mixture of redand blue-green-emitting clusters Ag8(H2MSA)8 and Ag7(H2MSA)7 (H2MSA: mercaptosuccinic acid), respectively, was synthesized in gram quantities by an interfacial etching reaction conducted at an aqueous/ organic interface starting from H2MSA-protected silver nanoparticles (Ag@H2MSA) [19] as precursor (for details see the Experimental Section and Figure S1 in the Supporting Information). During the reaction, the optical absorption spectrum of the aqueous phase showed gradual disappearance of the surface plasmon resonance at 400 nm (Figure 1A) of metallic silver nanoparticles. The color of the aqueous phase gradually changed from brown to yellow and finally to orange. The particles of Ag@MSA are polydisperse (Figure 1Ca) and form smaller clusters in the aqueous phase upon etching (Figure 1Cb) with complete disappearance of the nanoparticles. The unetched particles move to the junction of the two phases and form a self-assembled film of monodisperse nanoparticles, resembling two-dimensional superlattices (Figure 1Cc), which appears blue in color. The smaller clusters formed in the reaction upon longer electron-beam irradiation coalesce to form nanoparticles (Figure S2). It is known that such clusters are unstable to high-energy electrons. The peak at 600 nm, which appears at shorter reaction time (60 min) and may be due to interplasmon coupling, disappears slowly, and a new feature is seen at 550 nm after 48 h of reaction (Figure 1A). In accordance with previous studies on silver clusters, we assign this peak to interband Figure 1. A) Time-dependent UV/Vis spectra of the clusters synthesized during interfacial etching at room temperature. B) UV/Vis absorption spectra of the clusters obtained from the two bands in PAGE. The inset shows a photograph of the wet gel after electrophoresis in UV light at room temperature, and the inset to the inset an image of the first band at 273 K. C) HRTEM images of a) assynthesized Ag@(H2MSA), b) the product obtained after interfacial etching, and c) particles in the blue layer at the interface. Individual clusters are not observable by TEM, but aggregates are seen faintly (b, shown in circles). Insets of (a) and (b) are photographs of Ag@MSA and crude cluster samples. d) Photographs of aqueous of cluster solutions of first (cluster 1) and second (cluster 2) PAGE bands at 273 K and room temperature, respectively. D) Luminescence emission of cluster 1 and cluster 2 in water, excited at 550 and 350 nm, respectively.

Ordered arrays of dual-diameter nanopillars for maximized optical absorption.

Abstract: Optical properties of highly ordered Ge nanopillar arrays are tuned through shape and geometry control to achieve the optimal absorption efficiency. Increasing the Ge materials filling ratio is shown to increase the reflectance while simultaneously decreasing the transmittance, with the absorbance showing a strong diameter dependency. To enhance the broad band optical absorption efficiency, a novel dual-diameter nanopillar structure is presented, with a small diameter tip for minimal reflectance and a large diameter base for maximal effective absorption coefficient. The enabled single-crystalline absorber material with a thickness of only 2 μm exhibits an impressive absorbance of ∼99% over wavelengths, λ = 300−900 nm. These results enable a viable and convenient route toward shape-controlled nanopillar-based high-performance photonic devices.

Electronic structure of few-layer graphene: experimental demonstration of strong dependence on stacking sequence.

Abstract: The electronic structure of few-layer graphene (FLG) samples with crystalline order was investigated experimentally by infrared absorption spectroscopy for photon energies ranging from 0.2-1 eV. Distinct optical conductivity spectra were observed for different samples having precisely the same number of layers. The different spectra arise from the existence of two stable polytypes of FLG, namely, Bernal (AB) stacking and rhombohedral (ABC) stacking. The observed absorption features, reflecting the underlying symmetry of the two polytypes and the nature of the associated van Hone singularities, were reproduced by explicit calculations within a tight-binding model. The findings demonstrate the pronounced effect of stacking order on the electronic structure of FLG.

