Relative Scattering Cross Sections Research Articles

Long-Term Continuous Gas Diagnostics For Fuel Cell Systems: Development Of An Automatized Raman Metrology

Knowledge of the composition of gas flows in fuel cell systems is crucial for their understanding. In current research, this data is rarely recorded inline and in real-time due to the lack of suitable metrology. Here, we present an optical gas sensor. Green light (532 nm) from a 450 mW laser excites Raman scattering. A spectrometer analyzes the scattered light and a Python software extracts gas composition data automatized and in real-time. We designed a measurement cell that allows optical inline access to gas flow systems and is easy to implement. Therefore, common signal enhancement methods with more complex setups can not be applied. Literature data on scattering cross sections does not offer a reliable basis for calibration what constitutes the need for a gas conditioning system that allows fast preparation of well defined mixtures. We used it to present two principles of calibration. First, calibration based on relative scattering cross sections is the more stable possibility, because it is still valid in cases where laser power and integration time are changed. However, in consequence the results for one species are not independent from the other species. A poor signal, e.g. following from a low mole fraction in a single component, will introduce uncertainty in all components. We avoid that problem through direct linear correlation of signal strength of one species to its partial pressure, which is the second calibration variant. In this case, there is an influence of laser power and integration time. The additional information on overall pressure can be used in model systems, where the presence of undetected components can be ruled out. We can compare that information to pressure measurements to do plausibility checks on the Raman results in real-time. We apply the sensor in fuel cell system research, where measurement intervals of one second yield results of sufficient precision. Increasing integration time can further decrease the uncertainties where favorable. The measurement and data evaluation routines work stable and constant enough to monitor gas compositions in longterm operation. To date, the sensor can measure mixtures of nitrogen, oxygen, hydrogen and water vapor.

Confocal Raman Microscopy for Investigating Synthesis and Characterization of Individual Optically Trapped Vinyl-Polymerized Surfactant Particles

Small polymeric particles are increasingly employed as adsorbent materials, as molecular carriers, as delivery vehicles, and in preconcentration applications. The rational development of these materials requires in situ methods of analysis to characterize their synthesis, structure, and applications. Optical-trapping confocal Raman microscopy is a spectroscopic method capable of acquiring information at several stages of the development of such dispersed particulate materials. In the present study, an example material is developed and tested using confocal Raman microscopy for characterization at each stage of the process. Specifically, the method is used to investigate the synthesis, structure, and applications of individual polymeric surfactant particles produced by the vinyl polymerization of sodium 11-acrylamidoundecanoate (SAAU). The kinetics of polymerization can be monitored over time by measuring the loss of the acrylamide C=C functional groups using confocal Raman microscopy of particles optically trapped by the excitation laser, where, within the limits of detecting the vinyl functional group, the complete polymerization of the SAAU monomer was achieved. The polymerized SAAU particles are spherical, and they exhibit uniform access to water throughout their structure, as tested by the penetration of heavy water (D2O) and collection of spatially resolved Raman spectra from the interior of the particle. These porous particles contain hydrophobic domains that can be used to accumulate molecules for adsorption or carrier applications. This property was tested by using confocal Raman microscopy to measure the accumulation equilibria and kinetics of a model compound, dioxybenzone. The partitioning of this compound into the polymer surfactant could be determined on a quantitative basis using relative scattering cross sections of the SAAU monomer and the adsorbate. The study points out the utility of optical-trapping confocal Raman microscopy for investigating the synthesis, structure, and potential carrier applications of polymeric particle materials.

