Showing papers by "Jonathan Tennyson published in 2008"

New ab initio potential energy surface and the vibration-rotation-tunneling levels of (H2O)2 and (D2O)2.

Abstract: We report a new full-dimensional potential energy surface (PES) for the water dimer, based on fitting energies at roughly 30 000 configurations obtained with the coupled-cluster single and double, and perturbative treatment of triple excitations method using an augmented, correlation consistent, polarized triple zeta basis set. A global dipole moment surface based on Moller-Plesset perturbation theory results at these configurations is also reported. The PES is used in rigorous quantum calculations of intermolecular vibrational frequencies, tunneling splittings, and rotational constants for (H2O)2 and (D2O)2, using the rigid monomer approximation. Agreement with experiment is excellent and is at the highest level reported to date. The validity of this approximation is examined by comparing tunneling barriers within that model with those from fully relaxed calculations.

A new ab initio ground-state dipole moment surface for the water molecule.

Abstract: A valence-only (V) dipole moment surface (DMS) has been computed for water at the internally contracted multireference configuration interaction level using the extended atom-centered correlation-consistent Gaussian basis set aug-cc-pV6Z. Small corrections to these dipole values, resulting from core correlation (C) and relativistic (R) effects, have also been computed and added to the V surface. The resulting DMS surface is hence called CVR. Interestingly, the C and R corrections cancel out each other almost completely over the whole grid of points investigated. The ground-state CVR dipole of H(2) (16)O is 1.8676 D. This value compares well with the best ab initio one determined in this study, 1.8539+/-0.0013 D, which in turn agrees well with the measured ground-state dipole moment of water, 1.8546(6) D. Line intensities computed with the help of the CVR DMS shows that the present DMS is highly similar to though slightly more accurate than the best previous DMS of water determined by Schwenke and Partridge [J. Chem. Phys. 113, 16 (2000)]. The influence of the precision of the rovibrational wave functions computed using different potential energy surfaces (PESs) has been investigated and proved to be small, due mostly to the small discrepancies between the best ab initio and empirical PESs of water. Several different measures to test the DMS of water are advanced. The seemingly most sensitive measure is the comparison between the ab initio line intensities and those measured by ultralong pathlength methods which are sensitive to very weak transitions.

Water line lists close to experimental accuracy using a spectroscopically determined potential energy surface for H2(16)O, H2(17)O, and H2(18)O.

Abstract: Line lists of vibration-rotation transitions for the H(2) (16)O, H(2) (17)O, and H(2) (18)O isotopologues of the water molecule are calculated, which cover the frequency region of 0-20 000 cm(-1) and with rotational states up to J=20 (J=30 for H(2) (16)O) These variational calculations are based on a new semitheoretical potential energy surface obtained by morphing a high accuracy ab initio potential using experimental energy levels This potential reproduces the energy levels with J=0, 2, and 5 used in the fit with a standard deviation of 0025 cm(-1) Linestrengths are obtained using an ab initio dipole moment surface That these line lists make an excellent starting point for spectroscopic modeling and analysis of rotation-vibration spectra is demonstrated by comparison with recent measurements of Lisak and Hodges [J Mol Spectrosc (unpublished)]: assignments are given for the seven unassigned transitions and the intensity of the strong lines are reproduced to with 3% It is suggested that the present procedure may be a better route to reliable line intensities than laboratory measurements

On the chemistry and distribution of HOC+ in M 82 More evidence for extensive PDRs