Coherent dual-comb spectroscopy at high signal-to-noise ratio

Abstract: Two coherent frequency combs are used to measure the full complex response of a sample in a configuration analogous to a dispersive Fourier transform spectrometer, infrared time domain spectrometer, or a multiheterodyne laser spectrometer. This dual-comb spectrometer retains the frequency accuracy and resolution of the reference underlying the stabilized combs. We discuss the specific design of our coherent dual-comb spectrometer and demonstrate the potential of this technique by measuring the overtone vibration of hydrogen cyanide, centered at 194 THz (1545 nm). We measure the fully normalized, complex response of the gas over a 9 THz bandwidth at 220 MHz frequency resolution yielding 41,000 resolution elements. The average spectral signal-to-noise ratio (SNR) over the 9 THz bandwidth is 2500 for both the magnitude and phase of the measured spectral response and the peak SNR is 4000. This peak SNR corresponds to a fractional absorption sensitivity of 0.05% and a phase sensitivity of 250 microradians. As the spectral coverage of combs expands, coherent dual-comb spectroscopy could provide high-frequency accuracy and resolution measurements of a complex sample response across a range of spectral regions. Work of U. S. government, not subject to copyright.

Complex interactions of pillar[5]arene with paraquats and bis(pyridinium) derivatives

Abstract: The complexation behavior of a series of paraquats (G1·2PF6–G5·2PF6) and bis(pyridinium) derivatives (G6·2PF6–G14·2PF6) with pillar[5]arene (P5A) host has been comprehensively investigated by 1H NMR, ESI mass and UV-vis absorption spectroscopy. It is found that P5A forms 2 : 1 external complexes with N,N′-dialkyl-4,4′-bipyridiniums (G1–G4·2PF6); while it forms 1 : 1 pseudorotaxane-type inclusion complexes with methylene [–(CH2)n–] linked bis(pyridinium) derivatives possessing appropriate chain lengths (n = 3–6, G7–G10·2PF6). Host–guest association constants in dimethyl sulfoxide (DMSO) were determined, indicating G7–G10·2PF6 axles form stable [2]pseudorotaxanes with P5A wheel in this very high polarity solvent and 1,4-bis(pyridinium)butane (G8·2PF6) was the most suitable axle unit. Meanwhile, the nature of the substituents attached to 1,4-bis(pyridinium)butane dramatically affects the molecular recognition behavior. The introduction of pyridyls (G13·2PF6) increases not only the Ka value (4.5 × 102→7.4 × 102 M−1), but also the charge transfer (CT) absorption (colorless→yellow). Furthermore, the solvent effects have also been investigated, showing they significantly influence the association strength during the course of host–guest complexation. Particularly, the Ka value of P5A–G13·2PF6 in 1 : 1 (v:v) acetone-d6/DMSO-d6 is enhanced by a factor of 7.3 compared with pure DMSO-d6 (7.4 × 102→5.4 × 103 M−1).

Synthesis and characterization of porphyrin sensitizers with various electron-donating substituents for highly efficient dye-sensitized solar cells

Abstract: A series of porphyrin dyes with an electron-donating group (EDG) attached at a meso-position (YD1–YD8) have been designed and synthesized for use as sensitizers in dye-sensitized solar cells (DSSC). The nature of the EDG exerts a significant influence on the spectral, electrochemical and photovoltaic properties of these sensitizers. Absorption spectra of porphyrins having an amino group show broadened Soret band and red-shifted Q bands with respect to those of reference porphyrin YD0. This phenomenon is more pronounced for porphyrins YD7 and YD8 that have a π-conjugated triphenylamine at the meso-position opposite the anchoring group. Upon introduction of an EDG at the meso-position, the potential for the first oxidation alters significantly to the negative whereas that for the first reduction changes inappreciably, indicating a decreased HOMO-LUMO gap. Results of density-functional theory (DFT) calculations support the spectroelectrochemical data for a delocalization of charge between the porphyrin ring and the amino group in the first oxidative state of diarylamino-substituted porphyrins YD1–YD4, which exhibit superior photovoltaic performance among all porphyrins under investigation. With long-chain alkyl groups on the diarylamino substituent, YD2 shows the best cell performance with JSC = 13.4 mA cm−2, VOC = 0.71 V, and FF = 0.69, giving an overall efficiency 6.6% of power conversion under simulated one-sun AM1.5 illumination.

Mid-infrared Fourier transform spectroscopy with a broadband frequency comb.

