A full nine-dimensional potential-energy surface for hydrogen molecule-water collisions (original) (raw)
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Collisional excitation of water in warm astrophysical media
Astronomy and Astrophysics, 2008
Context. The interpretation of water line emission from infrared and submillimetre observations requires a detailed knowledge of collisional rate coefficients over a wide range of levels and temperatures. Aims. We attempt to determine rotational and rovibrational rate coefficients for H 2 O colliding with both H 2 and electrons in warm, molecular gas. Methods. Pure rotational rates are derived by extrapolating published data using a new method partly based on the information (phase space) theory of Levine and co-workers. Ro-vibrational rates are obtained using vibrational relaxation data available in the literature and by assuming a complete decoupling of rotation and vibration. Results. Rate coefficients were obtained for the lowest 824 ro-vibrational levels of H 2 O in the temperature range 200−5000 K. Our data is expected to be accurate to within a factor of ∼5 for the highest rates (> ∼ 10 −11 cm 3 s −1). Smaller rates, including the rovibrational ones, should be generally accurate to within an order of magnitude. As a first application of this data, we show that vibrationally excited water emission observed in evolved stars is expected to be at least partly excited by means of collisions.
Quasi-classical rate coefficient calculations for the rotational (de)excitation of H2O by H2
Astronomy & Astrophysics, 2007
Context. The interpretation of water line emission from existing observations and future HIFI/Herschel data requires a detailed knowledge of collisional rate coefficients. Among all relevant collisional mechanisms, the rotational (de)excitation of H 2 O by H 2 molecules is the process of most interest in interstellar space. Aims. To determine rate coefficients for rotational de-excitation among the lowest 45 para and 45 ortho rotational levels of H 2 O colliding with both para and ortho-H 2 in the temperature range 20−2000 K. Methods. Rate coefficients are calculated on a recent high-accuracy H 2 O−H 2 potential energy surface using quasi-classical trajectory calculations. Trajectories are sampled by a canonical Monte-Carlo procedure. H 2 molecules are assumed to be rotationally thermalized at the kinetic temperature. Results. By comparison with quantum calculations available for low lying levels, classical rates are found to be accurate within a factor of 1−3 for the dominant transitions, that is those with rates larger than a few 10 −12 cm 3 s −1. Large velocity gradient modelling shows that the new rates have a significant impact on emission line fluxes and that they should be adopted in any detailed population model of water in warm and hot environments.
Journal of Quantitative Spectroscopy and Radiative Transfer, 2013
This is the third of a series of articles reporting critically evaluated rotationalvibrational line positions, transition intensities, and energy levels, with associated critically reviewed labels and uncertainties, for all the main isotopologues of water. This paper presents experimental line positions, experimental-quality energy levels, and validated labels for rotational-vibrational transitions of the most abundant isotopologue of water, H 2 16 O. The latest version of the MARVEL (Measured Active Rotational-Vibrational Energy Levels) line-inversion procedure is used to determine the rovibrational energy levels of the electronic ground state of H 2 16 O from experimentally measured lines, together with their self-consistent uncertainties, for the spectral region up to the first dissociation limit. The spectroscopic network of H 2 16 O contains two components, an ortho (o) and a para (p) one. For o-H 2 16 O and p-H 2 16 O, experimentally measured, assigned, and labeled transitions were analyzed from more than 100 sources. The measured lines come from one-photon spectra recorded at room temperature in absorption, from hot samples with temperatures up to 3000 K recorded in emission, and from multiresonance excitation spectra which sample levels up to dissociation. The total number of transitions considered is 184 667 of which 182 156 are validated: 68 027 between para states and 114 129 ortho ones. These transitions give rise to 18 486 validated energy levels, of which 10 446 and 8040 belong to o-H 2 16 O and p-H 2 16 O, respectively. The energy levels, including their labeling with approximate (J. Tennyson). Journal of Quantitative Spectroscopy & Radiative Transfer 117 (2013) 29-58
Quantum study of the bending relaxation of H2O by collision with H
Monthly Notices of the Royal Astronomical Society
Vibrationally excited levels of the H2O molecule are currently detected in various environments of the interstellar medium (ISM), and collisional data for H2O, including vibration with the main colliders of the ISM, are needed. The present study focuses on the bending relaxation of H2O by collision with H when taking bending–rotation coupling explicitly into account with the rigid-bender close-coupling (RB-CC) method. With this aim, a new four-dimensional potential energy surface including the H2O bending mode is developed from a large grid of ab initio energies computed using a high level of theory. For purely rotational transitions, our RB-CC rates show very good agreement with rigid-rotor calculations performed using our new potential energy surface (PES) and with those available in the literature. Calculations for pure rotational transitions inside the excited bending level ν2 = 1 of H2O are performed and compared with their equivalents inside ν2 = 0. Vibrational quenching of H2...
Collisionally Assisted Spectroscopy of Water from 27 000 to 34 000 cm −1
The Journal of Physical Chemistry A, 2008
We report here an experimental approach that enables measurement of weak transitions to a wide range of rovibrational levels of water in the energy region 27 000-34 200 cm -1 . We have previously demonstrated the use of laser double-resonance overtone excitation to access highly excited vibrational levels from single rovibrational states. Although this approach simplifies the assignment of the spectra, it strongly reduces the number of observed transitions and hence our ability to test theoretical predictions. Here, we increase significantly the number of observed transitions by allowing rotational relaxation of H 2 O at intermediate levels of the double-resonance excitation scheme to the levels of the same nuclear spin (ortho or para). Our recently developed semiempirical potential energy surface PES 12 enables assignment of the resulting complex spectra and reproduction of the measured transitions with accuracy better than 1 cm -1 .
