Showing papers in "Journal of Quantitative Spectroscopy & Radiative Transfer in 1995"

A multispectrum nonlinear least squares fitting technique

Abstract: An extension of the nonlinear least squares spectrum fitting technique has been developed for the simultaneous fitting of multiple spectra. This procedure's smaller number of fitted parameters as compared to fitting one spectrum at a time improves the determination of spectroscopic parameters. A more reliable evaluation of the errors associated with the solution is possible. Correlations among fitted parameters may preclude their determination from a single spectrum fit. If the correlations differ from spectrum to spectrum, however, separation of these parameters is often possible when including the spectra in one solution. Overfilling is also avoided when combining spectra with sufficiently different experimental physical conditions.

THE CORRELATED k-DISTRIBUTION TECHNIQUE AS APPLIED TO THE AVHRR CHANNELS

Abstract: Correlated k-distributions have been created to account for the molecular absorption found in the spectral ranges of the five Advanced Very High Resolution Radiometer (AVHRR) satellite channels. The production of the k-distributions was based upon an exponential-sum fitting of transmissions (ESFT) technique which was applied to reference line-by-line absorptance calculations. To account for the overlap of spectral features from different molecular species, the present routines made use of the multiplication transmissivity property which allows for considerable flexibility, especially when altering relative mixing ratios of the various molecular species. To determine the accuracy of the correlated k-distribution technique as compared to the line-by-line procedure, atmospheric flux and heating rate calculations were run for a wide variety of atmospheric conditions. For the atmospheric conditions taken into consideration, the correlated k-distribution technique has yielded results within about 0.5% for both the cases where the satellite spectral response functions were applied and where they were not. The correlated k-distribution's principal advantages is that it can be incorporated directly into multiple scattering routines that consider scattering as well as absorption by clouds and aerosol particles.

Argon ion linelist and level energies in the hollow-cathode discharge

Abstract: Lines of Ar I, II, and III are identified in high-resolution Fourier Transform spectra of a hollow-cathode source covering the range 222–5865 nm. Ar II and III level energies are extracted and compared with earlier measurements. Selected Ar II lines suitable for use as secondary wavelength standards are measured relative to precise wavelengths of CO molecular lines.

Discrete ordinates quadrature schemes for multidimensional radiative transfer

Abstract: The fundamental problem of applying the method of discrete ordinates to radiative transfer predictions is the selection of the discrete directions and their associated weights. Both the accuracy of the solution and the computational effort depend on the angular discretization. This paper provides a sound mathematical methodology for the derivation of angular quadratures. By applying the collocation principle, the errors introduced by a quadrature are analysed and the constituting equations of angular quadratures are identified. Special emphasis is placed on the rotational invariance of the quadrature schemes. Multidimensional radiative transfer in participating media with isotropic and anisotropic scattering is accounted for throughout the analysis. A major goal of the present study is the construction of a new principle for multidimensional angular quadratures which is essentially a generalization of the principles employed for the well-known S n quadratures. The new construction principle has two major advantages. First, it enables a very flexible tailoring of quadratures according to the actual requirements. Second, compared to the S n quadratures, the new types of quadratures provide a higher accuracy while using the same number of nodal points.

Xuv opacity measurements and comparison with models

Abstract: The opacities of aluminium, iron and holmium were measured spectrally resolved in the energy range of 70 to 280 eV. For this purpose the iodine laser ASTERIX IV at the MPQ ( 200J 0.4 ns at 440 nm) was focused into a spherical gold Hohlraum with a dia of 3 mm. The generated radiation with a temperature of 60 eV heated thin tampered absorber foils. The transmission was measured spectrally and temporally resolved by means of a backlighter source with a delay of 0.8 ns to the main beam. At this delay time the density in the sample material is about 0.01 g/cm3 and the temperature is about 20 eV. We have compared the experimental data with the opacity codes IMP, JIMENA, OPAL, SAPHIR and STA.

