arXiv subjects

Jonathan Tennyson

Publications and source records attributed to Jonathan Tennyson.

At least 145 records · Page 8Linked to original sources

arXiv2017

A parallel algorithm for Hamiltonian matrix construction in electron-molecule collision calculations: MPI-SCATCI

Construction and diagonalization of the Hamiltonian matrix is the rate-limiting step in most low-energy electron -- molecule collision calculations. Tennyson (J Phys B, 29 (1996) 1817) implemented a novel algorithm for Hamiltonian construction which took advantage of the structure of the wavefunction in such calculations. This algorithm is re-engineered to make use of modern computer architectures and the use of appropriate diagonalizers is considered. Test calculations demonstrate that significant speed-ups can be gained using multiple CPUs. This opens the way to calculations which consider higher collision energies, larger molecules and / or more target states. The methodology, which is implemented as part of the UK molecular R-matrix codes (UKRMol and UKRMol+) can also be used for studies of bound molecular Rydberg states, photoionisation and positron-molecule collisions.

physics.comp-ph↗

arXiv2017

ExoMol molecular line lists - XXIII: spectra of PO and PS

Comprehensive line lists for phosphorus monoxide ($^{31}$P$^{16}$O) and phosphorus monosulphide ($^{31}$P$^{32}$S) in their $X$ $^2Π$ electronic ground state are presented. The line lists are based on new ab initio potential energy (PEC), spin-orbit (SOC) and dipole moment (DMC) curves computed using the MRCI+Q-r method with aug-cc-pwCV5Z and aug-cc-pV5Z basis sets. The nuclear motion equations (i.e. the rovibronic Schrödinger equations for each molecule) are solved using the program Duo. The PECs and SOCs are refined in least-squares fits to available experimental data. Partition functions, $Q(T)$, are computed up to $T=$ 5000 K, the range of validity of the line lists. These line lists are the most comprehensive available for either molecule. The characteristically sharp peak of the $Q$-branches from the spin-orbit split components give useful diagnostics for both PO and PS in spectra at infrared wavelengths. These line lists should prove useful for analysing observations and setting up models of environments such as brown dwarfs, low-mass stars, O-rich circumstellar regions and potentially for exoplanetary retrievals. Since PS is yet to be detected in space, the role of the two lowest excited electronic states ($a$ $^4Π$ and $B$ $^2Π$) are also considered. An approximate line list for the PS $X - B$ electronic transition, which predicts a number of sharp vibrational bands in the near ultraviolet, is also presented. he line lists are available from the CDS (cdsarc.u-strasbg.fr) and ExoMol (www.exomol.com) databases.

astro-ph.SR↗

arXiv2017

A hybrid line list for CH$_4$ and hot methane continuum

Molecular line lists are important for modelling absorption and emission processes in atmospheres of different astronomical objects, such as cool stars and exoplanets. In order to be applicable for high temperatures, line lists for molecules like methane must contain billions of transitions, which makes their direct (line-by-line) application in radiative transfer calculations impracticable. Here we suggest a new, hybrid line list format to mitigate this problem, based on the idea of temperature-dependent absorption continuum. The line list is partitioned into a large set of relatively weak lines and a small set of important, stronger lines. The weaker lines are then used either to construct a temperature-dependent (but pressure-independent) set of intensity cross sections or are blended into a greatly reduced set of super-lines. The strong lines are kept in the form of temperature independent Einstein A coefficients. A line list for methane is constructed as a combination of 17 million strong absorption lines relative to the reference absorption spectra and a background methane continuum in two temperature-dependent forms, of cross sections and super-lines. This approach eases the use of large high temperature line lists significantly as the computationally expensive calculation of pressure dependent profiles only need to be performed for a relatively small number of lines. Both the line list and cross sections were generated using a new 34 billion methane line list (34to10), which extends the 10to10 line list to higher temperatures (up to 2000 K). The new hybrid scheme can be applied to any large line lists containing billions of transitions. We recommend to use super-lines generated on a high resolution grid based on resolving power (R = 1,000,000) to model the molecular continuum as a more flexible alternative to the temperature dependent cross sections.