Abstract: We present a first implementation of optical-frequency-comb-based rapid trace gas detection in the molecular fingerprint region in the mid-infrared Near-real-time acquisition of broadband absorption spectra with 00056 cm(-1) maximum resolution is demonstrated using a frequency comb Fourier transform spectrometer which operates in the 2100-to-3700-cm(-1) spectral region We achieve part-per-billion detection limits in 30 seconds of integration time for several important molecules including methane, ethane, isoprene, and nitrous oxide Our system enables precise concentration measurements even in gas mixtures that exhibit continuous absorption bands, and it allows detection of molecules at levels below the noise floor via simultaneous analysis of multiple spectral features

Synthesis and characterization of tunable rainbow colored colloidal silver nanoparticles using single-nanoparticle plasmonic microscopy and spectroscopy

Abstract: Noble metal nanoparticles (NPs) possess size- and shape-dependent optical properties, suggesting the possibility of tuning desired optical properties of ensemble NPs at single NP resolution and underscoring the importance of probing the sizes and shapes of single NPs in situ and in real-time. In this study, we synthesized twelve colloids of Ag NPs. Each colloid contains various sizes and shapes of single NPs, showing rainbow colors with peak-wavelength of absorption spectra from 393 to 738 nm. We correlated the sizes and shapes of single NPs determined by high-resolution transmission electron microscopy (HRTEM) with scattering localized surface plasmon resonance (LSPR) spectra of single NPs characterized by dark-field optical microcopy and spectroscopy (DFOMS). Single spherical (2–39 nm in diameter), rod (2–47 nm in length with aspect ratios of 1.3–1.6), and triangular (4–84 nm in length with thickness of 2–27 nm) NPs show LSPR spectra (λmax) at 476 ± 5 or 533 ± 12, 611 ± 23, and 711 ± 40 nm, respectively. Notably, we observed new cookie-shaped NPs, which exhibit LSPR spectra (λmax) at 725 ± 10 nm with a shoulder peak at 604 ± 5 nm. Linear correlations of sizes of any given shape of single NPs with their LSPR spectra (λmax) enable the creation of nano optical rulers (calibration curves) for identification of the sizes and shapes of single NPs in solution in real time using DFOMS, offering the feasibility of using single NPs as multicolored optical probes for study of dynamics events of interest in solutions and living organisms at nm scale in real time.

Precursor Conversion Kinetics and the Nucleation of Cadmium Selenide Nanocrystals

Abstract: The kinetics of cadmium selenide (CdSe) nanocrystal formation was studied using UV−visible absorption spectroscopy integrated with an automated, high-throughput synthesis platform. Reaction of anhydrous cadmium octadecylphosphonate (Cd-ODPA) with alkylphosphine selenides (1, tri-n-octylphosphine selenide; 2, di-n-butylphenylphosphine selenide; 3, n-butyldiphenylphosphine selenide) in recrystallized tri-n-octylphosphine oxide was monitored by following the absorbance of CdSe at λ = 350 nm, where the extinction coefficient is independent of size, and the disappearance of the selenium precursor using {1H}31P NMR spectroscopy. Our results indicate that precursor conversion limits the rate of nanocrystal nucleation and growth. The initial precursor conversion rate (Qo) depends linearly on [1] (Qo(1) = 3.0−36 μM/s) and decreases as the number of aryl groups bound to phosphorus increases (1 > 2 > 3). Changes to Qo influence the final number of nanocrystals and thus control particle size. Using similar methods, w...

X-ray Absorption Spectroscopy

Abstract: This review gives a brief description of the theory and application of X-ray absorption spectroscopy, both X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), especially, pertaining to photosynthesis. The advantages and limitations of the methods are discussed. Recent advances in extended EXAFS and polarized EXAFS using oriented membranes and single crystals are explained. Developments in theory in understanding the XANES spectra are described. The application of X-ray absorption spectroscopy to the study of the Mn4Ca cluster in Photosystem II is presented.

Advanced X-ray absorption and emission spectroscopy: in situ catalytic studies

Abstract: Knowledge of the structure of catalysts is essential to understand their behavior, which further facilitates development of an active, selective, and stable catalyst. Determining the structure of a functioning catalyst is essential in this regard. The structure of a catalyst is prone to change during the catalytic process and needs to be determined in its working conditions. In this tutorial review, we have summarized studies done at synchrotron radiation facilities that illustrate the capability to determine catalyst structure using X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES). These studies aim at facilitating the determination of the dynamic structure-performance relationships during a catalytic process.