Abstract: Context. The molecular gas composition in the inner 1 kpc disk of the starburst galaxy M 82 resembles that of Galactic Photon Dominated Regions (PDRs). In particular, large abundances of the reactive ions HOC + and CO + have been measured in the nucleus of this galaxy. Two explanations have been proposed for such high abundances: the influence of intense UV fields from massive stars, or a significant role of X-Rays. Aims. Our aim is to investigate the origin of the high abundances of reactive ions in M 82. Methods. We have completed our previous 30 m HOC + J = 1 → 0 observations with the higher excitation HCO + and HOC + J = 4 → 3a nd 3→ 2 rotational lines. In addition, we have obtained with the IRAM Plateau de Bure Interferometer (PdBI) a 4 �� resolution map of the HOC + emission in M 82, the first ever obtained in a Galactic or extragalactic source. Results. Our HOC + interferometric image shows that the emission of the HOC + 1 → 0 line is mainly restricted to the nuclear disk, with the maxima towards the E and W molecular peaks. In addition, line excitation calculations imply that the HOC + emission arises in dense gas (n ≥ 10 4 cm −3 ). Therefore, the HOC + emission is arising in the dense PDRs embedded in the M 82 nuclear disk, rather than in the intercloud phase and/or wind. Conclusions. We have improved our previous chemical model of M 82 by (i) using the new version of the Meudon PDR code; (ii) updating the chemical network; and (iii) considering two different types of clouds (with different thickness) irradiated by the intense interstellar UV field (G0 = 10 4 in units of the Habing field) prevailing in the nucleus of M 82. Most molecular observations (HCO + , HOC + ,C O + , CN, HCN, H3O + ) are well explained assuming that ∼87% of the mass of the molecular gas is forming small clouds (Av = 5 mag) while only ∼13% of the mass is in large molecular clouds (Av = 50 mag). Such a small number of large molecular clouds suggests that M 82 is an old starburst, where star formation has almost exhausted the molecular gas reservoir.

Calculation of the O-H stretching vibrational overtone spectrum of the water dimer.

Abstract: The O-H stretching vibrational overtone spectrum of the water dimer has been calculated with the dimer modeled as two individually vibrating monomer units. Vibrational term values and absorption intensities have been obtained variationally with a computed dipole moment surface and an internal coordinate Hamiltonian, which consists of exact kinetic energy operators within the Born-Oppenheimer approximation of the monomer units. Three-dimensional ab initio potential energy and dipole moment surfaces have been calculated using the internal coordinates of the monomer units using the coupled cluster method including single, double, and perturbative triple excitations [CCSD(T)] with the augmented correlation consistent valence

On the chemistry and distribution of HOC+ in M82: More evidence for extensive PDRs

Abstract: The molecular gas composition in the inner 1 kpc disk of the starburst galaxy M82 resembles that of Galactic Photon Dominated Regions (PDRs). In particular, large abundances of the reactive ions HOC+ and CO+ have been measured in the nucleus of this galaxy. To investigate the origin of the large abundances of reactive ions in M82, we have completed our previous 30m HOC+ J=1-0 observations with the higher excitation HCO+ and HOC+ J=4-3 and 3-2 rotational lines. In addition, we have obtained with the IRAM Plateau de Bure Interferometer (PdBI) a 4" resolution map of the HOC+ 1-0 emission, the first ever obtained in a Galactic or extragalactic source. Our HOC+ interferometric image shows that the emission of the HOC+ 1-0 line is mainly restricted to the nuclear disk, with the maxima towards the E. and W. molecular peaks. In addition, line excitation calculations imply that the HOC+ emission arises in dense gas. Therefore, the HOC+ emission is arising in the dense PDRs embedded in the M82 nuclear disk, rather than in the intercloud phase and/or wind. We have improved our previous chemical model of M82 by (i) using the new version of the Meudon PDR code, (ii) updating the chemical network, and (iii) considering two different types of clouds (with different thickness). Most molecular observations (HCO+, HOC+, CO+, CN, HCN, H3O+) are well explained assuming that ~ 87 % of the mass of the molecular gas is forming small clouds (Av=5 mag) while only ~ 13 % of the mass is in large molecular clouds (Av=50 mag). Such small number of large molecular clouds suggests that M82 is an old starburst, where star formation has almost exhausted the molecular gas reservoir.