Laser-induced fluorescence measurements of NO and OH mole fraction in fuel-lean, high-pressure (1–10 atm) methane flames: Fluorescence modeling and experimental validation

Abstract: A method for quantifying laser-induced fluorescence (LIF) signals and planar laser-induced fluorescence (PLIF) images of the OH and NO radicals in high pressure flames is presented. The fluorescence signal per unit radical mole fraction is modeled as a function of temperature, pressure, overall flame stoichiometry and laser spectral bandwidth. A recently developed model ( JQSRT , 51, 511; Appl. Phys. B , 57, 249) 1,2 for electronic quenching cross-sections of OH and NO is utilized to express the fluorescence yield as a function of these parameters. The models are confirmed using single-point measurements in the burnt gas region of a flat flame burner at up to 10 atm. The measurements are performed at points in the flame where the temperature, pressure, OH and NO mole fraction are all known. For fuel-lean flames at elevated pressure, interference from the O 2 Schumann-Runge system was found with NO A ← X (0, 0) fluorescence measurements. This interference must be considered when selecting an appropriate NO transition in this type of environment.

Correlated-k fictitious gas model for H2O infrared radiation in the Voigt regime

Abstract: The ability of the correlated-k fictitious gas (ckfg) approach to accurately predict radiative transfer in strongly nonhomogeneous media is studied for the case of water vapor absorption lines with Voigt profiles. In the ckfg approach, the real gas is considered as a mixture of fictitious gases, each one characterized by absorption lines with similar transition lower level energies. The correlated-k approach is then applied to each gas and the spectra of different gases are assumed to be uncorrelated. The model is studied for a range of temperatures (180–2500 K) and pressures (10-2 atm). In addition, the model takes into account the nonlorentzian behavior of the far wings. The model parameters are obtained from line-by-line calculations based on approximate spectroscopic data suitable for high temperature and low spectral resolution applications. The choice of the quadrature for spectral integrations and of the lower level energy E″ ranges is discussed. ckfg results are compared to the reference line-by-line calculations and to the results from the correlated-k (ck) approach, and from available statistical narrow-band (SNB) models in the Voigt regime. It is shown that a 10 point quadrature may be sufficiently accurate, even for very small line widths. ckfg results agree well with line-by-line calculations while ck and SNB models may strongly underestimate the intensity of radiation emitted by a hot gaseous column and transmitted through a long cold column.

Quantitative measurement of mid-z opacities

Abstract: Results of recent experiments measuring x-ray absorption by a hot, dense, germanium plasma are presented. A general discussion of the experimental technique is given showing the requirements that must be met in order to extract quantitative transmission data. The resulting spectrally resolved absorption measurements can then be used to test the capabilities of LTE opacity codes. Meaningful comparisons require that the sample be in LTE, and that the temperature and density of the sample be uniform and accurately measured. Comparisons between the experiment and calculations are shown.

An inverse problem of simultaneous estimation of radiation phase function, albedo and optical thickness

Abstract: An inverse analysis is presented for simultaneous estimation of optical thickness, single scattering albedo and the coefficients of phase function for an anisotropic scattering plane-parallel medium. The Levenberg-Marquardt method of minimization is used to solve the resulting nonlinear system of algebraic equations. Forward and backward scattering phase functions are considered, and the effects of the magnitudes of single scattering albedo and optical thickness on the accuracy of the estimations are examined. The confidence bounds are established for the estimated values of the single scattering albedo, the optical thickness and the coefficients of the phase function.

Review of the third international opacity workshop and code comparison study

Abstract: The Third International Opacity Workshop and Code Comparison Study Work Op-III:94 was held in March 1994 at the Max-Planck-Institut fur Quantenoptik (MPQ) in Garching, Germany. The meeting was organized by A. Rickert, J. Meyer-ter-Vehn and K. Eidmann from MPQ with strong support from Lawrence Livermore National Laboratory, especially by F. J. D. Serduke and C. A. Iglesias. Twenty-one different codes participated in the comparison and results for up to 35 test cases with elements throughout the periodic table and two astrophysical mixtures were submitted prior to the meeting. There were 485 individual plasma calculations with about 150 Mbytes of information submitted. At the meeting, radiative opacities were compared in detail and differences in calculated results were traced back to the physics input such that their origin became transparent. A review of these comparisons is given and detailed comparisons for some of the test cases are presented.

Water vapor absorption in the atmospheric window at 239 GHz

Abstract: Absolute absorption rates of pure water vapor and mixtures of water vapor and nitrogen have been measured in the atmospheric window at 239 GHz. The dependence on pressure as well as temperature has been obtained. The experimental data are compared with several theoretical or empirical models, and satisfactory agreement is obtained with the models involving a continuum; in the case of pure water vapor, the continuum contribution based upon recent theoretical developments gives good results. The temperature dependence is stronger than that proposed in a commonly used atmospheric transmission model.