Comments on the Paper "Detection of the Absorption Spectra of Water Clusters under Atmospheric Conditions" by V. I. Serdyukov, L. N. Sinitsa, and Yu. A. Poplavskii, Pis'ma Zh. Eksp. Teor. Fiz. 89, 12 (2009) (JETP Lett. 89, 10 (2009))

Abstract: A numerical simulation has been performed, indicating that the statement on the detection of water dimer absorption by V.I. Serdyukov et al. in Pis’ma Zh. Eksp. Teor. Fiz. 89, 12 (2009) [JETP Lett. 89, 10 (2009)] had been premature. Most of the revealed induced absorption can be attributed to water monomers.

Virus-templated assembly of porphyrins into light-harvesting nanoantennae.

Abstract: Biological molecules can be used as versatile templates for assembling nanoscale materials because of their unique structures and chemical diversities. Supramolecular organization of molecular pigments, as is found in the natural light-harvesting antenna, has drawn attention for its potential applications to sensors, photocatalytic systems, and photonic devices. Here we show the arrangement of molecular pigments into a one-dimensional light-harvesting antenna using M13 viruses as scaffolds. Chemical grafting of zinc porphyrins to M13 viruses induces distinctive spectroscopic changes, including fluorescence quenching, the extensive band broadening and small red shift of their absorption spectrum, and the shortened lifetime of the excited states. Based on these optical signatures we suggest a hypothetical model to explain the energy transfer occurring in the supramolecular porphyrin structures templated with the virus. We expect that further genetic engineering of M13 viruses can allow us to coassemble othe...

Interstellar OH+, H2O+ and H3O+ along the sight-line to G10.6–0.4

Abstract: We report the detection of absorption lines by the reactive ions OH + ,H 2O + and H3O + along the line of sight to the submillimeter continuum source G10.6−0.4 (W31C). We used the Herschel HIFI instrument in dual beam switch mode to observe the ground state rotational transitions of OH + at 971 GHz, H2O + at 1115 and 607 GHz, and H3O + at 984 GHz. The resultant spectra show deep absorption over a broad velocity range that originates in the interstellar matter along the line of sight to G10.6−0.4 as well as in the molecular gas directly associated with that source. The OH + spectrum reaches saturation over most velocities corresponding to the foreground gas, while the opacity of the H2O + lines remains lower than 1 in the same velocity range, and the H3O + line shows only weak absorption. For LSR velocities between 7 and 50 kms −1 we estimate total column densities of N(OH + ) ≥ 2.5 × 10 14 cm −2 , N(H2O + ) ∼6 × 10 13 cm −2 and N(H3O + ) ∼4.0 × 10 13 cm −2 . These detections confirm the role of O + and OH + in initiating the oxygen chemistry in diffuse molecular gas and strengthen our understanding of the gas phase production of water. The high ratio of the OH + by the H2O + column density implies that these species predominantly trace low-density gas with a small fraction of

Herschel/HIFI observations of interstellar OH+ and H2O+ towards W49N: a probe of diffuse clouds with a small molecular fraction

Abstract: We report the detection of absorption by interstellar hydroxyl cations and water cations, along the sight-line to the bright continuum source W49N. We have used Herschel's HIFI instrument, in dual beam switch mode, to observe the 972 GHz N = 1-0 transition of OH+ and the 1115 GHz 1(11)-0(00) transition of ortho-H2O+. The resultant spectra show absorption by ortho-H2O+, and strong absorption by OH+, in foreground material at velocities in the range 0 to 70 km s(-1) with respect to the local standard of rest. The inferred OH+/H2O+ abundance ratio ranges from similar to 3 to similar to 15, implying that the observed OH+ arises in clouds of small molecular fraction, in the 2-8% range. This conclusion is confirmed by the distribution of OH+ and H2O+ in Doppler velocity space, which is similar to that of atomic hydrogen, as observed by means of 21 cm absorption measurements, and dissimilar from that typical of other molecular tracers. The observed OH+/H abundance ratio of a few x10(-8) suggests a cosmic ray ionization rate for atomic hydrogen of 0.6-2.4 x 10(-16) s(-1), in good agreement with estimates inferred previously for diffuse clouds in the Galactic disk from observations of interstellar H-3(+) and other species.

