Showing papers by "Jonathan Tennyson published in 2002"

Electron impact dissociative excitation of water within the adiabatic nuclei approximation

Abstract: The R-matrix method is used to calculate dissociative excitation cross sections for the four lowest-lying electronically excited states of H2O in the energy range 5-15 eV. For the first time calculations are performed taking into account the nuclear motion by means of an adaptation of the adiabatic nuclei approximation. Cross sections are compared with previous and new fixed-nuclei results and also experiments. Resonance positions and widths are calculated for different geometries of the water molecule.

Calculated rates for the electron impact dissociation of molecular hydrogen, deuterium and tritium

Abstract: At temperatures below 15 000 K, the major pathway for the electron impact dissociation of H2 is through excitation to the b 3Σu+ excited electronic state. Total cross sections and energy differential cross sections for threshold energies as a function of vibrational states (v) for H2 v = 0–4, D2 v = 0–6 and T2 v = 0–7 are calculated. The rates of dissociation as a function of electron temperature for each state are parametrized. Near-threshold rates are shown to be so critically dependent on the vibrational level that dissociation from very high-lying vibrational levels must be included in calculations of the rate at local thermal equilibrium (LTE) even at low temperature. An adapted version of the extrapolation procedure of Stibbe and Tennyson (1999 Astrophys. J. 513 L147) is used to approximate the rates for all of the higher vibrational levels, which are then used to calculate the LTE rate. The LTE rate is an order of magnitude greater than the v = 0 rate. Calculations of energy differential cross sections suggest that impact dissociation of vibrationally excited molecules could be the source of low energy H atoms observed in tokamak plasmas.

Vibrational and rotational cooling of H3

Abstract: The vibrational relaxation of H{sub 3}{sup +} molecules from a conventional plasma ion source is studied performing Coulomb explosion imaging on the ions extracted from a storage ring after variable times of storage. Storage for 2 s is found sufficient for radiative relaxation of the breathing excitation and the fragment velocity distribution in the breathing coordinate then agrees well with simulations based on the calculated ground-state wave function. The radiative decay of the two lowest pure breathing levels (1,0{sup 0}) and (2,0{sup 0}) is seen to be considerably faster than expected from rotationless calculations. Assuming a high rotational excitation of the H{sub 3}{sup +} ions, as suggested already in earlier experiments, the theoretical transition probabilities of the University College London line list for H{sub 3}{sup +} [L. Neale, S. Miller, and J. Tennyson, Astrophys. J. 464, 516 (1996)] can explain the increase of the vibrational cooling rates and reproduce the observed decay curve for the lowest breathing-excited level, confirming the absolute transition probabilities of these line tables. The observations give evidence for a quasistable population of high-lying rotational levels in the stored ion beam, relevant for the interpretation of storage ring measurements on the rate coefficients for dissociative recombination ofmore » H{sub 3}{sup +} ions with low-energy electrons.« less

Opacity data for HCN and HNC from a new AB initio line list

Abstract: A new extensive ab initio rotation-vibration HCN/HNC line list is presented. The line list contains rotation-vibration energy levels, line frequencies, and line strengths for transitions between states with energy less than 18,000 cm-1 and with J ≤ 60. This line list greatly improves the quality and range of HCN/HNC data available. It is presently the most extensive and most accurate ab initio HCN/HNC line list in existence. It is hoped that this data set will be used in models of C star atmospheres and elsewhere.

Spectral analysis of water vapour in cool stars

Abstract: M star spectra, at wavelengths beyond 1.35 µm, are dominated by water vapour yet terrestrial water vapour makes it notoriously difficult to make accurate measurement from ground-based observations. We have used the short wavelength spectrometer on the Infrared Space Observatory at four wavelength settings to cover the 2.5–3.0 µm region for a range of M stars. The observations show a good match with previous ground-based observations and with synthetic spectra based on the Partridge & Schwenke (1997) line list though not with the SCAN (Jorgensen et al. 2001) line list. We used a least-squared minimisation technique to systematically find best fit parameters for the sample of stars. The temperatures that we find indicate a relatively hot temperature scale for M dwarfs. We consider that this could be a consequence of problems with the Partridge & Schwenke linelist which leads to synthetic spectra predicting water bands which are too strong for a given temperature. Such problems need to be solved in the next generation of water vapour line lists which will extend the calculation of water vapour to higher energy levels with the good convergence necessary for reliable modelling of hot water vapour. Then water bands can assume their natural role as the primary tool for the spectroscopic analysis of M stars.

