Showing papers by "Jonathan Tennyson published in 2011"

The 2009 edition of the GEISA spectroscopic database

Abstract: The updated 2009 edition of the spectroscopic database GEISA (Gestion et Etude des Informations Spectroscopiques Atmospheriques; Management and Study of Atmospheric Spectroscopic Information) is described in this paper. GEISA is a computer-accessible system comprising three independent sub-databases devoted, respectively, to: line parameters, infrared and ultraviolet/visible absorption cross-sections, microphysical and optical properties of atmospheric aerosols. In this edition, 50 molecules are involved in the line parameters sub-database, including 111 isotopologues, for a total of 3,807,997 entries, in the spectral range from 10−6 to 35,877.031 cm−1. The successful performances of the new generation of hyperspectral sounders depend ultimately on the accuracy to which the spectroscopic parameters of the optically active atmospheric gases are known, since they constitute an essential input to the forward radiative transfer models that are used to interpret their observations. Currently, GEISA is involved in activities related to the assessment of the capabilities of IASI (Infrared Atmospheric Sounding Interferometer; http://smsc.cnes.fr/IASI/index.htm) on board the METOP European satellite through the GEISA/IASI database derived from GEISA. Since the Metop-A (http://www.eumetsat.int) launch (19 October 2006), GEISA is the reference spectroscopic database for the validation of the level-1 IASI data. Also, GEISA is involved in planetary research, i.e., modeling of Titan's atmosphere, in the comparison with observations performed by Voyager, or by ground-based telescopes, and by the instruments on board the Cassini–Huygens mission. GEISA, continuously developed and maintained at LMD (Laboratoire de Meteorologie Dynamique, France) since 1976, is implemented on the IPSL/CNRS (France) “Ether” Products and Services Centre WEB site (http://ether.ipsl.jussieu.fr), where all archived spectroscopic data can be handled through general and user friendly associated management software facilities. More than 350 researchers are registered for on line use of GEISA.

A variationally computed line list for hot NH3

Abstract: A B S T R A C T We present ‘BYTe’, a comprehensive ‘hot’ line list for the ro-vibrational transitions of ammonia, 14 NH 3, in its ground electronic state. This line list has been computed variationally using the program suite TROVE , a new spectroscopically determined potential energy surface and an ab initio dipole moment surface. BYTe, is designed to be used at all temperatures up to 1500 K. It comprises 1138 323 351 transitions in the frequency range from 0 to 12 000 cm −1 , constructed from 1373 897 energy levels below 18 000 cm −1 having J values ≤36. Comparisons with laboratory data confirm the accuracy of the line list which is suitable for modelling a variety of astrophysical problems including the atmospheres of extrasolar planets and brown dwarfs.

A global, high accuracy ab initio dipole moment surface for the electronic ground state of the water molecule.

Abstract: A highly accurate, global dipole moment surface (DMS) is calculated for the water molecule using ab initio quantum chemistry methods. The new surface is named LTP2011 and is based on all-electron, internally contracted multireference configuration interaction, including size-extensivity corrections in the aug-cc-pCV6Z basis set. Dipoles are computed as energy derivatives and small corrections due to relativistic effects included. The LTP2011 DMS uses an appropriate functional form that guarantees qualitatively correct behaviour even for most high energies configuration (up to about 60,000 cm(-1)), including, in particular, along the water dissociation channel. Comparisons with high precision experimental data show agreement within 1% for medium-strength lines. The new DMS and all the ab initio data are made available in the supplementary material.

