Near-Threshold and Resonance Effects in Rotationally Inelastic Scattering of DO with -H.

We present a combined experimental and theoretical study on the rotationally inelastic scattering of heavy water, DO, with -H. Crossed-molecular beam measurements are performed in the collision energy range between 10 and 100 cm, corresponding to the near-threshold regime in which scattering resonances are most pronounced. State-to-state excitation cross-sections are obtained by probing three low-lying rotational levels of DO using the REMPI technique.

Quenching transitions for the rovibrational transitions of water: Ortho-HO in collision with ortho- and para-H.

We present here the first full computation of the rovibrational quenching of a polyatomic molecule (water) by a rotating molecular projectile (H). The computation is performed for quenching from the first bending mode of water at ν ≃ 1595 cm with a rotation energy of up to ∼400 cm in the bending mode. Molecular hydrogen is in its para and ortho modifications; it is rotating with a rotational quantum number of up to 4 and 3, respectively.

Quantum nature of molecular vibrational quenching: Water-molecular hydrogen collisions.

Rates of conversions of molecular internal energy to and from kinetic energy by means of molecular collision allow us to compute collisional line shapes and transport properties of gases. Knowledge of ro-vibrational quenching rates is necessary to connect spectral observations to physical properties of warm astrophysical gasses, including exo-atmospheres. For a system of paramount importance in this context, the vibrational bending mode quenching of HO by H, we show here that the exchange of vibrational to rotational and kinetic energy remains a quantum process, despite the large numbers of quantum levels involved and the large vibrational energy transfer.

A Molecular Candle Where Few Molecules Shine: HeHHe.

HeHHe + is the only potential molecule comprised of atoms present in the early universe that is also easily observable in the infrared. This molecule has been known to exist in mass spectrometry experiments for nearly half-a-century and is likely present, but as-of-yet unconfirmed, in cold plasmas. There can exist only a handful of plausible primordial molecules in the epochs before metals (elements with nuclei heavier than 4 He as astronomers call them) were synthesized in the universe, and most of these are both rotationally and vibrationally dark.

Potential Energy Surface for the CH-H van der Waals Interaction.

Methane is an ubiquitous molecule, present as a minor component in many environments, including the Earth and planet atmospheres. Its van der Waals interaction with the main gases is an important ingredient for the understanding of radiative properties for those atmospheres. We present here the first precise determination of the interaction between CH and H.

van der Waals interaction of HNCO and H: Potential Energy Surface and Rotational Energy Transfer.

Isocyanic acid (HNCO) is the most stable of all its isomers; it has been observed repeatedly in many different conditions of the Interstellar Media, and its chemistry is poorly known. To quantitatively estimate the abundance of HNCO with respect to other organic molecules, we compute its rotational quenching rates colliding with H, the most common gas in the gaseous Interstellar Media. We compute ab initio the van der Waals interaction HNCO-H, in the rigid molecules approximation, with a CCSD(T)-F12a method.

Collision energy dependence of state-to-state differential cross sections for rotationally inelastic scattering of HO by He.

The inelastic scattering of HO by He as a function of collision energy in the range 381 cm to 763 cm at an energy interval of approximately 100 cm has been investigated in a crossed beam experiment using velocity map imaging. Change in collision energy was achieved by varying the collision angle between the HO and He beam. We measured the state-to-state differential cross section (DCS) of scattered HO products for the final rotational states J = 1, 1, 2 and 4.

Near-resonant rotational energy transfer in HCl-H2 inelastic collisions.

We present a new four-dimensional (4D) potential energy surface for the HCl-H2 van der Waals system. Both molecules were treated as rigid rotors. Potential energy surface was obtained from electronic structure calculations using a coupled cluster with single, double, and perturbative triple excitations method.

The H + CO ⇌ HCO reaction studied by ab initio benchmark calculations.

The title reaction has been calculated using complete active space self-consistent field and internally contracted multi-reference configuration interaction, including Davidson correction, calculations. Dunning's correlation consistent atomic basis sets, together with several complete basis set extrapolation schemes, were employed. Core-valence and scalar relativistic effects were also taken into account, as well as anharmonicity of the vibrational modes.

Spin-orbit quenching of the C+(2P) ion by collisions with para- and ortho-H2.

Spin-orbit (de-)excitation of C(+)((2)P) by collisions with H2, a key process for astrochemistry, is investigated. Quantum-mechanical calculations of collisions between C(+) ions and para- and ortho-H2 have been performed in order to determine the cross section for the C(+) (2)P3∕2 → (2)P1∕2 fine-structure transition at low and intermediate energies. The calculation are based on new ab initio potential energy surfaces obtained using the multireference configuration interaction method.