Synthesis and Mechanistic Studies of Organic Chromophores with Different Energy Levels for p-Type Dye-Sensitized Solar Cells

Abstract: A series of donor-pi-acceptor dyes with different electron-withdrawing groups were designed and synthesized for p-type dye-sensitized Solar cells. The modification of dye structures shows significant influence on the photophysical, electrochemical, and photovoltaic performance of the dyes. DSSCs based on these dyes show maximum 63% and minimum 6% of incident monochromatic photon-to-current conversion efficiencies. The two dyes with the highest (P1) and lowest (P3) efficiencies were Studied by femtosecond transient absorption spectroscopy, which shows a fast injection rate of more than (250 fs)(-1) for both dyes. Such fast injection corresponds to more than 90% injection efficiency. The photoinduced absorption Spectroscopy Study of sensitized NiO films in the presence of electrolyte showed poor regeneration of 113 due to all insufficient driving force. This, together with aggregation of the dye on the NiO film, explained the poor solar cell performance.

Mid-infrared dual-comb spectroscopy with 2.4 μm Cr2+:ZnSe femtosecond lasers

Abstract: The mid-infrared part of the electromagnetic spectrum is the so-called molecular fingerprint region because gases have tell-tale absorption features associated with molecular rovibrations. This region can be for instance exploited to detect small traces of environmental and toxic vapors in atmospheric and industrial applications. Novel Fourier-transform spectroscopy without moving parts, based on time-domain interferences between two comb sources, can in particular benefit optical diagnostics and precision spectroscopy. To date, high-resolution and -sensitivity proof-of-principle experiments have only been reported in the near-infrared region where frequency-comb oscillators are conveniently available. However, as most of the molecular transitions in this region are due to weak overtone bands, this spectral domain is not ideal for sensitive and rapid detection. Here we present a proof-of-principle experiment of frequency-comb Fourier-transform spectroscopy with two Cr2+:ZnSe femtosecond oscillators directly emitting in the 2.4 μm mid-infrared region. The acetylene absorption spectrum in the region of the $ u_{1}+ u_{5}^{1}$ band, extending from 2370 to 2525 nm, could be recorded within a 10 μs acquisition time without averaging with 12 GHz resolution.

Stark Effects after Excited-State Interfacial Electron Transfer at Sensitized TiO2 Nanocrystallites

Abstract: Photophysical studies were performed with [Ru(dtb)(2)(dcb)](PF(6))(2) and cis-Ru(dcb)(dnb)(NCS)(2,) where dtb is 4,4'-(C(CH(3))(3))(2)-2,2'-bipyridine, dcb is 4,4'-(COOH)(2)-2,2'-bipyridine, and dnb is 4,4'-(CH(3)(CH(2))(8))(2)-2,2'-bipyridine), anchored to anatase TiO(2) particles ( approximately 15 nm in diameter) interconnected in a mesoporous, 10 mum thick film immersed in Li(+)-containing CH(3)CN electrolytes with iodide or phenothiazine donors. Pulsed-laser excitation resulted in rapid excited-state injection and donor oxidation to yield TiO(2)(e(-))s and oxidized donors, while the metal-to-ligand charge-transfer (MLCT) absorption spectrum of the Ru(II) coordination compounds differed from that which was initially excited. The spectral data were consistent with an underlying Stark effect and indicated that the surface electric field was not completely screened from the molecular sensitizer. The magnitude of the electric field was estimated to be approximately 270 MV/m from Li(+) titration experiments, corresponding to a approximately 40 mV potential drop. With iodide donors, the amplitude of the Stark effect decreased over time periods where charge recombination was absent, behavior attributed to "screening" of the electric field by interfacial ionic reorganization. The screening kinetics were nonexponential but were well described by the Kohlrausch-Williams-Watts model, from which a characteristic rate constant, tau(o)(-1), of approximately 1.5 x 10(5) s(-1) was abstracted. At least seven other sensitizers and five different cations, as well as on SnO(2) nanoparticle films, exhibited similar transient absorption behavior with iodide donor molecules indicating that the effect was quite general. In the presence of phenothiazine donors (or in the absence of an external donor), there was no clear evidence for screening, and the Stark effect disappeared concurrent with interfacial charge recombination. Complementary spectroelectrochemical studies of these same sensitized films displayed similar absorption spectra when the TiO(2) thin film was partially reduced with a forward bias. Spectral modeling in the absence of donor molecules as well as studies of TiO(2) thin films sensitized with two different Ru(II) compounds demonstrated that the electric field created by excited-state injection from one sensitizer influenced the absorption spectra of other sensitizers that had not undergone photoinduced electron injection.