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[en] It is not often that I have the opportunity to write something that is not a scientific study, and I am thankful for this opportunity to express my thoughts with regard to pursuits that are greater than those of science. In this van de Hulst assay, to honor Hendrik van de Hulst, I briefly summarize a few points from Hendrik's life that I find especially interesting, including his interests in spiritual (or religious) aspects of life, and his decision to avoid involvement in nucleation problems, a critical and basic uncertainty of current climate models. Finally, after that, I present briefly a few episodes from my own experiences as an apprentice of science and life.
[en] In this paper, we report experimental measurements of spectral line shape parameters (air-broadened width, shift, and line mixing coefficients) for several transitions in the ν3 Q branch of methane in the 3000–3023 cm-1 region. 13 high-resolution, room temperature laboratory spectra of pure methane and air-broadened methane recorded with two different Fourier transform spectrometers are fitted. 12 of these spectra were acquired at 0.01 cm-1 resolution with the McMath-Pierce FTS at the National Solar Observatory on Kitt Peak, and one higher-resolution (~0.0011 cm-1) low pressure methane spectrum was obtained with the Bruker IFS-120HR FTS at the Pacific Northwest National Laboratory, in Richland, Washington. All the spectra were obtained using high purity natural samples of CH4 and lean mixtures of the same natural CH4 in dry air. For the 12 spectra recorded at Kitt Peak, three different absorption cells (L= 5, 25 and 150 cm) were used while the methane spectrum at PNNL was obtained using a 19.95 cm long absorption cell. For the analysis, an interactive multispectrum nonlinear least squares fitting software was employed where all the 13 spectra were fitted simultaneously. An accurate and self-consistent set of line parameters were determined by constraining a few of those for severely blended transitions. Line mixing was measured for fourteen transition pairs for the CH4-air collision system. Lastly, a constant speed dependence parameter, consistent with measured speed dependence values obtained in other methane bands, was applied to all the transitions included in the fitted region. The present measurements are compared to values reported in the literature.
[en] Linear particle transport in stochastic media is key to such relevant applications as neutron diffusion in randomly mixed immiscible materials, light propagation through engineered optical materials, and inertial confinement fusion, only to name a few. We extend the pioneering work by Adams, Larsen and Pomraning (recently revisited by Brantley) by considering a series of benchmark configurations for mono-energetic and isotropic transport through Markov binary mixtures in dimension d. The stochastic media are generated by resorting to Poisson random tessellations in 1d slab, 2d extruded, and full 3d geometry. For each realization, particle transport is performed by resorting to the Monte Carlo simulation. The distributions of the transmission and reflection coefficients on the free surfaces of the geometry are subsequently estimated, and the average values over the ensemble of realizations are computed. Reference solutions for the benchmark have never been provided before for two- and three-dimensional Poisson tessellations, and the results presented in this paper might thus be useful in order to validate fast but approximated models for particle transport in Markov stochastic media, such as the celebrated Chord Length Sampling algorithm. (authors)
[en] A tensorial formalism adapted to the case of symmetric-top molecules has been recently developed in Dijon (El Hilali et al., J Quant Spectrosc Radiat Transf 2010;111,1305-1315). It is based on the O3 (is a subset of) C∞ν (is a subset of) C3ν group chain and allows a systematic expansion of the Hamiltonian and dipole moment operators, including all possible interactions for a given ro-vibrational polyad system. This suite of programs is used to analyze and simulate the mid-infrared spectrum of methyl iodide in order to test its performance. The v6 fundamental band has been recorded using the FTIR Bruker HR125 at the AILES beamline of the SOLEIL Synchrotron facility with the Globar internal source. 3882 lines up to J=61 were assigned and fitted with only 16 tensorial parameters. (authors)
[en] The X-rays emitted by a rubidium plasma source created by the PHELIX laser at an intensity of about 6*10"1"4 W/cm"2 were studied. The lines have been measured using Bragg crystals in the wavelength range between 3.8 and 7.3 Angstroms and identified by means of a numerical method developed to describe highly charged rubidium ions in LTE plasma. The experimental plasma temperature, density and charge state distributions have been estimated using non-LTE codes such as CHIVAS and AVERROES. The LTE plasma temperature and density used in the calculations are those allowing to reproduce the calculated NLTE charge state distribution. In order to optimize the use of computational resources, a criterion is established to select the configurations contributing most to the spectra among all those obtained in detailed level accounting based on the MCDF code. Seventy Rb-X-rays have been identified among which forty-nine are reported for the first time. The capabilities of our method are demonstrated by the good agreement of our identifications with previously published data when available. (authors)
[en] An improved spectroscopy is used to implement and optimize the retrieval strategy of ethane (C2H6) from ground-based Fourier Transform Infrared (FTIR) solar spectra recorded at the high-altitude station of Jungfraujoch (Swiss Alps, 46.51 N, 8.01 E, 3580 ma.s.l.). The improved spectroscopic parameters include C2H6 pseudo-lines in the 2720-3100 cm-1 range and updated line parameters form ethyl-chloride and ozone.These improved spectroscopic parameters allow for substantial reduction of the fitting residuals as well as enhanced in formation content. They also contribute to limiting oscillations responsible for un-geophysical negative mixing ratio profiles. This strategy has been successfully applied to the Jungfraujoch solar spectra available from 1994 onwards. The resulting time series is compared with C2H6 total columns simulated by the state-of-the-art chemical transport model GEOS-Chem. Despite very consistent seasonal cycles between both datasets, a negative systematic bias relative to the FTIR observations suggests that C2H6 emissions are underestimated in the current inventories implemented in GEOS-Chem. Finally, C2H6 trends are derived from the FTIR time series, revealing a statistically-significant sharp increase of the C2H6 burden in the remote atmosphere above Jungfraujoch since 2009. Evaluating cause of this change in the C2H6 burden, which may be related to the recent massive growth of shale gas exploitation in North America, is of primary importance for atmospheric composition and air quality in the Northern Hemisphere. (authors)
[en] Using a semiclassical approach for the Stark broadening of atomic lines, we have calculated electron and proton impact line widths and shifts of 56 neutral He lines in the ultraviolet, visible and i.r. region of the spectrum. The comprehensive set of results obtained is used for investigation of Stark-broadening parameter regularities within the spectral series. (author)
[en] A brief description is given of the different quantum-mechanical models and approximations used in hot plasma calculations. The subject is elucidated by presentation of computed data for iron and astrophysical plasmas. (author)
[en] A method for obtaining values of the widths and other parameters describing the Gaussian and Lorentzian components of peaks, which have the shape of the convolution of those functions, is described. Results of the method are given for X-ray data containing several peaks with different ratios of the composite widths in separate spectra. (author)
[en] The intensities of lines emitted from the solar atmosphere and observed on earth are calculated by considering electron-impact excitation followed by radiative emission. In order to determine the contribution of dielectronic-recombination-excitation (DRE), the calculations have been repeated with the inclusion of DRE as a process for excitation, along with electron-impact excitation; the excited electron is assumed to deexcite through radiative emission. The DRE is found to contribute significantly and the contribution generally increases with a decrease in wavelength and an increase of ionic charge. (author)