Methane in the Atmosphere of the Transiting Hot Neptune GJ436B

Abstract: We present an analysis of seven primary transit observations of the hot Neptune GJ436b at 3.6, 4.5, and 8 μm obtained with the Infrared Array Camera on the Spitzer Space Telescope. After correcting for systematic effects, we fitted the light curves using the Markov Chain Monte Carlo technique. Combining these new data with the EPOXI, Hubble Space Telescope, and ground-based V, I, H, and Ks published observations, the range 0.5-10 μm can be covered. Due to the low level of activity of GJ436, the effect of starspots on the combination of transits at different epochs is negligible at the accuracy of the data set. Representative climate models were calculated by using a three-dimensional, pseudospectral general circulation model with idealized thermal forcing. Simulated transit spectra of GJ436b were generated using line-by-line radiative transfer models including the opacities of the molecular species expected to be present in such a planetary atmosphere. A new, ab-initio-calculated, line list for hot ammonia has been used for the first time. The photometric data observed at multiple wavelengths can be interpreted with methane being the dominant absorption after molecular hydrogen, possibly with minor contributions from ammonia, water, and other molecules. No clear evidence of carbon monoxide and carbon dioxide is found from transit photometry. We discuss this result in the light of a recent paper where photochemical disequilibrium is hypothesized to interpret secondary transit photometric data. We show that the emission photometric data are not incompatible with the presence of abundant methane, but further spectroscopic data are desirable to confirm this scenario.

EChO - Exoplanet Characterisation Observatory

Abstract: A dedicated mission to investigate exoplanetary atmospheres represents a major milestone in our quest to understand our place in the universe by placing our Solar System in context and by addressing the suitability of planets for the presence of life. EChO -the Exoplanet Characterisation Observatory- is a mission concept specifically geared for this purpose. EChO will provide simultaneous, multi-wavelength spectroscopic observations on a stable platform that will allow very long exposures. EChO will build on observations by Hubble, Spitzer and groundbased telescopes, which discovered the first molecules and atoms in exoplanetary atmospheres. EChO will simultaneously observe a broad enough spectral region -from the visible to the mid-IR- to constrain from one single spectrum the temperature structure of the atmosphere and the abundances of the major molecular species. The spectral range and resolution are tailored to separate bands belonging to up to 30 molecules to retrieve the composition and temperature structure of planetary atmospheres. The target list for EChO includes planets ranging from Jupiter-sized with equilibrium temperatures Teq up to 2000 K, to those of a few Earth masses, with Teq ~300 K. We have baselined a dispersive spectrograph design covering continuously the 0.4-16 micron spectral range in 6 channels (1 in the VIS, 5 in the IR), which allows the spectral resolution to be adapted from several tens to several hundreds, depending on the target brightness. The instrument will be mounted behind a 1.5 m class telescope, passively cooled to 50 K, with the instrument structure and optics passively cooled to ~45 K. EChO will be placed in a grand halo orbit around L2. We have also undertaken a first-order cost and development plan analysis and find that EChO is easily compatible with the ESA M-class mission framework.

An upper limit for water dimer absorption in the 750 nm spectral region and a revised water line list

Abstract: Absorption of solar radiation by water dimer molecules in the Earth's atmosphere has the potential to act as a positive feedback effect for climate change There seems little doubt from the results of previous laboratory and theoretical studies that significant concentrations of the water dimer should be present in the atmosphere, yet attempts to detect water dimer absorption signatures in atmospheric field studies have so far yielded inconclusive results Here we report spectral measurements in the near-infrared around 750 nm in the expected region of the | 0〈f | 4〉b|0 〉 overtone of the water dimer's hydrogen-bonded OH stretching vibration The results were obtained using broadband cavity ringdown spectroscopy (BBCRDS), a methodology that allows absorption measurements to be made under controlled laboratory conditions but over absorption path lengths representative of atmospheric conditions In order to account correctly and completely for the overlapping absorption of monomer molecules in the same spectral region, we have also constructed a new list of spectral data (UCL08) for the water monomer in the 750–20 000 cm−1 (13 μm–500 nm) range Our results show that the additional lines included in the UCL08 spectral database provide an improved representation of the measured water monomer absorption in the 750 nm region No absorption features other than those attributable to the water monomer were detected in BBCRDS experiments performed on water vapour samples containing dimer concentrations up to an order of magnitude greater than expected in the ambient atmosphere The absence of detectable water dimer features leads us to conclude that, in the absence of significant errors in calculated dimer oscillator strengths or monomer/dimer equilibrium constants, the widths of any water dimer absorption features present around 750 nm are of the order of 100 cm−1 HWHM, and certainly greater than the 25–30 cm−1 HWHM reported in the literature for lower energy water dimer transitions up to 8000 cm−1

Radiative cooling functions for primordial molecules

Abstract: Cooling of primordial gas plays a crucial role in the birth of the first structures in our Universe. Due to the low fractional abundance of molecular species at high redshifts, spontaneous emission rather than collisions represents the most efficient way to cool the pristine plasma. In the present work, radiative cooling functions are evaluated for the diatomic species HD, HD + ,H eH + , LiH and LiH + . Cooling functions for the triatomic ions H + and H2D + are also considered. Analytic fits as functions of temperature are provided.