astro-ph.EP↗

arXiv2017

Laboratory spectra of hot molecules: data needs for hot super-Earth exoplanets

The majority of stars are now thought to support exoplanets. Many of those exoplanets discovered thus far are categorized as rocky objects with an atmosphere. Most of these objects are however hot due to their short orbital period. Models suggest that water is the dominant species in their atmospheres. The hot temperatures are expected to turn these atmospheres into a (high pressure) steam bath containing remains of melted rock. The spectroscopy of these hot rocky objects will be very different from that of cooler objects or hot gas giants. Molecules suggested to be important for the spectroscopy of these objects are reviewed together with the current status of the corresponding spectroscopic data. Perspectives of building a comprehensive database of linelist/cross sections applicable for atmospheric models of rocky super-Earths as part of the ExoMol project are discussed. The quantum-mechanical approaches used in linelist productions and their challenges are summarized.

astro-ph.EP↗

arXiv2017

ExoMol Line List XXI: Nitric Oxide (NO)

Line lists for the ground electronic ground state for six major isotopologues of nitric oxide are presented. The line lists are constructed using empirical energy levels (and line positions) and high-level {\it ab inito} intensities. The energy levels were obtained using a combination of two approaches, from an effective Hamiltonian and from solving the rovibronic Schrödinger equation variationally. The effective hamiltonian model was obtained through a fit to the experimental line positions of NO available in the literature for all six isotopologues using the programs SPFIT and SPCAT. The variational model was built through a least squares fit of the \textit{ab inito} potential and spin-orbit curves to the experimentally derived energies and experimental line positions of the main isotopologue only using the Duo program. The \textit{ab inito} potential energy, spin-orbit and dipole moment curves (PEC, SOC and DMC) are computed using high-level {\it ab inito} methods and the MARVEL method is used to obtain energies of NO from experimental transition frequencies. Each line list covers a wavenumber range from 0 - 40,000 \cm with approximately 22,000 rovibronic states and 2.3-2.6 million transitions extending to $J_{max} = 184.5$ and $v_{max} = 51$. Partition functions are also calculated up to a temperature of 5000 K. The calculated absorption line intensities at 296 K using these line lists show excellent agreement with those included in the HITRAN and HITEMP databases. The computed NO line lists are the most comprehensive to date, covering a wider wavenumber and temperature range compared to both the HITRAN and HITEMP databases. These line lists are also more accurate than those used in HITEMP. The full line lists are available from the CDS and ExoMol databases; data will also be available from CDMS.

astro-ph.EP↗

arXiv2017

Lightning Chemistry on Earth-like Exoplanets

We present a model for lightning shock induced chemistry that can be applied to atmospheres of arbitrary H/C/N/O chemistry, hence for extrasolar planets and brown dwarfs. The model couples hydrodynamics and the STAND2015 kinetic gas-phase chemistry. For an exoplanet analogue to the contemporary Earth, our model predicts NO and NO2 yields in agreement with observation. We predict height-dependent mixing ratios during a storm soon after a lightning shock of NO ~ 1e-3 at 40 km and NO2 ~ 1e-4 below 40 km, with O3 reduced to trace quantities (<< 1e-10). For an Earth-like exoplanet with a CO2/N2 dominated atmosphere and with an extremely intense lightning storm over its entire surface, we predict significant changes in the amount of NO, NO2, O3, H2O, H2, and predict significant abundance of C2N. We find that, for the Early Earth, O2 is formed in large quantities by lightning but is rapidly processed by the photochemistry, consistent with previous work on lightning. The effect of persistent global lightning storms are predicted to be significant, primarily due to NO2, with the largest spectral features present at ~3.4 μm and ~6.2 μm. The features within the transmission spectrum are on the order of 1 ppm and therefore are not likely detectable with JWST. Depending on its spectral properties, C2N could be a key tracer for lightning on Earth-like exoplanets with a N2/CO2 bulk atmosphere, unless destroyed by yet unknown chemical reactions.

astro-ph.EP↗

arXiv2017

Simulating electric field interactions with polar molecules using spectroscopic databases

