Wavelength dependence of the Raman cross section for liquid water

We report measurements of the wavelength dependence of the 3400-cm(-1) Raman scattering cross section of liquid water for excitation wavelengths between 215 and 550 nm. Using previous measurements of the absolute Raman scattering cross section, we have determined an expression for the wavelength dependence of the absolute Raman cross section for water.     

Rotational vibrational-rotational Raman differential absorption lidar for atmospheric ozone measurements: methodology and experiment

A single-laser Raman differential absorption lidar (DIAL) for ozone measurements in clouds is proposed. An injection-locked XeCl excimer laser serves as the radiation source. The ozone molecule number density is calculated from the differential absorption of the anti-Stokes rotational Raman return signals from molecular nitrogen and oxygen as the on-resonance wavelength and the vibrational-rotational Raman backscattering from molecular nitrogen or oxygen as the off-resonance wavelength. Model calculations show that the main advantage of the new rotational vibrational-rotational (RVR) Raman DIAL over conventional Raman DIAL is a 70-85% reduction in the wavelength-dependent effects of cloud-particle scattering on the measured ozone concentration; furthermore the complexity of the apparatus is reduced substantially. We describe a RVR Raman DIAL setup that uses a narrow-band interference-filter polychromator as the lidar receiver. Single-laser ozone measurements in the troposphere and lower stratosphere are presented, and it is shown that on further improvement of the receiver performance, ozone measurements in clouds are attainable with the filter-polychromator approach.     

Measurements of the yield and chlorine utilization of singlet oxygen generator by use of Raman spectroscopy

A Raman scattering system was used to measure the O2 (a1delta) yield end the chlorine utilization in a singlet oxygen generator of chemical oxygen iodine laser with nitrogen diluent. We present the results from the tests that conducted on a 0.1-mol/s jet-type singlet oxygen-iodine generator. On the basis of the current reported uncertainty of the Raman cross section, the error in the yield measurement is calculated to be less than 8%, and the error of the chlorine utilization is 12%.     

Ozone and water-vapor measurements by Raman lidar in the planetary boundary layer: error sources and field measurements

A new lidar instrument has been developed to measure tropospheric ozone and water vapor at low altitude. The lidar uses Raman scattering of an UV beam from atmospheric nitrogen, oxygen, and water vapor to retrieve ozone and water-vapor vertical profiles. By numerical simulation we investigate the sensitivity of the method to both atmospheric and device perturbations. The aerosol optical effect in the planetary boundary layer, ozone interference in water-vapor retrieval, statistical error, optical cross talk between Raman-shifted channels, and optical cross talk between an elastically backscattered signal in Raman-shifted signals and an afterpulse effect are studied in detail. In support of the main conclusions of this model study, time series of ozone and water vapor obtained at the Swiss Federal Institute of Technology in Lausanne and during a field campaign in Crete are presented. They are compared with point monitor and balloon sounding measurements for daytime and nighttime conditions.     

Monitoring O3 with solar-blind Raman lidars

The benefits of retrieving ozone concentration profiles by a use of a single Raman signal rather than the Raman differential absorption lidar (DIAL) technique are investigated by numerical simulations applied either to KrF- (248 nm) or to quadrupled Nd:YAG- (266 nm) based Raman lidars, which are used for both daytime and nighttime monitoring of the tropospheric water-vapor mixing ratio. It is demonstrated that ozone concentration profiles of adequate accuracy and spatial and temporal resolution can be retrieved under low aerosol loading by a single Raman lidar because of the large value of the ozone absorption cross section both at 248 nm and at 266 nm. Then experimental measurements of Raman signals provided by the KrF-based lidar operating at the University of Lecce (40 degrees 20'N, 18 degrees 6'E) are used to retrieve ozone concentration profiles by use of the Raman DIAL technique and the nitrogen Raman signal.     

Dual-pump coherent anti-Stokes Raman scattering measurements of nitrogen and oxygen in a laminar jet diffusion flame

Dual-pump coherent anti-Stokes Raman scattering (CARS) has been demonstrated for the simultaneous measurement of gas-phase temperature and concentrations of molecular nitrogen and oxygen. A polarization technique was used to vary the relative intensities of the two CARS signals and expand the dynamic range of the relative concentration measurements. Detailed temperature and oxygen mole fraction measurements were performed in the stabilization region of a hydrogen-nitrogen jet diffusion flame. These results indicate that there is a region below the nozzle exit where significant amounts of oxygen are found on the fuel side of the peak flame temperature profile.     