A H13CN/HN13C linelist, model atmospheres and synthetic spectra for carbon stars

Abstract: ‘The definitive version is available at www.blackwell-synergy.com’. Copyright Blackwell Publishing DOI: 10.1111/j.1365-2966.2008.13642.x

Spectrum of hot water in the 4750–13 000 cm−1 wavenumber range (0.769–2.1 μm)

Abstract: A B S T R A C T The high resolution laboratory spectrum of hot water vapour has been recorded in the 500– 13000 cm −1 wavenumber range and we report on the analysis of the 4750–13000 cm −1 (0.769–2.1 µm)portion.Theemissionspectrumwasrecordedusinganoxy-acetylenewelding torch and a Fourier transform spectrometer. Line assignments in the laboratory spectrum as well as in an absorption spectrum of a sunspot umbra were made with the help of the BT2 line-list. Our torch spectrum is the first laboratory observation of the 9300 A ‘steam bands’ seen in M-stars and brown dwarfs.

Water line parameters from refitted spectra constrained by empirical upper state levels : Study of the 9500-14,500 cm- 1 region

Abstract: A fitting technique is proposed which gives more reliable spectral line parameters for blended and very weak lines. The technique uses common upper state energies to fix frequencies in cases where these are poorly determined. Room temperature Fourier Transform absorption spectra of pure water vapour due to Schermaul et al. [The water vapour spectrum in the region 8600– 15 , 000 cm - 1 : experimental and theoretical studies for a new spectral line database I: laboratory measurements. J Mol Spectrosc 2001;208:32–42; The water vapour spectrum in the region 8600– 15 , 000 cm - 1 : experimental and theoretical studies for a new spectral line database I: laboratory measurements. J Mol Spectrosc 2002;211:169–78] recorded at several pathlengths are refitted. Parameters are obtained for over 10,700 transitions of H 2 O 16 covering the frequency region 9500– 14 , 500 cm - 1 . Only 73 of the lines remain unassigned. Comparisons with previous studies on water vapour absorption in this region suggest that the new parameters give a more consistent representation of the underlying spectrum.

Excitations to the electronic continuum of 3HeT+ in investigations of T2 β-decay experiments

Abstract: The final-state probability distribution for excitations to the electronic continuum of 3 HeT + resulting from the β-decay of T2 is calculated. The R-matrix method is used to obtain a geometry dependant treatment of the resonance states of 3 HeT + allowing for nuclear motion effects of these states to be accounted for explicitly in the determination of the final-state distribution. A discretized distribution for the background continuum is used to obtain this part of the decay probability. When combined with previous studies of other energy ranges, 99.94% of the decay probability is recovered, a significant improvement on previous results. (Some figures in this article are in colour only in the electronic version)

Potential energy surface of HDO up to 25,000 cm-1.

Abstract: A new spectroscopically determined potential energy surface (PES) for HD(16)O is presented. This surface is constructed by adjusting the high accuracy ab initio PES of Polyansky et al. [Science 299, 539 (2003)] by fitting to both published experimental data and our still unpublished data. This refinement used experimentally derived term values up to 25,000 cm(-1) and with J< or =8: a data set of 3478 energy levels once some levels with ambiguous assignment is excluded. To improve the extrapolation properties of the empirical PES, the restraint that the resulting PESs remain close to the ab initio surface was imposed. The new HDO_07 PES reproduces the experimental data, including high J levels not included in the fit, with a root mean square error of 0.035 cm(-1). Predictions for rotation-vibration term values up to J=12 are made.

Electron-impact study of NeF using the R-matrix method

Abstract: a configuration-interaction expansion which is shown to yield accurate potential energy curves and target properties. Elastic and inelastic collision cross sections for the four states are calculated ab initio using the R-matrix method. Special care is needed to treat the large dipole moments found for the charge-transfer states which are predicted to have electron collision cross sections almost two orders of magnitude bigger than the lower states. Differential and momentum transfer cross sections are also considered for the electron impact on the repulsive ground state. Rate constants for electron deexcitation of the excimer states of NeF have been calculated for electron temperature corresponding up to 10 eV. The superelastic processes are dominated by the 1 2+ →X 2+ transition with thermal rate constant of 2–5 10 �9 cm 3 s �1 in the entire range of electron temperature considered.