Positron collisions with molecular hydrogen: cross sections and annihilation parameters calculated using the R-matrix with pseudo-states method

Abstract: The molecular R-matrix with pseudo-states (MRMPS) method is employed to study positron collisions with H2. The calculations employ pseudo-continuum orbital sets containing up to h (l = 5) functions. Use of these high l functions is found to give converged eigenphase sums. Below the positronium formation threshold, the calculated cross sections agree with other high-accuracy theories and generally with the measurements. Calculation of the positron annihilation parameter Zeff with the MRMPS wavefunctions gives values significantly higher than other R-matrix wavefunctions but still do not completely converge with h functions. Extrapolation to higher l-values leads to a predicted value of Zeff for H2 of about 10.4. The MRMPS method is both completely general and ab initio; it can therefore be applied to positron collisions with other molecular targets.

VAMDC - The Virtual Atomic and Molecular Data Centre - A new way to disseminate atomic and molecular data - VAMDC level 1 release

Abstract: The Virtual Atomic and Molecular Data Centre (VAMDC, http://www.vamdc.eu/) is a European‐Union‐funded collaboration between groups involved in the generation, evaluation, and use of atomic and molecular data. VAMDC aims to build a reliable, open, flexible and interoperable e‐science interface to existing atomic and molecular data. The project will cover establishing the core consortium, the development and deployment of the infrastructure and the development of interfaces to the existing atomic and molecular databases. This paper describes the organisation of the project and the achievements during its first year.

Bound and continuum states of molecular anions C2H― and C3N―

Abstract: Recently a number of molecular anions, closed-shell linear carbon chains of the form CnH− and CnN−, have been detected in space. The molecules C2H− and C3N− are investigated by using the R-matrix method to consider electron scattering from the corresponding neutral targets. Initial target calculations are conducted and refined in order to produce target state characteristics similar to the experimental data. A number of different scattering models are tested including static exchange and close-coupling models, and the use of Hartree–Fock or natural orbitals in the close-coupling calculations. The calculations concentrate on bound and resonances states for the anions as well as eigenphase sums, elastic cross-sections and electronic excitation cross-sections for electron collisions with the neutral. It is found that electronic resonances are all too high in energy to be important for anion formation in the interstellar medium. However, C3N−, unlike C2H−, supports a number of very weakly bound excited states, which may well provide the route to electron attachment for this system.

Positron collisions with acetylene calculated using the R-matrix with pseudo-states method

Abstract: Eigenphase sums, total cross sections and differential cross sections are calculated for low-energy collisions of positrons with C2H2. The calculations demonstrate that the use of appropriate pseudo-state expansions very significantly improves the representation of this process giving both realistic eigenphases and cross sections. Differential cross sections are strongly forward peaked in agreement with the measurements. These calculations are computationally very demanding; even with improved procedures for matrix diagonalization, fully converged calculations are too expensive with current computer resources. Nonetheless, the calculations show clear evidence for the formation of a virtual state but no indication that acetylene actually binds a positron at its equilibrium geometry.

High Accuracy Rotation — Vibration Calculations on Small Molecules

Abstract: The first-principles calculation of the spectra of triatomic and increasingly tetratomic molecules is beginning to make a significant impact on high-resolution spectroscopy as well as astrophysics and atmospheric physics The variational nuclear motion methods combined with high accuracy ab initio electronic structure calculations can be used to make spectral predictions of increasing accuracy with very few a priori assumptions This work is important for spectral analysis and is becoming particularly so for dipole transition intensities, which are often very difficult to obtain reliably from experiment but are essential for modeling and remote sensing In addition, demands for very extended linelists covering several millions of transitions that are required to reproduce spectra of hot sources are best met using high-quality theoretical models High accuracy can only be obtained by explicitly considering the so-called minor effects such as relativistic effects on the electronic motion and failure of the Born–Oppenheimer approximation H3+ and its isotopologues are discussed as a benchmark system for high-accuracy calculations Keywords: variational methods; electronic structure calculations; transition intensities; Born–Oppenheimer approximation; relativistic corrections