Scattering of Stark-decelerated OH radicals with rare-gas atoms

We present a combined experimental and theoretical study on the rotationally inelastic scattering of OH (X2Π3/2, J = 3/2, f) radicals with the collision partners He, Ne, Ar, Kr, Xe, and D2 as a function of the collision energy between ∼70 cm−1 and 400 cm−1. The OH radicals are state selected and velocity tuned prior to the collision using a Stark decelerator, and field-free parity-resolved state-to-state inelastic relative scattering cross sections are measured in a crossed molecular beam configuration. For all OH-rare gas atom systems excellent agreement is obtained with the cross sections predicted by coupled channel scattering calculations based on accurate ab initio potential energy surfaces. This series of experiments complements recent studies on the scattering of OH radicals with Xe [J.J. Gilijamse, S. Hoekstra, S.Y.T. van de Meerakker, G.C. Groenenboom, G. Meijer, Science 313, 1617 (2006)], Ar [L. Scharfenberg, J. Kłos, P.J. Dagdigian, M.H. Alexander, G. Meijer, S.Y.T. van de Meerakker, Phys. Chem. Chem. Phys. 12, 10660 (2010)], He, and D2 [M. Kirste, L. Scharfenberg, J. Kłos, F. Lique, M.H. Alexander, G. Meijer, S.Y.T. van de Meerakker, Phys. Rev. A 82, 042717 (2010)]. A comparison of the relative scattering cross sections for this set of collision partners reveals interesting trends in the scattering behavior.

Genetic-Algorithm-Based Parameter Estimation Technique for Fragmenting Radar Meteor Head Echoes

Meteoroid fragmentation presents a serious problem for Doppler estimation using Fourier transform techniques. Radar returns from multiple closely spaced bodies traveling at nearly identical speeds result in an interference pattern which makes it difficult to estimate properties of individual bodies by traditional techniques. Here, we present a genetic-algorithm-based procedure to determine the properties of the individual fragments, such as relative scattering cross section, speed, and deceleration. The radar meteor observations presented here were made using the Poker Flat (Alaska) Incoherent Scatter Radar operating at 449.3 MHz.

On the relationship between energy fluxes, dielectric properties, and microwave scattering over snow covered first‐year sea ice during the spring transition period

In this research we investigate the seasonal nature of the co‐variability in surface energy balance variables, volume dielectrics, and microwave scattering (ERS 1) of a snow‐covered first‐year sea ice surface during a spring transitional period. Variables required to derive the components of the energy balance and dielectric properties were measured during the Seasonal Sea Ice Monitoring and Modeling site in the Canadian Arctic Archipelago in 1992. We observed that both the energy terms and dielectric properties followed a pattern similar to the total relative scattering cross section (σ°) over the seasonal transition from winter to spring. We explain this relationship through the impact of surface fluxes on dielectric and geophysical properties of the snow‐covered first‐year sea ice. We speculate that ice surface scattering dominated the total scattering cross section σ° prior to Julian day 120 and that the snow volume contributed an increasing amount of scattering to σ° over the remainder of the season. From a multivariate statistical analysis we find that the surface temperature Ts and the net shortwave energy flux K* explained a statistically significant amount of the variation observed in the seasonal evolution of σ°. An inverse relationship existed between both Ts and K* relative to σ,° and the influence of Ts was approximately twice that of K* in explaining the observed variation in σ°.

Crossed beam rovibrational energy transfer from S1 glyoxal. IV. Reduced mass effects and an overview of the inelastic scattering characteristics from four initial levels

Crossed molecular beam studies of rotationally and rovibrationally inelastic scattering of S1 glyoxal from H2 and He have been extended to one additional light gas, D2, and to two heavy gases of identical masses, Kr and cyclohexane, C6H12 (84 amu). Laser excitation was used to prepare glyoxal in its 00 level with K′=0 and 0≤J′≤10. Dispersed fluorescence detection was used to observe the final K′ and vibrational states of the inelastic scattering. The relative scattering cross sections for D2 and He collisions are identical to within experimental error and differ substantially from those of H2. The Kr and C6H12 cross sections are also a matched set. These results show that the competition among the approximately 25 observable scattering channels is far more sensitive to the reduced mass of the collision than to variation in the intermolecular potential or even the internal structure of the target gas. An overview of rotational and rovibrational scattering in glyoxal from four vibrational levels (00, 72, 51, and 81) extending to εvib=735 cm−1 is used to uncover generalities and insights about the energy transfer. For all four initial levels the vibrational state changes are highly selective. The detectable channels are always limited to ±1 quantum change in only one of the 12 modes, specifically ν7′ = 233 cm1, the lowest frequency mode. The cross sections for vibrational state change are surprisingly large relative to those for pure rotationally inelastic scattering. Many cases occur with the light target gases where the ΔK resolved cross sections for rovibrational interactions are nearly equal to those for pure rotationally inelastic scattering with equivalent energy transfer ΔE. Scattering from 72, K′=0 glyoxal contains examples with both H2 and He where the rovibrational cross sections actually exceed those for rotational scattering. Plots of the entire set of cross sections [rotational (ΔK) plus rovibrational (Δυ7=+1)] against ΔE are essentially superimposible for He scattering from 00, 51, and 81 glyoxal. In contrast, scattering from 72 glyoxal with the active mode initially excited is distinctive. For all initial levels, the distribution of cross sections for different ΔK within rotational channels differs from that within rovibrational channels. It is further seen in these comparisons that the change in angular momentum ΔK rather than ΔE controls the relative sizes of cross sections within these channels. The theoretical predictions of Clary, Kroes, and Rettschnick are in accord with these trends and distinctions, agreeing even on some rather subtle points.