In Situ Method Correlating Raman Vibrational Characteristics to Chemical Expansion via Oxygen Nonstoichiometry of Perovskite Thin Films

Effective integration of perovskite films into devices requires knowledge of their electro‐chemomechanical properties. Raman spectroscopy is an excellent tool for probing such properties as the films' vibrational characteristics couple to the lattice volumetric changes during chemical expansion. While lattice volumetric changes are typically accessed by analyzing Raman shifts as a function of pressure, stress, or temperature, such methods can be impractical for thin films and do not capture information on chemical expansion. An in situ Raman spectroscopy technique using an electrochemical titration cell to change the oxygen nonstoichiometry of a model perovskite film, Sr(Ti,Fe)O_3?y , is reported and the lattice vibrational properties are correlated to the material's chemical expansion. How to select an appropriate Raman vibrational mode to track the evolution in oxygen nonstoichiometry is discussed. Subsequently, the frequency of the oxygen stretching mode around Fe⁴⁺ is tracked, as it decreases during reduction as the material expands and increases during reoxidation as the material shrinks. This methodology of oxygen pumping and in situ Raman spectroscopy of oxide films enables future in operando measurements even for small material volumes, as is typical for applications of films as electrodes or electrolytes utilized in electrochemical energy conversion or memory devices.     

Simultaneous temperature and relative nitrogen-oxygen concentration measurements in air with pure rotational coherent anti-Stokes Raman scattering for temperatures to as high as 2050 K

The accuracy of temperature and simultaneous relative N(2) -O(2) concentration measurements of accumulated as well as of single-pulse rotational coherent anti-Stokes Raman spectra has been investigated in air in the temperature range from 300 to 2050 K. The experimental spectra were taken in a high-temperature oven at atmospheric pressure for a constant oxygen concentration of 20.9% (air). The evaluation procedure is based on the energy-corrected sudden-power scaling law. The agreement of the thermocouple readings with the mean values of the evaluated coherent anti-Stokes Raman spectroscopy temperatures is higher than 50 K and independent of the temperature. The evaluated oxygen concentration is found to be in the range from 20.0 to 21.7% and is also independent of the temperature.     

Highly Water Soluble C60 Derivatives: A New Synthesis

Unprecedented protocol, consisting in reduction of Buckminsterfullerene with Na/K alloy and successive stirring in presence of O₂, affords polyhydroxylated fullerenols which are highly water soluble, from strongly basic to medium acidic conditions. The products have been characterized by means of cross polarization magic angle spinning (CPMAS) NMR, ¹H-NMR, electron, FT-IR, Raman and mass spectroscopies. Possible paths of reaction and the role of molecular oxygen in fullerene cage-oxidation are discussed.     

Bandgap opening in oxygen plasma-treated graphene

We report a change in the semimetallic nature of single-layer graphene after exposure to oxygen plasma. The resulting transition from semimetallic to semiconducting behavior appears to depend on the duration of the exposure to the plasma treatment. The observation is confirmed by electrical, photoluminescence and Raman spectroscopy measurements. We explain the opening of a bandgap in graphene in terms of functionalization of its pristine lattice with oxygen atoms. Ab initio calculations show more details about the interaction between carbon and oxygen atoms and the consequences on the optoelectronic properties, that is, on the extent of the bandgap opening upon increased functionalisation density.     

Lamp mapping technique for independent determination of the water vapor mixing ratio calibration factor for a Raman lidar system

We have investigated a technique that allows for the independent determination of the water vapor mixing ratio calibration factor for a Raman lidar system. This technique utilizes a procedure whereby a light source of known spectral characteristics is scanned across the aperture of the lidar system's telescope and the overall optical efficiency of the system is determined. Direct analysis of the temperature-dependent differential scattering cross sections for vibration and vibration-rotation transitions (convolved with narrowband filters) along with the measured efficiency of the system, leads to a theoretical determination of the water vapor mixing ratio calibration factor. A calibration factor was also obtained experimentally from lidar measurements and radiosonde data. A comparison of the theoretical and experimentally determined values agrees within 5%. We report on the sensitivity of the water vapor mixing ratio calibration factor to uncertainties in parameters that characterize the narrowband transmission filters, the temperature-dependent differential scattering cross section, and the variability of the system efficiency ratios as the lamp is scanned across the aperture of the telescope used in the Howard University Raman Lidar system.     