Temperature dependent partition functions and equilibrium constant for HCN and HNC

Abstract: Ab initio vibration-rotation energy levels are summed to estimate a partition function for the total HCN system. By assigning individual levels to HCN and HNC, separate partition functions are obtained for the isomers. These are used to give a temperature dependent equilibrium constant which suggests that at temperatures typical of cool carbon stars, about 20% of the HCN system is actually HNC. Errors in the partition functions and equilibrium constant are estimated.

Dipole Moments of Highly Vibrationally Excited Water

Abstract: The intensity of water absorption in the region of the solar spectrum plays a dominant role in atmospheric energy balance and hence strongly influences climate. Significant controversy exists over how to model this absorption accurately. We report dipole moment measurements of highly vibrationally excited water, which provide stringent tests of intensities determined by other means. Our measurements and accompanying calculations suggest that the best currently available potential and dipole surfaces do not accurately model intensities in the optical spectrum of water.

Electron collisions with the CF2 radical using the R-matrix method

Abstract: The R-matrix method is used to treat electron collisions with the molecular radical CF2. These calculations concentrate on obtaining low-energy (less than 10 eV) elastic and excitation cross sections of the six lowest-lying electronically excited states of the CF2 molecule. These states have symmetry 3B1, 1B1, 3A2, 1A2, 3B2 and 1B2 and vertical excitation energies in the range 2.44-10 eV. Two shape resonances of 2A1 and 2B1 symmetries are found at 5.61 and 0.95 eV respectively. Calculations which stretch one C-F bond show that the 2B1 resonance becomes bound at a bond length beyond 3.2 a0. No other bound CF2- states are found.

Beyond the Born–Oppenheimer approximation: High-resolution overtone spectroscopy of H2D+ and D2H+

Abstract: Transitions to overtone 2ν2 and 2ν3, and combination ν2+ν3 vibrations in jet-cooled H2D+ and D2H+ molecular ions have been measured for the first time by high-resolution IR spectroscopy. The source of these ions is a pulsed slit jet supersonic discharge, which allows for efficient generation, rotational cooling, and high frequency (100 KHz) concentration modulation for detection via sensitive lock-in detection methods. Isotopic substitution and high-resolution overtone spectroscopy in this fundamental molecular ion permit a systematic, first principles investigation of Born–Oppenheimer “breakdown” effects due to large amplitude vibrational motion as well as provide rigorous tests of approximate theoretical methods beyond the Born–Oppenheimer level. The observed overtone transitions are in remarkably good agreement (<0.1 cm−1) with non-Born–Oppenheimer ab initio theoretical predictions, with small but systematic deviations for 2ν2, ν2+ν3, and 2ν3 excited states indicating directions for further improvement...

Laboratory spectroscopy of hot water near 2 microns and sunspot spectroscopy in the H-band region

Abstract: The infrared spectrum of sunspots is analyzed in the H-band region (5540-6997 cm-1) with the aid of a new, hot (T = 1800 K) laboratory emission spectrum of water covering 4878-7552 cm-1. There are 682 lines in the sunspot spectrum and 5589 lines in the laboratory spectrum assigned quantum numbers corresponding to transitions due to H216O using a combination of previously known experimental energy levels for water and variational line lists. A further 201 unassigned lines common to both spectra can also be associated with water.

The metallicity of CM Draconis

Abstract: ‘The definitive version is available at www.blackwell-synergy.com '. Copyright Blackwell Publishing / Royal Astronautical Society. DOI: 10.1046/j.1365-8711.2002.04982.x [Full text of this article is not available in the UHRA]

Variational calculations of vibrational energy levels for XY4 molecules: 2. Bending states of methane

Abstract: A variational method for calculating excited bending states of symmetric tetrahedral pentaatomic molecules is presented based on the use of Radau coordinates and Jacobi polynomials as the basis functions. Symmetry is used both to reduce the size of secular matrix to be diagonalized and to calculate potential energy matrix elements over a reduced grid of quadrature points. Methods of treating the redundant coordinate are investigated and fitting is found to be more effective than the use of Taylor expansions. Test results are presented for methane, for which stretch-bend coupling and the contribution due to the redundant coordinate are found to be significant. Converged results are obtained for bending states significantly higher than considered in previous calculations. These states will be used as a basis for bending motions in a fully coupled stretch-bend calculation.