An approximate data base of H2O infrared lines for high temperature applications at low resolution. Statistical narrow-band model parameters

Abstract: An approximate spectroscopic H2O data base suitable for high temperature and a few tens cm-1 spectral resolution applications has been developed as an extrapolation of HITRAN92 and Flaud and Camy-Peyret bases for the spectral and temperature ranges 150–9000 cm-1 and 300–2500 K, respectively. The energies of high rovibrational levels have been extrapolated by using analytical expressions derived from Tyuterev's Hamiltonian. The intensities of lines issued from the fundamental vibrational level and from high rotational levels have been extrapolated empirically. The intensities of lines issued from other vibrational levels are calculated from the intensities of the corresponding lines issued from the fundamental vibrational level by using the independent rigid harmonic oscillator approach. Satisfactory agreement between line-by-line calculations based on the extended data base and experimental data is obtained. Statistical narrow-band model parameters have been adjusted using line-by-line calculations; they are available under request.

Electron number density and temperature measurement in a laser-induced hydrogen plasma

Abstract: A Nd:YAG laser operated at 1064 nm and 7.5 nsec pulse duration is used to create optical breakdown in gaseous hydrogen. Time-resolved spectral measurements of the hydrogen Balmer series are reported and analyzed to characterized the electron number density and excitation temperature of the decaying plasma. The electron density is inferred from the Stark broadened Hα linewidths; excitation temperatures are estimated using Boltzmann plots of the Balmer series. In the first few microseconds following laser breakdown, electron densities are found to be in the range of 1019-1016cc-1, with corresponding excitation temperatures in the range of approx. 100,000–6600 K.

Effective interpolation technique for line-by-line calculations of radiation absorption in gases

Abstract: An interpolation technique for Line-by-Line calculations of absorption coefficients is developed. The technique gives a possibility considerably to increase the calculation speed. This work may be particularly useful in the calculation of radiation transfer in the atmosphere.

Self-broadening coefficients and line strengths in the v3 band of CH335C1 at low temperature

Abstract: Self-broadened linewidths have been measured in the P- and R-branches of the v 3 band of CH 3 35 Cl at 200 K using a tunable diode-laser spectrometer. The lines under study are located in the spectral range 706–751 cm -1 , with J values ranging from 3 to 27 and K from 3 to 6. The collisional half-widths were estimated along with the corresponding line strengths by fitting measured shapes of the individual lines with Voigt profiles convolved with the instrumental function. The line strengths at 200 K are in satisfactory agreement with results derived from previous measurements. Even though semi-classical calculations of line-broadening involving electrostatic interactions have yielded half-widths at 200 and 296 K that are greater than experimental data for low or medium values of J , they seem to predict correctly the K -dependence of the broadening coefficients of lines with the same J transition as well as their temperature dependence.

Collisional broadening of spectral lines in the a-x (0-0) system of no by n2, ar, and he at elevated pressures measured by laser-induced fluorescence

Abstract: Lineshape parameters for NO A-X(0,0) transitions broadened by He, Ar, and N2 have been determined at room temperature from LIF at pressures up to 5 bar. Isolated line shapes have been extracted from pressure broadened LIF spectra using a deconvolution technique. It was found that a formalism based on a combined impact and statistical theory leads to an adequate representation of the asymmetric lineshapes. Pressure broadening and shift coefficients are in good agreement with published data obtained at much lower pressures.

Ozone measurements by zenith-sky spectrometers: An evaluation of errors in air-mass factors calculated by radiative transfer models

Abstract: Calculations of air-mass factors (AMFs) for ground-based zenith-sky UV-visible spectrometers are presented and discussed. Causes and size of errors in AMFs of ozone in the visible are evaluated. Errors can be caused by approximations in the calculation (intensity-weight approximation, ignoring the finite field of view of the instrument); by approximation in the scheme of the calculation (single scattering, ignoring refraction); or by variable geophysical parameters (vertical profile of constituents). These relative errors in AMF cause identical relative errors in vertical columns of ozone deduced from measurements by zenith-sky spectrometers. The mean of the relative errors of ozone AMFs due to using one set of AMFs for all seasons and locations is ±2.4% when averaged over the commonly used range of solar zenith angles.