Ro-vibrational Stark-associated phenomena of small polyatomic molecules are modelled using extensive spectroscopic data generated as part of the ExoMol project. The external field Hamiltonian is built from the computed ro-vibrational line list of the molecule in question. The Hamiltonian we propose is general and suitable for any polar molecule in the presence of an electric field. By exploiting precomputed data, the often prohibitively expensive computations associated with high accuracy simulations of molecule-field interactions are avoided. Applications to strong terahertz field-induced ro-vibrational dynamics of PH$_3$ and NH$_3$, and spontaneous emission data for optoelectrical Sisyphus cooling of H$_2$CO and CH$_3$Cl are discussed.

physics.chem-ph↗

arXiv2017

QDB: a new database of plasma chemistries and reactions

One of the most challenging and recurring problems when modelling plasmas is the lack of data on key atomic and molecular reactions that drive plasma processes. Even when there are data for some reactions, complete and validated datasets of chemistries are rarely available. This hinders research on plasma processes and curbs development of industrial applications. The QDB project aims to address this problem by providing a platform for provision, exchange, and validation of chemistry datasets. A new data model developed for QDB is presented. QDB collates published data on both electron scattering and heavy-particle reactions. These data are formed into reaction sets, which are then validated against experimental data where possible. This process produces both complete chemistry sets and identifies key reactions that are currently unreported in the literature. Gaps in the datasets can be filled using established theoretical methods. Initial validated chemistry sets for SF$_6$/CF$_4$/O$_2$ and SF$_6$/CF$_4$/N$_2$/H$_2$ are presented as examples.

physics.plasm-ph↗

arXiv2017

Molecular line shape parameters for exoplanetary atmospheric applications

We describe the recent updates to the ExoMol database regarding the molecular spectral line shapes. ExoMol provides comprehensive molecular line lists with a special emphasis on the applications involving characterization of atmospheres of exoplanets and cool stars. Among important requirements of such applications are (i) the broadening parameters for hydrogen and helium dominating atmospheres and (ii) very broad ranges of temperature and pressures. The current status of the available line shape data in the literature, demands from the exoplanetary community and their specific needs are discussed.

astro-ph.EP↗

arXiv2017

ExoMol molecular line lists XX: a comprehensive line list for H$_3^+$

H$_3^+$ is a ubiquitous and important astronomical species whose spectrum has been observed in the interstellar medium, planets and tentatively in the remnants of supernova SN1897a. Its role as a cooler is important for gas giant planets and exoplanets, and possibly the early Universe. All this makes the spectral properties, cooling function and partition function of H$_3^+$ key parameters for astronomical models and analysis. A new high-accuracy, very extensive line list for H$_3^+$ called MiZATeP was computed as part of the ExoMol project alongside a temperature-dependent cooling function and partition function as well as lifetimes for %individual excited states. These data are made available in electronic form as supplementary data to this article and at http://www.exomol.com

astro-ph.GA↗

arXiv2017

MARVEL analysis of the measured high-resolution rovibronic spectra of $^{48}$Ti$^{16}$O

Accurate, experimental rovibronic energy levels, with associated labels and uncertainties, are reported for 11 low-lying electronic states of the diatomic \TiO\ molecule, determined using the \Marvel\ (Measured Active Rotational-Vibrational Energy Levels) algorithm. All levels are based on lines corresponding to critically reviewed and validated high-resolution experimental spectra taken from 24 literature sources. The transition data are in the 2 $-$ 22,160 \cm{} region. Out of the 49,679 measured transitions, 43,885 are triplet-triplet, 5710 are singlet-singlet and 84 are triplet-singlet transitions. A careful analysis of the resulting experimental spectroscopic network (SN) allows 48,590 transitions to be validated. The transitions determine 93 vibrational band origins of \TiO\, including 71 triplet and 22 singlet ones. There are 276 (73) triplet-triplet (singlet-singlet) band-heads derived from \Marvel\ experimental energies, 123 (38) of which have never been assigned in low or high resolution experiments. The highest $J$ value, where $J$ stands for the total angular momentum, for which an energy level is validated is 163. The number of experimentally-derived triplet and singlet \TiO\ rovibrational energy levels is 8682 and 1882, respectively. The lists of validated lines and levels for \TiO\ are deposited in the Supporting Information to this paper.