Exoplanet Atmospheres: Physical Processes, by Sara Seager

Abstract: Exoplanet Atmospheres: Physical Processes, by Sara Seager, Princeton, Princeton University Press, 2010, 264 pp, £3095 (hardback), ISBN 978-0-691-14645-4 Scope: reference Level: postgraduate, ea

Rotational Cooling of HD+ by Superelastic Collisions with Electrons

Abstract: Rotational cooling of HD+ by superelastic collisions (SEC) with electrons was observed at the Heidelberg test storage ring by merging a beam of rotationally hot HD+ ions with an electron beam at zero relative energy. Neutral fragments resulting from DR events were recorded at different electron densities using a high resolution imaging detector and a large-area, energy sensitive detector. The data allowed to deduce the time dependence of the population of three groups of rotational angular momentum states J built on the vibrational ground state of the ion together with the corresponding DR rate coefficients. The latter are found to be (statistical uncertainties only) α0,1,2 = 3.8(1), α3,4 = 4.0(2), and α5,6,7 = 9.0(1.3) in units of 10−8 cm3/s, in reasonable agreement with the average values derived within the MQDT approach. The time evolution of the population curves clearly reveals that rotational cooling by SEC takes place, which can be well described by using theoretical SEC rate coefficients obtained by combining the molecular R-matrix approach with the adiabatic nuclear rotation approximation. We verify the ΔJ = −2 coefficients, which are predicted to be dominant as opposed to the ΔJ = −1 coefficients and to amount to (1 − 2) 10−6 cm3/s, to within 30%.

Electron collisions with BF+: bound and continuum states of BF

Abstract: Rydberg and continuum states of the BF molecule are studied as a function of geometry using an electron collision formalism in the framework of the R-matrix method. Up to 14 BF+ target states are used in a close-coupling expansion and bound states are searched for as negative energy solutions of the scattering calculation. Potential energy curves and quantum defects are obtained for the excited states of BF. Resonance positions and widths are also calculated for Feshbach resonances in the system. The data obtained can be used to model dissociative recombination of the BF+ molecular ion.

R-matrix with Pseudo-States (RMPS) method: application to CH + resonances curves

Abstract: In a series of calculations on both electron and positron collisions with small molecules the R-Matrix with Pseudo-States (RMPS) method has been found to recover polarisation effects neglected in other close-coupling methods including the standard R-matrix procedure. The molecular R-Matrix and RMPS methods is being applied to determine low-lying resonance states of CH+ as a function of internuclear separation. Initial results are presented for both a standard R-matrix close-coupling model and for an RMPS calculation. Eigenphase sums and resonances below the 3Π threshold are presented for 2Π total symmetry. These resonances are classified by their quantum defects and compared to previous results. Prospects for these and other calculations using the RMPS method are discussed.

Characterising the Atmospheres of Transiting Planets with a Dedicated Space Telescope

Abstract: Exoplanetary science is among the fastest evolving fields of today's astronomical research. Ground-based planet-hunting surveys alongside dedicated space missions (Kepler, CoRoT) are delivering an ever-increasing number of exoplanets, now numbering at ~690, with ESA's GAIA mission planned to bring this number into the thousands. The next logical step is the characterisation of these worlds: what is their nature? Why are they as they are? The use of the HST and Spitzer Space Telescope to probe the atmospheres of transiting hot, gaseous exoplanets has demonstrated that it is possible with current technology to address this ambitious goal. The measurements have also shown the difficulty of understanding the physics and chemistry of these environments when having to rely on a limited number of observations performed on a handful of objects. To progress substantially in this field, a dedicated facility for exoplanet characterization with an optimised instrument design (detector performance, photometric stability, etc.), able to observe through time and over a broad spectral range a statistically significant number of planets, will be essential. We analyse the performances of a 1.2/1.4m space telescope for exoplanet transit spectroscopy from the visible to the mid IR, and present the SNR ratio as function of integration time and stellar magnitude/spectral type for the acquisition of spectra of planetary atmospheres in a variety of scenarios: hot, warm, and temperate planets, orbiting stars ranging in spectral type from hot F to cool M dwarfs. We include key examples of known planets (e.g. HD 189733b, Cancri 55 e) and simulations of plausible terrestrial and gaseous planets, with a variety of thermodynamical conditions. We conclude that even most challenging targets, such as super-Earths in the habitable-zone of late-type stars, are within reach of a M-class, space-based spectroscopy mission.