Contribution to the development of radiative transfer models for high spectral resolution observations in the infrared

Abstract: The capability of the Automatized Atmospheric Absorption atlas method for raditaive transfer modelling to simulate analytic Jacobians (first partial derivative of the observations with respect to the geophysical parameters) is analyzed. Several applications based on analytical Jacobians are described. The sensitivity of a ‘1200 resolution’ infrared spectrum to the water vapor content in the lower atmosphere is found to be weak, however a significant sensitivity to the methane give us good confidence in a successful retrieval of its total atmospheric content from high spectral resolution nadir viewing sounders. The degree of linearity of the radiative transfer equation with respect to the temperature and the water vapour is quantified. The continuum absorption is found to contribute to the non-linear behavior of channels located around 750 cm−1 for wet atmospheres. The highest degree of non-linearity is found at the end of the CO2-4.3 μm absorption band. The rest of the spectrum can be considered to linearly answer to individual perturbations

Non-LTE treatment of FE II in astrophysical plasmas

Abstract: We describe our implementation of an extremely detailed model atom of singly ionized iron for NLTE computations in static and moving astrophysical plasmas. Our model atom includes 617 levels, 13,675 primary permitted transitions and up to 1.2 million secondary transitions. Our approach guarantees that the total iron opacity is included at the correct wavelength with reasonable memory and CPU requirements. We find that the lines saturate the wavelength space, such that special wavelength points inserted along the detailed profile functions may be replaced with a statistical sampling method. We describe the results of various test calculations for novae and supernovae.

A parametric potential for ions from helium to iron isoelectronic sequences

Abstract: It is shown that a new family of analytical potentials with one, two or three parameters produces similar results to those obtained with more complex models. The parameters of the potential are obtained by an iterative procedure with a selfconsistent relativistic potential that has corrections at large distances, and considers the exchange and correlation energy by means of the local density approximation. The analysis of the results obtained with the family of analytical potentials (eigenvalues, eigenfunctions and atomic energies) allowed us to propose a parametric potential with one or two parameters that depends on the electron number of ion. These parameters were fitted to power series in Z for intermediate and highly ionized atoms from helium to iron isoelectronic sequences. Finally, using this analytic potential, we obtained oscillator strengths for lithium and soduim like gold, which are compared with other models.

Computed infrared absorption properties of hot water vapour

Abstract: Using first principles quantum mechanics we have computed a list of 6.2 million infrared transitions for H 16 2 O. This dataset is used to study the absorption properties of water as a function of temperature between 300 and 3000 K. Comparison is made with experimental data of Ludwig ( Appl. Opt. 10 , 1057) and the 30, 117 lines contained in the HITRAN database. AT 300 K the HITRAN data is the most reliable, but at higher temperatures HITRAN contains too few lines to be useful. Ludwig's data is too coarse to be reliable for many applications and shows a number of systematic errors when compared with our dataset or HITRAN. Methods of further improving our linelist are discussed.

An improved iterative method for solving a class of coupled conductive-radiative heat-transfer problems

Abstract: Newton's method of iteration is used along with the P N method, also called the spherical-harmonics method, and Hermite cubic splines to develop an iterative solution to a class of nonlinear problems in radiative transfer. Anisotropic scattering and specularly and diffusely reflecting boundaries are allowed for the steady-state, combined-mode, conductive-radiative, heat-transfer problem considered.

Simulations of spectra from dense aluminium plasmas

Abstract: We describe a computer code used to predict spectra arising from neutral and near neutral species in a dense, cool plasma. The ionization potential depression and excited states of the plasma ions must be included to obtain the correct ionization balance. Good agreement between the code predictions and observed spectra is obtained. This provides a verification of the code prediction so that it could be used for plasma diagnostics.

Measurement of the self-broadening rate coefficients of the cesium resonance lines

Abstract: The self-broadening rate coefficients of the cesium D 1 [6 2 S 1 2 − 6 2 P 1 2 ] and D 2 [6 2 S 1 2 − 6 2 P 3 2 ] resonance lines are determined from the 6 2 P 1 2 → 6 2 D 3 2 , 6 2 P 3 2 → 7 2 D 3 2 and 6 2 P 3 2 → 7 2 D 5 2 transition lineshapes, which are mapped out using single-mode cw lasers. The self-broadening rates are found from Voigt function fits (including hyperfine structure) to the experimental lineshapes. Our measured D1 and D2 broadening coefficients, kbr(D1) = (5.7 ± 1.0) × 10−7 cm3 sec−1 and kbr(D2) = (6.7 ± 1.1) × 10−7 cm3 sec−1, are compared to previous experimental and theoretical values in the literature. The present D2 broadening coefficient is smaller, and the D1 broadening coefficient is larger, than theoretical values.