Resonant Raman-scattering measurements of trichloroethene in a thermal boundary layer

Concentration profiles of trichloroethene were measured in a boundary-layer flow over a heated ceramic surface. Raman scattering was excited with the fifth harmonic of a Nd:YAG laser at 213 nm. This wavelength took advantage of a resonance in the trichloroethene molecule to significantly enhance the C2HCl3 scattering cross section. The resonant Raman system was calibrated in a heated flow. The optical system was optimized so that measurements could be obtained close to the solid surface, normally a significant challenge for a spontaneous Raman-scattering setup. Measured concentrations indicated the lack of catalytic activity on a bare alumina surface. However, the results showed that a surface that was coated with Cr2O3-based zeolite was catalytically active.     

High temperature Raman spectroscopy of silver tetratungstate,Ag8W4O16

Polarized Raman spectra have been obtained on oriented single crystals of Ag₈W₄O₁₆ from 20°C to the melting point (620°C) and also of the melt, both in air and in an oxygen atmosphere. The temperature dependences of the low-frequency Ag⁺-ion vibrations show no anomalies, in contrast to those in ionic conductors such as α-AgI. Polarization versus temperature measurements have been made on the crystals under the same conditions as the Raman measurements. The results indicate that there is decomposition of the crystals in air which produces irreversible conducting phases. The oxygen atmosphere impedes this decomposition, and the results in this case confirm an earlier prediction that there is no crystallographic transformation of the crystals up to the melting point and that the (W₄O₁₆)^8? ions continue to be noncentrosymmetric in the melt.     

石墨表层对四面体非晶炭膜中受激电子的石墨建序化作用

对渗气阴极真空电弧法制备的四而体非晶炭(ta-c)膜实施氧等离子体刻蚀,消除其表面石墨层后,发现:原沉积膜中ta-C石墨表层的消除会影响其受激电子的石墨建序化.应用发射电子能耗谱,表面增强拉曼光谱和表面敏化X光吸收光谱等测量方法,测定了其表层的淌除(程度).样品的氧等离子体刻蚀阻迟了受激电子的石墨化作用,可能归因于多相成核过程中石墨晶核的缺失之故.     

A Raman spectroscopic study of structural evolution of electrochemically deposited ZnO films with deposition time

Zinc oxide thin films were fabricated on ITO substrates by electrodeposition method. The electrolyte used was a 0.2 M zinc nitrate aqueous solution. The substrates were maintained at room temperature and the deposition performed for different times between 10 and 30 min. X-ray diffraction measurements indicated the formation of polycrystalline ZnO film with hexagonal wurtzite structure. The structure and crystallinity of the films was also confirmed by Raman spectroscopy. Further, the degree of disorder was estimated both from the phonon correlation length calculated from the Raman spectra using the spatial correlation model and from the intensity ratios of the phonons. The variation with deposition time followed the same trend as the crystallite sizes obtained from X-ray diffraction. X-ray photoelectron spectroscopy measurements indicated oxygen deficiency in the films. A combination of annealing and optimum deposition time improves the quality of the electrodeposited ZnO films.     

Validation of a novel ultraviolet lidar system with relative Raman-scattering cross sections determined from atmospheric measurements

We have developed an ultraviolet lidar system in which the upwelled laser beam and the telescope field of view can be made to overlap at any specified location in space. We refer to this system as the Selected Overlap Lidar Experiment. We discuss validation of our system by calculating relative Raman-scattering cross sections (with respect to the nitrogen scattering cross section) for oxygen and water vapor using data collected during field operations of our lidar. Our relative cross sections are consistent with those obtained by other researchers making similar measurements in laboratory environments.     