Modeling synthetic aperture radar (SAR) scattering from a seasonally varying snow-covered sea ice volume at 5.3 and 9.25 GHz

A series of sensitivity analyses using dielectric, mixture and microwave scattering models is presented. Data from the Seasonal Sea Ice Monitoring and Modeling Site (SIMMS) in 1990 and 1991 are used to initialize the models. The objective of the research is to investigate the role of various geophysical and electrical properties in specifying the total relative scattering cross section (???) of snow covered first-year sea ice during the spring period. The seasonal transition period from the Winter SAR scattering season to Early Melt was shown to signal a transition in dielectric properties which caused the snow volume to become a factor in the microwave scattering process. The effect of the thermal insulation of a snow cover on sea ice was shown to be significant for both ?? and ???. Higher atmospheric temperatures caused proportionally greater changes in the dielectric properties of the sea ice at the base of the snow cover. Model ?0 was computed for a range of sensor, sensor-earth geometry, and geophysical properties. In the Winter season the surface roughness terms (ohand L) were shown to have a significant impact on ?0 when the ice surface was the primary scattering mechanism. Once the snow cover began to warm and water was available in a liquid phase, the ice surface became masked because of the decrease in microwave penetration depths. During this period the water volume variable dominated ?0, both from its impact on ?v0, and due to its control over the dielectric mismatch created at the air/snow interface.

Kinetic theory of rarefied atmospheres— What a realistic kinetic theory of ionized gases prescribes about the heating of the solar corona

Abstract The observed inversions of kinetic temperature in all known planetary thermospheres and the solar corona have similarities which appear to be a natural and unavoidable consequence of the manner in which a real gas stratifies its properties in a gravitational field. The classical kinetic theory of a gas bound by a gravitational field has severe limitations when the gas becomes too rarefied, and it incorrectly predicts that the field should induce no vertical gradients of kinetic temperature in an atmosphere. Correcting the classical theory to apply to rarefied gases implies a contrary conclusion, a kinetic temperature which increases with altitude at the average rate mg/3k in the transition region of a neutral atmosphere. The correction rests upon the experimentally supported postulate that in a sufficiently rarefied atmosphere, in which the mean free path exceeds about one-tenth percent of its scale height, the distribution of speed of the particles varies with altitude according to a modified Boltzmann probability distribution in which the kinetic energy of the individual particles replaces their average kinetic energy or temperature in the latter distribution. In an ionized atmosphere the temperature increases at a much greater rate which is determined by the relative scattering cross sections of its particles compared to those of a neutral atmosphere under the same conditions. Furthermore, the strong electrical forces between particles cause any departures from Maxwellian conditions throughout the region of transition to be very small, which is in contrast to the greater departures which may occur in neutral planetary atmospheres. The present theory appears correctly to predict simultaneously and without any arbitrary parameters the observed profiles as a function of altitude of both the solar coronal density of hydrogen and of the temperature. From near the base of the region of transition of the solar corona to just below the altitude at which the temperature profile saturates, the present theory prescribes that the temperature increases as the one-third power of the normalized distance from that base, 6amgx/kT in which a is a known parameter from plasma theory, m is the mass of the hydrogen atom, g is the solar gravitational acceleration, k is Boltzmann's constant, T is the temperature at the base, and x is the geometric distance. The coronal density of hydrogen decreases as the inverse one-half power of that same normalized distance.