Rotational excitation of HDO and D2O by H2: experimental and theoretical differential cross-sections.

We present state-to-state differential cross sections (DCSs) for rotationally inelastic scattering of HDO by normal- and para-H(2) at collision energies of 580 cm(-1) and 440 cm(-1). (2+1) resonance enhanced multiphoton ionization is used to detect rotationally cold HDO molecules before collision and as scattering products, which occupy higher rotational states due to collision with H(2). Relative integral cross sections of HDO are obtained by integrating its DCSs measured at the same experimental conditions.

Potential energy surface and rotational cross sections for methyl formate colliding with helium.

A potential energy surface for helium interacting with methyl formate has been computed using high-level electronic structure methods. The interaction energies obtained on a three-dimensional grid have been fitted by an analytic function of interatomic distances with correct asymptotic behavior for large intermonomer separations. This potential has then been refitted using partial wave expansion in terms of the distance between centers of mass and spherical angles.

Theoretical investigation of the isomerization of trans-HCOH to H2CO: an example of a water-catalyzed reaction.

A concerted hydrogen atom transfer mechanism has been elucidated for the isomerization of trans-HCOH to H(2)CO using a variety of ab initio and density functional theory methods. This work places specific emphasis on the role water molecules can play as a catalyst for this reaction and the mechanism by which this is achieved. This is of particular importance in the context of molecular ices in the interstellar medium because the presence of water in this reaction reduces the activation energy by at least 80%, which is accompanied by a significant enhancement of the reaction rate, at ≤300 K.

Rotational quenching of monodeuterated water by hydrogen molecules.

Cross sections and rate coefficients for low lying rotational transitions in HDO induced by para and ortho-H(2) collisions are presented for the first time. Calculations have been performed at the close-coupling and coupled-states levels with the deuterated variant of the H(2)O-H(2) interaction potential of Valiron et al. [J.

Fast ion-molecule reactions in planetary atmospheres: a semiempirical capture approach.

The description of planetary and interstellar chemistry relies strongly on ion-molecule reaction rate data collected at room temperature or above. However, the temperature in the ionospheres of planets and in the interstellar medium can decrease down to 100 K and 10 K, respectively. We present here a simple semiempirical method to extend available measurements towards those temperatures.

Communication: Rotational excitation of interstellar heavy water by hydrogen molecules.

Cross sections and rate coefficients for low lying rotational transitions in D(2)O induced by para-H(2) collisions are presented for the first time. Calculations have been performed at the close-coupling level with the deuterated variant of the H(2)O-H(2) interaction potential of Valiron et al. [J.

Imaging the inelastic scattering of water with helium. Comparison of experiment and theory.

State-to-state differential cross sections for rotationally inelastic He-H(2)O scattering have been measured at 53.3 meV (429 cm(-1)) collision energy, using the crossed molecular beam technique. The inelastic events are detected by velocity map imaging of nascent H(2)O(+) ions, which are formed by state-selective (2 + 1) resonance enhanced multiphoton ionization (REMPI) of the scattered H(2)O molecules.

A semiempirical capture model for fast neutral reactions at low temperature.

The description of Titan's chemistry relies strongly on reaction rate data collected at room temperature or above. However, the temperature in the atmosphere of Titan ranges from 70 to 200 K. We describe here a simple theoretical method to extend the available measurements toward those temperatures.

R12-calibrated H2O-H2 interaction: full dimensional and vibrationally averaged potential energy surfaces.

The potential energy surface of H(2)O-H(2) is of great importance for quantum chemistry as a test case for H(2)O-molecule interactions. It is also required for a detailed understanding of important astrophysical processes, namely, the collisional excitation of water, including the pumping of water masers and the formation of molecular hydrogen on icy interstellar dust grains. We have calculated the interaction for H(2)O-H(2) by performing both rigid-rotor (five-dimensional) and non-rigid-rotor (nine-dimensional) calculations using the coupled-cluster theory at the level of singles and doubles with perturbative corrections for triple excitations [CCSD(T)] with moderately large but thoroughly selected basis set.

Electron-impact rotational excitation of H3+: relevance for thermalization and dissociation dynamics.

Electrons are known to be efficient in rotationally exciting molecular ions in low-density astrophysical plasmas. Rotational excitation of molecular ions has also been shown to affect the measured values of dissociative recombination (DR) rate coefficients. Thus, electron collisions with H3+ are expected to play a significant role in thermalization and dissociation dynamics of this ion, both in the laboratory and in space.