An-Wen Liu

Bio: An-Wen Liu is an academic researcher from University of Science and Technology of China. The author has contributed to research in topics: Cavity ring-down spectroscopy & Absorption spectroscopy. The author has an hindex of 22, co-authored 88 publications receiving 1631 citations. Previous affiliations of An-Wen Liu include Centre national de la recherche scientifique & Joseph Fourier University.

A b i n i t i o calculations of the spectral shapes of CO 2 isolated lines including non-Voigt effects and comparisons with experiments

Abstract: Wepresentafully ab initiomodelandcalculationsofthespectralshapesofabsorptionlinesinapuremolecular gas under conditions where the influences of collisions and of the Doppler effect are significant. Predictions of the time dependence of dipole autocorrelation functions (DACFs) are made for pure CO2 at room temperature using requantized classical molecular dynamics simulations. These are carried, free of any adjusted parameter, on the basis of an accurate anisotropic intermolecular potential. The Fourier-Laplace transforms of these DACFs then yield calculated spectra which are analyzed, as some measured ones, through fits using Voigt line profiles. Comparisonsbetweentheoryandvariousexperimentsnotonlyshowthatthemainline-shapeparameters(Lorentz pressure-broadening coefficients) are accurately predicted, but that subtle observed non-Voigt features are also quantitatively reproduced by the model. These successes open renewed perspectives for the understanding of the mechanisms involved (translational-velocity and rotational-state changes and their dependences on the molecular speed) and the quantification of their respective contributions. The proposed model should also be of great help for the test of widely used empirical line-shape models and, if needed, the construction of more physically based ones.

The 1.58 μm transparency window of methane (6165–6750 cm−1): Empirical line list and temperature dependence between 80 and 296 K

Abstract: The empirical line parameters of over 12,000 methane transitions have been obtained at 80 K in the 1.58 μm transparency window (6165–6750 cm −1 ) which is of importance for planetary applications. This line list (WKC-80K) was constructed from high sensitivity spectra of normal abundance methane recorded by CW-Cavity Ring Down Spectroscopy at low temperature. The minimum intensity reported is on the order of 5×10 −30 cm/molecule. High resolution Fourier transform spectra have also been recorded using enriched CH 3 D samples at 90–120 K in order to facilitate identification of monodeuterated methane features in the methane line list at 80 K. The CH 3 D relative contribution in the considered region is observed to be much larger at 80 K than at room temperature. In particular, CH 3 D is found dominant in a narrow spectral window near 6300 cm −1 corresponding to the highest transparency region. Using a similar line list constructed at room temperature (Campargue A, Wang L, Liu AW, Hu SM and Kassi S. Empirical line parameters of methane in the 1.63–1.48 μm transparency window by high sensitivity Cavity Ring Down Spectroscopy. Chem Phys 2010;373:203–10.), the low energy values of the transitions observed both at 80 K and at room temperature were derived from the variation of their line intensities. Empirical lower states and J -values have been obtained for 5671 CH 4 and 1572 CH 3 D transitions representing the most part of the absorbance in the region. The good quality of these derived energy values is demonstrated by the marked propensity of the corresponding CH 4 lower state J values to be close to integers. The WKC line lists at 80 K and room temperature provided as Supplementary Material allow one accounting for the temperature dependence of methane absorption between these two temperatures. The importance of the 80 K line list for the study of Titan and other methane containing planetary atmospheres is underlined and further improvements are proposed. The resulting information will advance the theoretical modeling of the methane spectrum in the 1.58 μm transparency window.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

The HITRAN 2008 molecular spectroscopic database

Abstract: This paper describes the contents of the 2016 edition of the HITRAN molecular spectroscopic compilation. The new edition replaces the previous HITRAN edition of 2012 and its updates during the intervening years. The HITRAN molecular absorption compilation is composed of five major components: the traditional line-by-line spectroscopic parameters required for high-resolution radiative-transfer codes, infrared absorption cross-sections for molecules not yet amenable to representation in a line-by-line form, collision-induced absorption data, aerosol indices of refraction, and general tables such as partition sums that apply globally to the data. The new HITRAN is greatly extended in terms of accuracy, spectral coverage, additional absorption phenomena, added line-shape formalisms, and validity. Moreover, molecules, isotopologues, and perturbing gases have been added that address the issues of atmospheres beyond the Earth. Of considerable note, experimental IR cross-sections for almost 300 additional molecules important in different areas of atmospheric science have been added to the database. The compilation can be accessed through www.hitran.org. Most of the HITRAN data have now been cast into an underlying relational database structure that offers many advantages over the long-standing sequential text-based structure. The new structure empowers the user in many ways. It enables the incorporation of an extended set of fundamental parameters per transition, sophisticated line-shape formalisms, easy user-defined output formats, and very convenient searching, filtering, and plotting of data. A powerful application programming interface making use of structured query language (SQL) features for higher-level applications of HITRAN is also provided.

The HITRAN2020 molecular spectroscopic database

Abstract: The HITRAN database is a compilation of molecular spectroscopic parameters. It was established in the early 1970s and is used by various computer codes to predict and simulate the transmission and emission of light in gaseous media (with an emphasis on terrestrial and planetary atmospheres). The HITRAN compilation is composed of five major components: the line-by-line spectroscopic parameters required for high-resolution radiative-transfer codes, experimental infrared absorption cross-sections (for molecules where it is not yet feasible for representation in a line-by-line form), collision-induced absorption data, aerosol indices of refraction, and general tables (including partition sums) that apply globally to the data. This paper describes the contents of the 2020 quadrennial edition of HITRAN. The HITRAN2020 edition takes advantage of recent experimental and theoretical data that were meticulously validated, in particular, against laboratory and atmospheric spectra. The new edition replaces the previous HITRAN edition of 2016 (including its updates during the intervening years). All five components of HITRAN have undergone major updates. In particular, the extent of the updates in the HITRAN2020 edition range from updating a few lines of specific molecules to complete replacements of the lists, and also the introduction of additional isotopologues and new (to HITRAN) molecules: SO, CH3F, GeH4, CS2, CH3I and NF3. Many new vibrational bands were added, extending the spectral coverage and completeness of the line lists. Also, the accuracy of the parameters for major atmospheric absorbers has been increased substantially, often featuring sub-percent uncertainties. Broadening parameters associated with the ambient pressure of water vapor were introduced to HITRAN for the first time and are now available for several molecules. The HITRAN2020 edition continues to take advantage of the relational structure and efficient interface available at www.hitran.org and the HITRAN Application Programming Interface (HAPI). The functionality of both tools has been extended for the new edition.