Electronic transition moment for the B(2)II-X(2)II system of NO

Abstract: The upsilon' = 0-3 and 7 vibrational levels of the NO B(2)II state have been selectively excited by laser radiation. The fluorescence spectra together with calculated Franck-Condon factors and r-centroids have been used to evaluate the electronic transition moment. The results for upsilon' = 0-3 are in very good agreement with recent chemiluminescence measurements and ab initio calculations. Furthermore, the data from upsilon' = 7 have been used to extend the empirically determined moment to limits 1.23 and 1.78 A, improving agreement with experimentally determined lifetimes.

Absorption coefficient estimation using a two-dimensional interpolation procedure

Abstract: A method of evaluating high resolution absorption coefficients for the purpose of integrating the radiative transfer equation is described. The method uses a two-dimensional interpolation scheme which operates on a table of pre-computed absorption coefficients. Computation time for the evaluation of the RT equation is significantly reduced whilst achieving accuracy comparable to conventional line-by-line methods.

x-ray spectroscopic diagnostics of mix in high growth factor spherical implosions

Abstract: Rayleigh-Taylor (RT) instability of the pusher-fuel interface occurring upon acceleration and deceleration of the pusher is of major concern for current and future ICF experiments. One common diagnostic technique for measuring pusher-fuel mix in spherical implosion experiments involves placing spectroscopic dopants both in the capsule fuel region and the innermost region of the capsule wall adjacent to the fuel. As the degree of pusher-fuel mix is increased the pusher dopant x-ray emission increases relative to that of the fuel dopant. Spherical implosion experiments of this type using Ar and Ti dopants in the fuel and pusher, respectively, are being carried out on Nova. We first show that the Ti He-α Ar He-β line ratio shows promise as a mix diagnostic for high growth factor targets. We then discuss some of the important physical processes underlying Ar and Ti spectral line formation in these targets and discuss how these processes affect the calculation of simulated spectra. The importance of radiative transfer as well as high-density plasma phenomena such as continuum lowering and Stark broadening is demonstrated. The simulated spectra are also observed to be sensitive to assumptions regarding the treatment of electron thermal conduction in the mix region. Spectral postprocessing of 2-D hydrodynamic simulations using detailed line transfer methods has been carried out and implies that simple escape factor treatments must be tested carefully before they can be relied upon. Preliminary comparisons of experimental data with simulation are presented. It is shown that the computed spectra is sensitive to the laser energy and pusher temperature. These comparisons to data also imply that the inclusion of convective effects in computing the electron temperature profile through the mix region is necessary in order to satisfactorily model experimental spectra.

Experimental stark shifts of several HeI and ArI spectral lines

Abstract: Stark shifts of four HeI and six ArI spectral lines have been measured in a linear pulsed arc plasma, superimposed to the glow discharge positive column plasma in helium and argon-helium mixture, respectively. The measured values were compared to the existing data calculated, according to the various theoretical approaches.

Laboratory measurements of absorption coefficients for the 727 nm band of methane at 77 K and comparison with results derived from spectra of the Giant planets

Abstract: A high resolution absorption spectrum of the 727 nm band of methane at 77 K is obtained using the intracavity laser spectroscopy technique. Absorption coefficients are determined and are reported as averages over 1 cm−1 intervals throughout the region 13,470–14,025 cm−1. An intensity of 753 cm−1 km−1 am−1 is obtained for the entire band. Positions of the vibration-rotation lines in this band are determined to an accuracy of ±0.02 cm−1 and, for the stronger lines, absorption coefficients at the center of the absorption line are reported. The results are compared with previous room temperature measurements on methane, with the gas phase band intensity calculated from measurements on liquid methane, and with absorption coefficients derived from methane features observed in the albedo spectra of the Giant planets and Titan.

An improved determination of the 216.615 nm absorption coefficient for methyl radicals

Abstract: Shock tube experiments which incorporate improved selection and handling of chemical precursors provide a corrected determination of 216.615 nm absorption coefficient for methyl radicals at high temperatures (1311–2506 K). The revised expression is approximately one-half that given by us previously.