Synthesis and optical properties of CeO<Subscript>2</Subscript> nanocrystalline films grown by pulsed electron beam deposition

The nanocrystalline cerium dioxide (CeO₂) thin films were deposited on soda lime (SLG) and Corning glass by pulsed e-beam deposition (PED) method at room temperature. The structure of the produced CeO₂ thin films was investigated by X-ray diffraction (XRD) analysis, X-ray photoelectron spectroscopy (XPS), and micro Raman spectroscopy. The surface topography of the films was examined by atomic force microscopy (AFM). Film thickness and growth morphologies were determined with FEG-SEM from the fracture cross sections. XPS studies gave a film composition composed of +4 and +3 valent cerium typical to nanocrystalline ceria structures deficient in oxygen. The ceria films were polycrystalline in nature with a lattice parameter (a) of 0.542 nm. The Raman characteristics of the source material and the films deposited were very similar in character. Raman lines for thin film and bulk CeO₂ was observed at 465 cm⁻¹. The optical properties of the CeO₂ films were deduced from reflectance and transmittance measurements at room temperature. From the optical model, the refractive index was determined as 1.8–2.7 in the photon energy interval from 3.5 to 1.25 eV. The optical indirect band gap (E _g) of CeO₂ nanocrystalline films was calculated as 2.58 eV.     

Application of the Raman scattering coefficient of water to calculations in marine optics

The relation between the total Raman scattering coefficient of water (b(r)) and the Raman scattering cross section of liquid water is clarified. Consideration of the experimental configurations used to derive the Raman scattering cross section of water relative to benzene leads to a revised value of b(r) for water. In certain applications in marine optics it is necessary to scale b(r) to effect a change from implicit units of energy to quanta.     

Methodology for the independent calibration of Raman backscatter water-vapor lidar systems

We present a method for the independent calibration of Raman backscatter water-vapor lidar systems. Particular attention is given to the resolution of instrumental changes in the short and the long terms. The method reposes on the decomposition of the instrument function, which allows the lidar calibration coefficient to be re-expressed as the product of two terms, one describing the instrumental transmission and detection efficiency and the other describing the wavelength-dependent convolution of the Raman backscatter cross sections with the instrument function. The origins of changes in instrument response necessitate the experimental determination of the system detection efficiency. Two external light sources for calibration are assessed: zenith observation of diffuse sunlight and a xenon arc lamp. The results favor use of the diffuse-sunlight measurement but highlight the need for simultaneous sunphotometer measurements to constrain modeled aerosol optical properties. Quantum mechanical models of the Raman cross sections are described, and errors in determining the cross sections and their convolution with the instrument function are discussed in detail. The calibration coefficients deduced by using the independent method are compared with coefficients deduced from Vaisala H-Humicap radiosonde measurements. These results agree to within current calibration errors (15%, unconstrained aerosol parameters), and a change in calibration coefficient following instrument modification is reproduced satisfactorily. Results from modeling and intercomparison studies are extended to estimate the calibration accuracy and the precision of the diffuse-sunlight method with constrained modeled aerosol parameters. Changes in the calibration coefficient in the short and the long terms should be resolved to 4(6)% and 6(9)%, respectively, which is comparable or better than the precision of existing dependent methods of calibration. The reduction of the absolute calibration error remains an outstanding issue for all calibration methods.     

Investigation of spatially resolved light absorption in a spark-ignition engine fueled with propane/air

UV absorption in the combustion phase of spark-ignition engines strongly influences laser-induced-fluorescence measurements and flame-emission techniques because of the attenuation of a laser and/or signal light. This absorption was assessed with spatial, spectral, and temporal resolutions in an optically accessible research engine. Absorption was measured along a line for different crank-angle positions throughout the combustion phase of the engine by use of spectrally resolved transmittance measurements of both broadband illumination from a deuterium lamp and emission of laser-excited hot oxygen. Evaluating the spatial patterns of absorptivity revealed that no increased absorption can be attributed to the flame-front region and that homogeneous absorption cross sections for the whole burned-gas region can be assumed. The temporal change of absorption was shown to depend on the pressure effect with only negligible changes in absorption cross sections. Results obtained from the absorption measurements are applied for spatially resolved corrections of two-dimensional laser-induced-fluorescence measurements of NO concentration fields obtained under different operating conditions.