Water vapor line width and shift calculations with accurate vibration–rotation wave functions

Abstract: Calculations of H216O rotation–vibration line broadening and shifting due to N2 pressure effects are performed using a semi-empirical approach. The calculations are based on impact theory modified by introducing additional parameters to extend the use of empirical data. These model parameters are determined by fitting the broadening and shifting coefficients to experimental data. The method is further developed by using anharmonic wavefunctions in the estimates of the line profiles. The main feature of the present calculation is the use of a complete set of high-accuracy vibration–rotation dipole transition moments calculated for all possible transitions using wavefunctions determined from variational nuclear motion calculations and an ab initio dipole moment surface. This approach explicitly takes into account all scattering channels induced by collisions. Results of these calculations clearly demonstrate improved agreement between observed and calculated parameters for both the line widths and the line shifts.

Polarization effects in electron collisions with Li2: application of the molecular R-matrix method with pseudostates

Abstract: Li2 has a huge polarizability and a huge elastic cross section for collisions with low-energy electrons. The recently developed molecular R-matrix with pseudostates (MRMPS) method is applied to electron collisions with Li2 at energies below 5 eV. The calculations, which are shown to be stable with respect to the choice of pseudostate basis, demonstrate the power of the MRMPS method for representing target polarizabilities and polarization effects in general. A previously identified low-lying 2?u shape resonance is found at about 0.05 eV and several other low-lying resonances and resonance-like features are identified. Cross sections for elastic and electronically inelastic electron collisions are calculated and compared with previous studies.

Water in the Near IR spectrum of Comet 8P/Tuttle

Abstract: High resolution spectra of Comet 8P/Tuttle were obtained in the frequency range 3440.6-3462.6 cm-1 on 3 January 2008 UT using CGS4 with echelle grating on UKIRT. In addition to recording strong solar pumped fluorescent (SPF) lines of H2O, the long integration time (152 miutes on target) enabled eight weaker H2O features to be assigned, most of which had not previously been identified in cometary spectra. These transitions, which are from higher energy upper states, are similar in character to the so-called 'SH' lines recorded in the post Deep Impact spectrum of comet Tempel 1 (Barber et al., 2007). We have identified certain characteristics that these lines have in common, and which in addition to helping to define this new class of cometary line, give some clues to the physical processes involved in their production. Finally, we derive an H2O rotational temperature of 62+/- K and a water production rate of (1.4+/-0.3)E28 molecules/s.

Resonant states of H3+ and D2H+

Abstract: Vibrational resonances for H3+ and D2H+, as well as H3+ at J=3, are calculated using a complex absorbing potential (CAP) method with an automated procedure to find stability points in the complex plane. Two different CAP functional forms and different CAP extents are used to analyze the consistency of the results. Calculations are performed using discrete variable representation continuum basis elements calculated to high levels of accuracy by diagonalizing large, dense, Hamiltonian matrices. For D2H+, two energy regions are analyzed: the one where D2+H+ is the only dissociation product and the one where HD+D+ can also be formed. Branching ratios are obtained in the latter case by using different CAPs. It is shown that H3+ and D2H+ support some narrow Feshbach-type resonances but that higher angular momentum states must be studied to model the pre-dissociation spectrum recorded by Carrington and co-workers [J. Chem. Phys. 98, 1073 (1993)].

Correlation–polarization effects in electron/positron scattering from acetylene: A comparison of computational models

Abstract: Different computational methods are employed to evaluate elastic (rotationally summed) integral and differential cross sections for low energy (below about 10 eV) positron scattering off gas-phase C 2 H 2 molecules. The computations are carried out at the static and static-plus-polarization levels for describing the interaction forces and the correlation–polarization contributions are found to be an essential component for the correct description of low-energy cross section behavior. The local model potentials derived from density functional theory (DFT) and from the distributed positron model (DPM) are found to produce very high-quality agreement with existing measurements. On the other hand, the less satisfactory agreement between the R-matrix (RM) results and measured data shows the effects of the slow convergence rate of configuration-interaction (CI) expansion methods with respect to the size of the CI-expansion. To contrast the positron scattering findings, results for electron–C 2 H 2 integral and differential cross sections, calculated with both a DFT model potential and the R-matrix method, are compared and analysed around the shape resonance energy region and found to produce better internal agreement.