astro-ph.SR↗

arXiv2017

ExoMol molecular line lists XIX: high accuracy computed hot line lists for H$_2$$^{18}$O and H$_2$$^{17}$O

Hot line lists for two isotopologues of water, \octo\ and \heto, are presented. The calculations employ newly constructed potential energy surfaces (PES) which take advantage of a novel method for using the large set of experimental energy levels for \hato\ to give high quality predictions for \octo\ and \heto. This procedure greatly extends the energy range for which a PES can be accurately determined, allowing accurate prediction of higher-lying energy levels than are currently known from direct laboratory measurements. This PES is combined with a high-accuracy, {\it ab initio} dipole moment surface of water in the computation of all energy levels, transition frequencies and associated Einstein A coefficients for states with rotational excitation up to $J=50$ and energies up to 30~000 \cm. The resulting HotWat78 line lists complement the well-used BT2 \hato\ line list (Barber et.al, 2006, MNRAS, {\bf 368}, 1087). Full line lists are made available in the electronic form as supplementary data to this article and at \url{www.exomol.com}.

astro-ph.EP↗

arXiv2017

Room temperature line lists for CO\2 symmetric isotopologues with \textit{ab initio} computed intensities

Remote sensing experiments require high-accuracy, preferably sub-percent, line intensities and in response to this need we present computed room temperature line lists for six symmetric isotopologues of carbon dioxide: $^{13}$C$^{16}$O$_2$, $^{14}$C$^{16}$O$_2$, $^{12}$C$^{17}$O$_2$, $^{12}$C$^{18}$O$_2$, $^{13}$C$^{17}$O$_2$ and $^{13}$C$^{18}$O$_2$, covering the range 0-8000 \cm. Our calculation scheme is based on variational nuclear motion calculations and on a reliability analysis of the generated line intensities. Rotation-vibration wavefunctions and energy levels are computed using the DVR3D software suite and a high quality semi-empirical potential energy surface (PES), followed by computation of intensities using an \abinitio\ dipole moment surface (DMS). Four line lists are computed for each isotopologue to quantify sensitivity to minor distortions of the PES/DMS. Reliable lines are benchmarked against recent state-of-the-art measurements and against the HITRAN2012 database, supporting the claim that the majority of line intensities for strong bands are predicted with sub-percent accuracy. Accurate line positions are generated using an effective Hamiltonian. We recommend the use of these line lists for future remote sensing studies and their inclusion in databases.

astro-ph.EP↗

arXiv2017

Predicted Landé $g$-factors for open shell diatomic molecules

The program {\sc Duo} (Yurchenko {\it et al.}, Computer Phys. Comms., 202 (2016) 262) provides direct solutions of the nuclear motion Schrödinger equation for the (coupled) potential energy curves of open shell diatomic molecules. Wavefunctions from {\sc Duo} are used to compute Landé $g$-factors valid for weak magnetic fields, the results are compared with the idealized predictions of both Hund's case (a) and Hund's case (b) coupling schemes. Test calculations are performed for AlO, NO, CrH and C$_2$. The computed $g_J$'s both provide a sensitive test of the underlying spectroscopic model used to represent the system and an indication of whether states of the molecule are well-represented by the either of the Hund's cases considered. The computation of Landé $g$-factors is implemented as a standard option in the latest release of {\sc Duo}.

physics.chem-ph↗

arXiv2016

High-resolution absorption measurements of NH3 at high temperatures: 2100 - 5500

High-resolution absorption spectra of \NH\ in the region 2100 - 5500 \cm\ at 1027 $^{\circ}$C and atmospheric pressure (1045 $\pm$ 3 mbar) are measured. An \NH\ concentration of 10\% in volume fraction is used in the measurements. Spectra are recorded in a high-temperature gas-flow cell using a Fourier Transform Infrared (FTIR) spectrometer at a nominal resolution of 0.09 \cm. The spectra are analysed by comparison to a variational line list, BYTe, and experimental energy levels determined using the MARVEL procedure. 2308 lines have been assigned to 45 different bands, of which 1755 and 15 have been assigned or observed for the first time in this work.