Modeling synthetic aperture radar (SAR) scattering from a seasonally varying snow-covered sea ice volume at 5.3 and 9.25 GHz

A series of sensitivity analyses using dielectric, mixture and microwave scattering models is presented. Data from the Seasonal Sea Ice Monitoring and Modeling Site (SIMMS) in 1990 and 1991 are used to initialize the models. The objective of the research is to investigate the role of various geophysical and electrical properties in specifying the total relative scattering cross section (???) of snow covered first-year sea ice during the spring period. The seasonal transition period from the Winter SAR scattering season to Early Melt was shown to signal a transition in dielectric properties which caused the snow volume to become a factor in the microwave scattering process. The effect of the thermal insulation of a snow cover on sea ice was shown to be significant for both ?? and ???. Higher atmospheric temperatures caused proportionally greater changes in the dielectric properties of the sea ice at the base of the snow cover. Model ?0 was computed for a range of sensor, sensor-earth geometry, and geophysical properties. In the Winter season the surface roughness terms (ohand L) were shown to have a significant impact on ?0 when the ice surface was the primary scattering mechanism. Once the snow cover began to warm and water was available in a liquid phase, the ice surface became masked because of the decrease in microwave penetration depths. During this period the water volume variable dominated ?0, both from its impact on ?v0, and due to its control over the dielectric mismatch created at the air/snow interface.

On the Links between Microwave and Solar Wavelength Interactions with Snow-Covered First-Year Sea Ice

Electromagnetic (EM) energy at solar and microwavelengths will interact with a snow-covered sea ice volume as a function of its geophysical properties. The seasonal metamorphosis of the snow cover modulates the relative distribution of the three main interaction mechanisms of EM energy: reflection, transmission, and absorption. We use a combination of modeling and observational data to illustrate how the total relative scattering cross section (Sigma 0) at microwavelengths can be used to estimate the surface climatological shortwave albedo and the transmitted Photosynthetically Active Radiation (PAR) for a snow-covered, first-year sea ice volume typical of the Canadian Arctic. Modeling results indicate that both 5.3 and 9.25 GHz frequencies, at HH polarization and incidence angles of 20 degrees, 30 degrees, and 40 degrees can be used to estimate the daily averaged integrated climatological albedo (Alpha). The models at 5.3 GHz, HH polarization, at 20 degree, 30 degree, and 40 degree incidence angles were equally precise in predications of Alpha. The models at 9.25 GHz were slightly less precise, particularly at the 40 degree incidence angle. The reduction in precision at the 40 degree incidence angle was attributed to the increased sensitivity at both 5.3 and 9.25 GHz to the snow surface scattering term (Sigma 0 ss) used in computation of the total relative scattering cross section (Sigma 0). Prediction of subsnow PAR was also possible using the same combination of microwave sensor variables utilized in prediction of Alpha, but because subice algal communities have evolved to be low light sensitive, the majority of the growth cycle occurs prior to significant changes in Sigma 0. A method of remote estimation of snow thickness is required to be scientifically useful. Observational data from the European ERS-1 SAR were used to confirm the appropriateness of the modeled relationships between Sigma 0, Alpha, and PAR. Over a time series spanning all conditions used in the modeled relationships, the same general patterns were observed between Sigma, Alpha, and PAR.Key words: microwave scattering models, snow, sea ice, climatological shortwave radiation, photosynthetically active radiation, microwave remote sensing

Solvent dependence of the relative Raman trace scattering intensity of the isolated CH stretching bands of cyclohexane- d11

The trace scattering Raman spectra of several solutions of cyclohexane- d 11 have been measured to observe the solvent dependence of the relative intensities of bands due to the isolated axial and equatorial CH stretching vibrations. The band positions and widths and their relative areas were determined. Significant changes in the relative band intensities were observed, both among the various solution spectra, and between the liquid and vapor phase spectra. These variations are attributed to changes in the relative scattering cross section for the two bands; however, a detailed explanation cannot be offered at this time.