Creating ultracold molecules by collisions with ultracold rare-gas atoms in an optical trap

Abstract: We study collisions of para-${\mathrm{H}}_{2}$ with five rare-gas atomic species (He, Ne, Ar, Kr, and Xe) over the range from $1\phantom{\rule{0.3em}{0ex}}\mathrm{K}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{K}$ and evaluate the feasibility of sympathetic cooling ${\mathrm{H}}_{2}$ with ultracold ground state rare-gas atoms co-trapped within a deep optical trap. Collision cross sections over this large temperature range show that all of these species could be used to cool ${\mathrm{H}}_{2}$ to ultracold temperatures and that argon and helium are the most promising species for future experiments. A simple yet accurate method for calculating the microscopic observables needed in the present analysis is also described, as well as the sensitivity of the calculated cross sections to the potential energy surface used.

Low and intermediate energy electron collisions with the C-2 molecular anion

Abstract: Calculations are presented which use the molecular R-matrix with pseudo-states (MRMPS) method to treat electron impact electron detachment and electronic excitation of the carbon dimer anion. Resonances are found above the ionization threshold of C?2 with 1?+g, 1?g and 3?g symmetry. These are shape resonances trapped by the effect of an attractive polarization potential competing with a repulsive Coulomb interaction. The ?g resonances are found to give structure in the detachment cross section similar to that observed experimentally. Both excitation and detachment cross sections are found to be dominated by large impact parameter collisions whose contribution is modelled using the Born approximation.

The electronic bands of CrD, CrH, MgD and MgH: application to the ‘deuterium test’

Abstract: We compute opacities for the electronic molecular band systems A 6 � + – X 6 � + of CrH and CrD, and A 2 � – X 2 � + of MgH and MgD. The opacities are computed by making use of existing spectroscopic constants for MgH and CrH. These constants are adjusted for the different reduced masses of MgD and CrD. Frank-Condon factors are used to provide intensities for the individual vibronic bands. These results are used in the computation of synthetic spectra between Teff = 1800 and 1200 K with an emphasis on the realisation of “deuterium test”, first proposed by Bejar et al. (1999) to distinguish brown dwarfs from planetary mass objects. We discuss the possible use of CrD and MgD electronic bands for the “deuterium test”. We find CrD to be the more promising of the two deuterides, potentially, the most useful bands of ′′ ′′ ′′ ! × "

A line list of allowed and forbidden rotational transition intensities for water

Abstract: Pure rotational lines are important for monitoring water concentrations in many environments both in space and on earth. A list of line intensities of rotational transitions for H2 16 O is calculated using variational nuclear-motion wave functions and an ab initio dipole moment surface. This methodology should be equally reliable for both allowed and forbidden rotational transitions. Extensive comparisons are made with available intensity data for these transitions including the HITRAN and JPL databases. Problems are identified with some of these data. A list of 555 allowed and 846 forbidden rotational transition lines within the ground vibrational state is made available.

Resonances in electron-impact electron detachment of C2-.

Abstract: Molecular $R$-matrix with pseudostate calculations are reported for the electron-impact ionization cross section of the carbon dimer anion. A $^{1}\ensuremath{\Sigma}_{g}^{+}$ resonance is found near the detachment threshold and two further resonances, of $^{3}\ensuremath{\Pi}_{g}$ and $^{1}\ensuremath{\Pi}_{g}$ symmetry, are found near 10 eV close to the structures observed experimentally. These unusual shape resonances are a result of the competition between the repulsive Coulomb interaction and the large, attractive polarizability of $\mathrm{C}_{2}{}^{\ensuremath{-}}$. Use of the Born approximation to allow for higher partial waves gives a total cross section close to that observed experimentally.