physics.chem-ph↗

arXiv2016

Definitive ideal-gas thermochemical functions of the H$_2$$^{16}$O molecule

$Q_{\rm int}$($T$), of the H$_2$$^{16}$O molecule is reported for temperatures between 0 and 6000 K. Determination of $Q_{\rm int}$($T$) is principally based on the direct summation technique involving all accurate experimental energy levels known for H$_2$$^{16}$O (almost 20~000 rovibrational energies including an almost complete list up to a relative energy of 7500 \cm), augmented with a less accurate but complete list of first-principles computed rovibrational energy levels up to the first dissociation limit, about 41~000 \cm\ (the latter list includes close to one million bound rovibrational energy levels up to $J = 69$, where $J$ is the rotational quantum number). Partition functions are developed for {\it ortho}- and {\it para}-H$_2$$^{16}$O as well as for their equilibrium mixture. Unbound rovibrational states above the first dissociation limit are considered using an approximate model treatment. The effect of the excited electronic states on the thermochemical functions is neglected, as their contribution to the thermochemical functions is negligible even at the highest temperatures considered. Based on the high-accuracy $Q_{\rm int}$($T$) and its first two moments, definitive results, in 1~K increments, are obtained for the following thermochemical functions: Gibbs energy, standardized enthalpy, entropy, and isobaric heat capacity. Reliable approximately two standard deviation uncertainties, as a function of temperature, are estimated for each quantity determined. These uncertainties emphasize that the present results are the most accurate ideal-gas thermochemical functions ever produced for H$_2$$^{16}$O. It is recommended that the new value determined for the standard molar enthalpy increment at 298.15~K, $9.90404 \pm 0.00001$ kJ~mol$^{-1}$, should replace the old CODATA datum, $9.905 \pm 0.005$ kJ~mol$^{-1}$.

physics.chem-ph↗

arXiv2016

Pressure-dependent water absorption cross sections for exoplanets and other atmospheres

Many atmospheres (cool stars, brown dwarfs, giant planets, extrasolar planets) are predominately composed of molecular hydrogen and helium. H$_2{}^{16}$O is one of the best measured molecules in extrasolar planetary atmospheres to date and a major compound in the atmospheres of brown-dwarfs and oxygen-rich cool stars, yet the scope of experimental and theoretical studies on the pressure broadening of water vapour lines by collision with hydrogen and helium remains limited. Theoretical H$_2$- and He-broadening parameters of water vapour lines (rotational quantum number $J$ up to 50) are obtained for temperatures in the range 300 - 2000 K. Two approaches for calculation of line widths were used: (i) the averaged energy difference method and (ii) the empirical expression for $J$\p $J$\pp-dependence. Voigt profiles based on these widths and the BT2 line list are used to generate high resolution ($Δ\tildeν$ = 0.01 \cm) pressure broadened cross sections for a fixed range of temperatures and pressures between 300 - 2000 K and 0.001 - 10 bar. An interpolation procedure which can be used to determine cross sections at intermediate temperature and pressure is described. Pressure broadening parameters and cross sections are presented in new ExoMol format.

astro-ph.EP↗

arXiv2016

Quantum-mechanical calculations of cross sections for electron collisions with atoms and molecules

An overview of quantum-mechanical methods to generate cross-section data for electron collisions with atoms and molecules is presented. Particular emphasis is placed on the time-independent close-coupling approach, since it is particularly suitable for low-energy collisions and also allows for systematic improvements as well as uncertainty estimates. The basic ideas are illustrated with examples for electron collisions with argon atoms and methane. For many atomic systems, such as e-Ar collisions, highly reliable cross sections can now be computed with quantified uncertainties. On the other hand, while electron collision calculations with molecules do provide key input data for plasma models, the methods and computer codes presently used require further development to make these inputs robust.

physics.plasm-ph↗