Electron-impact rotational excitation of symmetric-top molecular ions

Abstract: We present electron-impact rotational excitation calculations for polyatomic molecular ions. The theory developed in this paper is an extension of the work of Rabadan et al (Rabadan I, Sarpal B K and Tennyson J 1998 J. Phys. B: At. Mol. Opt. Phys. 31 2077) on linear molecular ions to the case of symmetric-top species. The H3+ and H3O+ ions, as well as their deuterated forms D3+ and D3O+, are used as test cases and cross sections are obtained at various levels of approximation for impact energies up to 5 eV. As in the linear case, the widely used Coulomb–Born (CB) approximation is found to be unreliable in two major aspects: transitions with ΔJ > 1 are entirely dominated by short-range interactions and threshold effects are important at very low energies. Electron collisional selection rules are found to be consistent with the CB theory. In particular, dominant transitions are those for which ΔJ ≤ 2 and ΔK = 0.

Calculated rates for the electron impact dissociation of molecular hydrogen: mixed isotopomers and scaling laws

Abstract: Following our calculations of electron impact dissociation rates of molecular H2, D2 and T2 (Trevisan and Tennyson 2002 Plasma Phys. Control. Fusion 44 1263–76), calculations for the mixed isotopomers HD, HT and DT have been made. Total cross sections and energy differential cross sections at threshold energies as a function of vibrational states (v) for HD: v = 0–5, for HT: v = 0–5 and for DT: v = 0–7 are calculated for the electron impact dissociation through excitation to the b 3Σu+ excited electronic state, which is the dominant dissociation process at such energies. The rates of dissociation as a function of electron temperature for each state are parametrized. Coinciding with our previous results, near-threshold rates are shown to be so critically dependent on the vibrational level that dissociation from very high-lying vibrational levels must be included in calculations of the rate at local thermal equilibrium (LTE) even at low temperature. Rates for the higher vibrational levels are obtained by extrapolation following the procedure discussed in our previous publication, and are then used to calculate the LTE rate. The LTE rate is an order of magnitude greater than the v = 0 rate. A scaling law for the electron impact dissociation cross sections of vibrationally excited H2 and its isotopomers has been derived.

Calculations of rotation-vibration states with the z axis perpendicular to the plane: High accuracy results for H 3 +

Abstract: A method of calculation of rotation–vibration states for a general triatomic that places the body-fixed z axis perpendicular to the plane of the molecule is implemented within a discrete variable representation (DVR) for the vibrational motion. Calculations are presented for water and H3+. For H3+ the new method improves on previous high accuracy ab initio treatments of the rotation–vibration energies of the molecule both in accuracy and the range of rotational states that can be treated. Reliable treatment of quasilinear geometries means that the method is also promising for treating very highly excited states.

R-matrix calculations for polyatomic molecular ions: electron scattering by H3+ and H3O+

Abstract: Electron impact calculations are presented for H3+ and H3O+ at their equilibrium geometry using coupled states expansions. The R-matrix method is employed as it is very efficient for characterizing the many resonances found in electron-ion collisions. Positions, widths and symmetries are obtained for the lowest ten Feshbach resonances for each ion. These resonances are important for explaining features observed in dissociative recombination experiments at energies of 5-10 eV. In addition a broad shape resonance at about 4.5 eV is identified for H3O+. Electronic excitation cross sections are obtained in reasonable agreement with recent storage ring experiments.

Variational calculations of vibrational energy levels for XY4 molecules 1. Stretching states

Abstract: A variational method for calculating excited stretching states of symmetric tetrahedral pentaatomic molecules is presented based on the use of Radau coordinates and Morse oscillator-like basis functions. Symmetry is used both to reduce the size of the secular matrix to be diag-onalized and to calculate the potential energy matrix elements over a reduced grid of quadrature points. Test results are presented for methane, silane and germane. For CH4, stretch-bend coupling is found to be significant, whereas it is less important for the more strongly local mode SiH4 and GeH4 molecules. Converged results are obtained for stretching states significantly higher than considered in previous calculations. These states will be used to represent stretching motions in a fully coupled stretch-bend calculation.

Weak line water vapor spectrum in the 13 200–15 000 cm−1 region

Abstract: New Fourier transform spectra of water vapor are presented in the range 6500–16 400 cm−1 obtained using pathlengths of up to 800 m and long integration times. These spectra have a significantly higher signal-to-noise than previous measurements in this wavenumber range. Wavenumbers, absolute intensities and self-broadening coefficients, all with associated uncertainties, are presented for 3604 lines in the region 13 200–15 000 cm−1. Analysis of these lines using variational linelists, along with other unassigned lines from previous studies, has been conducted. This leads to 952 new line assignments to transitions involving 35 different vibrational states of H216O. A smaller number of lines are assigned to H218O and H217O.

Low energy electron collisions with molecular hydrogen

Abstract: We discuss the theory of electron-molecule excitation processes of importance in fusion plasmas. We consider elastic scattering, rotational, vibrational, and electronic excitation in turn and also dissociative attachment and impact dissociation.