Low and intermediate energy electron collisions with the C$_2^-$ molecular anion

Abstract: Calculations are presented which use the molecular R-matrix with pseudo-states (MRMPS) method to treat electron impact electron detachment and electronic excitation of the carbon dimer anion. Resonances are found above the ionisation threshold of C$_2^-$ with $^1\Sigma^+_g$, $^1\Pi_g$ and $^3\Pi_g$ symmetry. These are shape resonances trapped by the effect of an attractive polarisation potential competing with a repulsive Coulomb interaction. The $\Pi_g$ resonances are found to give structure in the detachment cross section similar to that observed experimentally. Both excitation and detachment cross sections are found to be dominated by large impact parameter collisions whose contribution is modelled using the Born approximation.

Global plasma simulations using dynamically generated chemical models

Abstract: Extensive molecular data are a key requirement in understanding modern technical plasmas. A method for coupling molecular data with chemical models in a global plasma simulation to enable rapid testing and evaluation of new plasmas is presented. A global plasma model (GLOBALKIN) is extended using an expert system (Quantemol-P) to enable ad hoc simulations using new plasma recipes. A set of atomic and molecular species to be considered in the plasma simulation is specified by the user. The expert system generates a complete set of reaction pathways for both the gas and surface reactions in a plasma. This set is pruned by discarding unphysical reactions and reaction data not appropriate to technical plasmas (such as autodetachment). The species, gas phase reactions, surface reactions, and plasma properties can be adjusted to control the simulation. The reaction list is populated through a database of molecular parameters and cross sections; missing data can be calculated through molecular cross sections usi...

Water in the Near IR spectrum of Comet 8P/Tuttle

Abstract: High-resolution spectra of comet 8P/Tuttle were obtained in the frequency range 3449.0-3462.2 cm-1 on 2008 January 3 UT using CGS4 with echelle grating on United Kingdom Infrared Telescope. In addition to observing solar pumped fluorescent lines of H2O, the long integration time (152 min on target) enabled eight weaker H2O features to be assigned, most of which had not previously been identified in cometary spectra. These transitions, which are from higher energy upper states, are similar in character to the so-called SH lines recorded in the post Deep Impact spectrum of comet Tempel 1. We have identified certain characteristics that these lines have in common, and which in addition to helping to define this new class of cometary line give some clues to the physical processes involved in their production. Finally, we derive an H2O rotational temperature of and a water production rate of (1.4 +/- 0.3) × 1028 molecules s-1.

Polarisation effects in low-energy positron-molecule scattering

Abstract: The UK molecular R-matrix method has been adapted to treat positron collisions from polyatomic targets. A simple empirical enhancement factor which corrects for the underestimation of electron–positron polarisation and correlation effects in the calculations performed with the static-plus-polarization model at low scattering energies is presented. Application of this model to positron scattering from carbon dioxide at energies below 8 eV is discussed. Introduction of the enhancement factor improves the integral cross sections significantly and introduces structures in the differential cross sections consistent with other studies. The prospects for a fully ab initio treatment of this problem are discussed.

On the treatment of long-range interactions in global potential energy surfaces for chemically bound systems

Abstract: The convergence of undamped, inverse power series asymptotic van der Waals expansions for the hydrogen molecule is tested. The role of this kind of expansion in the description of global potential energy surfaces for strongly bound molecules is discussed. The single-bond breaking dissociation channel of the water molecule is shown to display the same behaviour. It is suggested that such expansions do not provide a good starting point for this problem as they only become reliable when the interaction energy is very small.

Shift of the centers of H 2 O absorption lines in the region of 1.06 μm

Abstract: The shift coefficients for the lines of the ν1 + ν2 + ν3 and ν2 + 2ν3 bands of H2O in the region from 9403 to 9413 cm−1 are measured and calculated. The measurements are performed using an intracavity laser spectrometer based on a neodymium laser with a determination error of the line center of 0.003–0.004 cm−1. The Ar, Kr, and Xe noble gases, as well as nitrogen, oxygen, and hydrogen were used as buffer gases. The coefficients of shifts in eight H2O absorption lines induced by oxygen, nitrogen, and atmospheric air pressures fall into the region from −0.004 to −0.069 cm−1/bar. The calculations are performed by a semiempirical method using variational wave functions, which, in contrast to other studies, correctly takes into account intramolecular interactions. The calculated values agree satisfactorily with experimental data.