Parity violation in proton scattering: low energy region

Measurements of the parity-violating longitudinal analyzing power, Az, in the scattering of low-energy protons (Ep < 50 MeV) are reviewed. The experiments are based on the determination of the relative scattering cross sections σ+ and σ− for incident protons of positive and hegative helicity, respectively. The first results were reported from Los Alamos, where a significant analyzing power [Az = (−1.7 ± 0.8) × 10−7] was found for p–p scattering at 15 MeV. All other results are for a proton energy near 45 MeV, primarily by the group working at Schweizerisches Institut für Nuklearforschung, which recently reported a new, very accurate result for p–p scattering of Az = (−1.50 ± 0.22) × 10−7. A review is presented of the methods that have been developed by this group over the last 10 years to reduce systematic errors in the determination of Az to a few times 10−9.

Investigation of particle elastic scattering analysis as a complementary technique to pixe for aerosol characterization

The possibility of analysis by the detection of elastically scattered particles for carbon, nitrogen, oxygen and also hydrogen determination in the fine mode of aerosols has been investigated. The main objectives in this study have been to find suitable projectiles (protons or alpha particles) and projectile energy, in order to obtain separated peaks for the elements of interest and to achieve high analytical sensitivity and precision. Sensitive PIXE analysis should also be obtained on an aerosol substrate suitable for both methods. Relative scattering cross sections at an angle of 170° for protons (3.5–5.8 MeV) and alpha particles (8–9 MeV) on 12C, 14N and 16O have been investigated. Lower and upper energy limits are set by the demand for resolved peaks from C, N and O and the accelerator available, respectively. Protons were found to be the most suitable projectiles and for one proton energy, 3.58 MeV, fulfilling the requirements mentioned above, the possibility for H determination at laboratory scattering angles between 29° and 59° has been investigated. Some aerosol filters and foils have also been investigated at this proton energy in order to find a suitable aerosol substrate. The lowest detection limits were obtained for thin aluminum foils, yielding 15, 10 and 350 ng cm −2 for C, N and O, respectively, and PIXE detection limits comparable to those obtained with a Nuclepore ® filter.

Results on relative scattering cross section of 140‐MHz auroral backscatter

Observations of power spectra and backscattered signal intensities associated with auroral electron density fluctuations with scale length of nearly 1 m have been made with the Scandinavian Twin Auroral Radar Experiment (STARE). The intensities of the backscattered signals, which are related to the radar scattering cross sections, have been analyzed to determine their angular distribution in the plane perpendicular to the magnetic field with respect to the direction of the electron drift velocity. In the analysis the Doppler spectral characteristics of the data are used in order to identify the type of irregularities. It is found that the primary irregularities, having narrow spectra centered at large Doppler shifts, have a highly anisotropic scattering cross section as compared with the secondary 1‐m waves. The strength of primaries is large and strongly dependent on flow angle θ; typically, the intensity decreases from 0.3 to 0.6 dB/deg with θ increasing. The intensities of the secondaries are relatively weak and rather insensitive to angular variations. We find the ac variation of the backscatter amplitudes associated with primary and secondary irregularities to be closely correlated. This indicates a common modulation mechanism to dominate for the two kinds of scatterers.

Forbidden Raman bands of Sf 6: collision induced Raman scattering

The frequencies, relative scattering cross sections and depolarizations ratios of forbidden fundamental and overtone Raman bands have been measured for the first time in the gaseous state. The absolute scattering cross sections of the allowed bands of gaseous SF 6 are given as well. The characteristic density dependence of the scattering cross sections of the forbidden fundamentals is explained in terms of collision induced Raman scattering. There is evidence for frame distortion of the molecules during the collision.

On the scattering cross section of passive linear arrays

A general formula is derived for the scattering cross section of a passive <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</tex> -element linear array consisting of isotropic radiators. When all the reactances are tuned out and scattering in the mirror direction is investigated, it is found that <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A_{sr}</tex> , the relative scattering cross section is equal to the square of the maximum gain the array can produce. As a consequence, for forward scattering in the limiting case of zero spacing between the elements, <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A_{sr} = n^{4}</tex> .