Communication: Mapping water collisions for interstellar space conditions
The Journal of Chemical Physics, 2010
We report a joint experimental and theoretical study that directly tests the quality of the potential energy surfaces used to calculate energy changing cross sections of water in collision with helium and molecular hydrogen, at conditions relevant for astrophysics. Fully state-to-state differential cross sections are measured for H 2 O -He and H 2 O-H 2 collisions at 429 and 575 cm −1 collision energy, respectively. We compare these differential cross sections with theoretical ones for H 2 O+H 2 derived from state-of-the-art potential energy surfaces ͓P. Valiron et al., J. Chem. Phys. 129, 134306 ͑2008͔͒ and quantum scattering calculations. This detailed comparison forms a stringent test of the validity of astrophysics calculations for energy changing rates in water. The agreement between theory and experiment is striking for most of the state-to-state differential cross sections measured.
Revised study of the collisional excitation of HCO+ by H2(j = 0)
Monthly Notices of the Royal Astronomical Society, 2014
Rotational excitation of the interstellar HCO + by para-H 2 (j = 0) is investigated. The scattering calculations are based on a new potential energy surface (PES) for the HCO +-H 2 van der Waals complex averaged over H 2 orientations and considering both molecules as rigid rotors. The new ab initio PES was obtained from electronic structure calculations using a single and double excitation coupled cluster method with perturbative contributions from connected triple excitations [CCSD(T)]. The five atoms were described using the augmented correlationconsistent triple zeta basis set. For a better description of the van der Waals interaction, bond functions were placed approximately at mid-distance between the HCO + and H 2 centres of mass. Inelastic cross sections between rotational levels up to rotational momentum j = 20 were computed using a close coupling approach. Rate coefficients for temperatures ranging from 10 to 500 K were computed. The new rate coefficients were compared with previous results obtained for the HCO + cation. Differences were found. We expect that the new rate coefficients will help to analyse present and future observations of this key ion for the interstellar chemistry.
Journal of Quantitative Spectroscopy and Radiative Transfer, 2014
This is the third of a series of articles reporting critically evaluated rotationalvibrational line positions, transition intensities, and energy levels, with associated critically reviewed labels and uncertainties, for all the main isotopologues of water. This paper presents experimental line positions, experimental-quality energy levels, and validated labels for rotational-vibrational transitions of the most abundant isotopologue of water, H 2 16 O. The latest version of the MARVEL (Measured Active Rotational-Vibrational Energy Levels) line-inversion procedure is used to determine the rovibrational energy levels of the electronic ground state of H 2 16 O from experimentally measured lines, together with their self-consistent uncertainties, for the spectral region up to the first dissociation limit. The spectroscopic network of H 2 16 O contains two components, an ortho (o) and a para (p) one. For o-H 2 16 O and p-H 2 16 O, experimentally measured, assigned, and labeled transitions were analyzed from more than 100 sources. The measured lines come from one-photon spectra recorded at room temperature in absorption, from hot samples with temperatures up to 3000 K recorded in emission, and from multiresonance excitation spectra which sample levels up to dissociation. The total number of transitions considered is 184 667 of which 182 156 are validated: 68 027 between para states and 114 129 ortho ones. These transitions give rise to 18 486 validated energy levels, of which 10 446 and 8040 belong to o-H 2 16 O and p-H 2 16 O, respectively. The energy levels, including their labeling with approximate (J. Tennyson). Journal of Quantitative Spectroscopy & Radiative Transfer 117 (2013) 29-58
Journal of Physics: Conference Series, 2017
Line shape parameters for hydrogen broadening of water vapor are needed to understand remote sensing measurements of planetary and exoplanet atmospheres. In order to address these needs, semiclassical calculations based on the Modified Complex Robert-Bonamy (MCRB) formalism were made. The intermolecular potential for the calculation is comprised of electrostatic, atom-atom (expanded to order 16 and rank 4), induction, and London dispersion terms. The trajectories were determined by numerical integration of the Hamilton's equations. The average over the Maxwell-Boltzmann distribution of velocities was performed by integration over 35 velocities corresponding to the temperature range 75K-27000K. The formalism is complex valued yielding the half-width and line shift from a single calculation. The calculations are reported at 7 temperatures from 200 to 700 K. The halfwidth temperature dependence coefficient n was determined using the relation γ (T) = γ (T 0) T 0 T [ ] n with T 0 =296K. The calculations are compared with the measurements of Brown and Plymate [JQSRT 56, 263, 1996]. Future plans for further refinement of the intermolecular potential are discussed.
The Chemistry of Vibrationally Excited H2IN the Interstellar Medium
The Astrophysical Journal, 2010
The internal energy available in vibrationally excited H 2 molecules can be used to overcome or diminish the activation barrier of various chemical reactions of interest for molecular astrophysics. In this article we investigate in detail the impact on the chemical composition of interstellar clouds of the reactions of vibrationally excited H 2 with C + , He + , O, OH, and CN, based on the available chemical kinetics data. It is found that the reaction of H 2 (v > 0) and C + has a profound impact on the abundances of some molecules, especially CH + , which is a direct product and is readily formed in astronomical regions with fractional abundances of vibrationally excited H 2 , relative to ground state H 2 , in excess of ∼ 10 −6 , independently of whether the gas is hot or not. The effects of these reactions on the chemical composition of the diffuse clouds ζ Oph and HD 34078, the dense PDR Orion Bar, the planetary nebula NGC 7027, and the circumstellar disk around the B9 star HD 176386 are investigated through PDR models. We find that formation of CH + is especially favored in dense and highly FUV illuminated regions such as the Orion Bar and the planetary nebula NGC 7027, where column densities in excess of 10 13 cm −2 are predicted. In diffuse clouds, however, this mechanism is found to be not efficient enough to form CH + with a column density close to the values derived from astronomical observations.