The R-matrix Calculations of Orientation and Coulomb Phase Effects in Electron–Molecule (Re-)Collisions

Abstract: Electron recollision in strong laser fields is usually studied with oriented molecules. This introduces orientation effects into the recollision problem which are generally not present in usual treatments of electron–molecule collisions. In addition this collision occurs with a molecular ion which means that the dominant electron– molecule interaction is the long-range Coulomb potential, which competes asymptotically with the strong laser field. Different workers have performed treatments varying between the complete inclusion of all asymptotic Coulomb effects to their complete neglect. Three possible treatments of the Coulomb problem are explored using H2 and CO2 as prototypical systems. Calculations based on R-matrix studies of the (re-)collision, which neglect the effects of the laser field, show that inclusion of the complete Coulomb interaction leads not only to the well-known singularity problems for forward scattering but also leads to the washing out of much of the detailed, angular structure in the differential cross section of the oriented molecules.

Electron-molecular cation reactive collisions : from channel mixing to competitive processes

Abstract: The competition between dissociative recombination, vibrational excitation, and dissociative excitation of molecular cations in electron-impact collisions is discussed within the formalism of the Multichannel Quantum Defect Theory. Illustrative results are given for the HD+/HD and CO+/CO systems.

Simulations of SF 6 plasma etching in the GEC Reference Cell

Abstract: Here we present 2D simulations of an inductively driven SF 6 silicon etch process in the GEC Reference Cell, performed with Quantemol-D and building upon previous calculations of SF 6 plasma chemistries using Quantemol-P. Pressure and power trends along with chamber wide contour plots of gas-phase species concentrations and fundamental plasma properties are considered. We find good agreement with experimental results, which both validates the underlying model and points to the important role of simulation-assisted plasma process development and optimization.

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Abstract: $EVWUDFWElectron recollision in strong laser fields is usually studied with oriented molecules. This introduces orientation effects into the recollision problem which are generally not present in usual treatments of electron–molecule collisions. In addition this collision occurs with a molecular ion which means that the dominant electron– molecule interaction is the long-range Coulomb potential, which competes asymptotically with the strong laser field. Different workers have performed treatments varying between the complete inclusion of all asymptotic Coulomb effects to their complete neglect. Three possible treatments of the Coulomb problem are explored using H2 and CO2 as prototypical systems. Calculations based onR-matrix studies of the (re-)collision, which neglect the effects of the laser field, show that inclusion of the complete Coulomb interaction leads not only to the well-known singularity problems for forward scattering but also leads to the washing out of much of the detailed, angular structure in the differential cross section of the oriented molecules. �,QWURGXFWLRQ The detailed treatment of the dynamics of a molecule trapped in a strong laser field presents a number of theoretical challenges. This is particularly true when considering electron recollision as for this case it is necessary to treat effects due to the field and the physics of the recollision process itself. In the context of electron–molecule collisions, the recollision problem actually involves the ionized electron recollid

Radiative cooling functions for primordial molecules

Abstract: Cooling of primordial gas plays a crucial role in the birth of the first structures in our Universe. Due to the low fractional abundance of molecular species at high redshifts, spontaneous emission rather than collisions represents the most efficient way to cool the pristine plasma. In the present work, radiative cooling functions are evaluated for the diatomic species HD, HD$^+$, HeH$^+$, LiH and LiH$^+$. Cooling functions for the triatomic ions H$_3^+$ and H$_2$D$^+$ are also considered. Analytic fits as functions of temperature are provided.