Effet Raman de resonance par excitation dans le continuum d'absorption : sections efficaces relatives de diffusion

With the lines λ = 4965, 4880, 4765, 4727, 4658, 4579 and 4545 Å of an argon ion laser, we have studied the exciting frequency dependence of the relative scattering cross section of the 213 cm -1 line of I 2 when the excitation lies in the absorption continuum associated with the 3Π + ← 1Σ + transition. The experimental results are compared with the theoretical prediction of Behringer and Mortensen.

Resonance Raman Effect and Resonance Fluorescence in Halogen Gases

The various output frequencies of the argon ion laser at 5145, 5017, 4965, 4880, and 4765 Å lie in the absorption bands of the heavier halogen gases. With the appropriate choice of exciting line, either resonance Raman effect or resonance fluorescence can be observed. The difference between the two types of spectra are discussed in some detail. In the case of a strong resonance Raman effect, overtone sequences up to the 14th harmonic could be observed. Raman frequencies, depolarization ratios, and relative scattering cross sections are given for the fundamentals and overtones of Cl2, Br2, I2, BrCl, ICI, and IBr at 4880 Å excitation.

High-Resolution Raman Spectroscopy of Gases with cw-Laser Excitation*

Several techniques of exciting Raman spectra of low-pressure gases and vapors with cw laser radiation are described. Both the He–Ne laser with output at 6328 a and the Ar+ ion laser with output at 4880 and 5145 a have been used. The use of the He–Ne laser with a multiple-pass Raman tube inside the laser cavity yields Raman spectra which can be photographed in exposure times that are equivalent to those required with the classical Hg-arc and large-volume Raman tube as the source of scattered radiation. The Ar+-ion laser allows the photoelectric recording of rotational Raman spectra at atmospheric pressure with a signal/noise ratio of 10:1 when the Raman tube is located outside the laser cavity, and about 100:1 when the Raman tube is inside the laser cavity. A comparison is given for the relative efficiency when the scattering gas is located inside and outside the laser cavity. Pure rotational Raman spectra for N2, O2, CO2, and H3CC≡CH (methylacetylene) are presented. The depolarization ratios of both the Rayleigh and Raman lines of N2, O2, and CO2 have been determined and relative scattering cross section for the pure rotational Raman scattering of oxygen has been obtained.

Electron-Electron Scattering at 15.7 Mev

The differential scattering cross section for electron-electron scattering has been measured at relativistic energies. A focused beam of 15.7-Mev monokinetic electrons from the 22-Mev betatron was scattered by 1- and 2-mil Nylon foils in a 20-inch scattering chamber. The incident current was collected by a faraday cage after it had passed through the scattering foil and was measured by a vibrating reed electrometer. The scattered beam was defined by gold-lined slits at a known angle to the incident beam. The number of scattered electrons in a given momentum interval chosen by a magnetic analyzer was counted by a Geiger counter.Energy analysis of the scattered electrons made it possible to separate the nuclear scattering events from the electron scattering events. The energy of the scattered electrons was measured as a function of the scattering angle and was found to be in agreement with the laws of relativistic mechanics within an experimental accuracy of 0.4 percent.For the electron-electron collisions the relative scattering cross sections agree with the M\o{}ller formula. For the electron-nuclear collisions they agree with the relativistic Mott theory (Born approximation) corrected for the radiative losses as calculated by Schwinger (3 to 8 percent). The experimental consistency of the cross sections was about 4 percent.The absolute differential scattering cross sections for the electron-nuclear scattering were measured at angles between 10\ifmmode^\circ\else\textdegree\fi{} and 43\ifmmode^\circ\else\textdegree\fi{}. The average of the absolute values was 2 percent lower than theory. This is probably due to systematic errors in the solid angle, and in the collection and counting efficiencies.The absolute differential scattering cross sections for electron-electron scattering were measured at the same set of angles, which correspond to the angular interval of 70\ifmmode^\circ\else\textdegree\fi{} to 150\ifmmode^\circ\else\textdegree\fi{} in the center-of-mass system. The average of these absolute cross sections was 7 percent lower than theory predicts. The bulk of the 5 percent additional departure from theory may be due to a systematic error in the measurement of the momentum interval defined by the counter.