Philippe Halvick

BASECOL2012: A collisional database repository and web service within the Virtual Atomic and Molecular Data Centre (VAMDC)

The BASECOL2012 database is a repository of collisional data and a web service within the Virtual Atomic and Molecular Data Centre (VAMDC, http://www.vamdc.eu). It contains rate coefficients for the collisional excitation of rotational, ro-vibrational, vibrational, fine, and hyperfine levels of molecules by atoms, molecules, and electrons, as well as fine-structure excitation of some atoms that are relevant to interstellar and circumstellar astrophysical applications. Submissions of new published collisional rate coefficients sets are welcome, and they will be critically evaluated before inclusion in the database. In addition, BASECOL2012 provides spectroscopic data queried dynamically from various spectroscopic databases using the VAMDC technology. These spectroscopic data are conveniently matched to the in-house collisional excitation rate coefficients using the SPECTCOL sofware package (http:// vamdc.eu/software), and the combined sets of data can be downloaded from the BASECOL2012 website. As a partner of the VAMDC, BASECOL2012 is accessible from the general VAMDC portal (http://portal.vamdc.eu) and from user tools such as SPECTCOL.

O+OH→O2+H: A key reaction for interstellar chemistry. New theoretical results and comparison with experiment

We report extensive, fully quantum, time-independent (TID) calculations of cross sections at low collision energies and rate constants at low temperatures for the O+OH reaction, of key importance in the production of molecular oxygen in cold, dark, interstellar clouds and in the chemistry of the Earths atmosphere. Our calculations are compared with TID calculations within the J-shifting approximation, with wave-packet calculations, and with quasiclassical trajectory calculations. The fully quantum TID calculations yield rate constants higher than those from the more approximate methods and are qualitatively consistent with a low-temperature extrapolation of earlier experimental values but not with the most recent experiments at the lowest temperatures.

Converged three‐dimensional quantum mechanical reaction probabilities for the F+H2 reaction on a potential energy surface with realistic entrance and exit channels and comparisons to results for three other surfaces

Accurate three‐dimensional quantum mechanical reaction probabilities are presented for the reaction F+H2→HF+H on the new global potential energy surface 5SEC for total angular momentum J=0 over a range of translational energies from 0.15 to 4.6 kcal/mol. We find that the v’=3 HF vibrational product state has a threshold as low as for v’=2. We also find considerable structure in the reaction probability and cumulative reaction probability curves which may be indicative of resonance structures. We compare these results to those for another potential energy surface 5SEC‐W, which differs from surface 5SEC in the magnitude of the van der Waals well in the entrance channel, and to those for two previous potential energy surfaces.

A theoretical study of the dynamics of the reaction C(3P)+NO(X2Π)→CN(X2Σ+)+O(3P)

Abstract A quasi-classical trajectory (QCT) treatment of the reaction C ( 3 P) + NO (X 2 Π)→CN(X 2 Σ + ) + O ( 3 P) is presented on two different analytical and global representations of the lowest potential energy surface of 2 A′ symmetry. We compare the dynamic information obtained theoretically with that obtained from the recent supersonic pulsed crossed beam scattering experiment performed in our laboratory. A satisfactory agreement is obtained, notably for the percentage of the vibrational energy and the population of the vibrational levels of CN(X 2 Σ + ).

On the statistical behavior of the O+OH→H+O2 reaction: A comparison between quasiclassical trajectory, quantum scattering, and statistical calculations

The dynamics of the O + OH reaction on the ground state potential energy surface (PES) is investigated by means of the quasiclassical trajectory method and two statistical methods: phase space theory and statistical quantum method. Preliminary calculations with an exact quantum method are also reported. The quasiclassical trajectory calculations show evidence for a phase space bottleneck inhibiting the intramolecular energy transfer between the O-H and O-O bonds. As a result, the probability of the intermediate complex dissociating back toward the reactants is high, thereby yielding a reaction probability significantly lower than expected for a barrierless and exothermic reaction. The features of the PES, which are the cause of this dynamical effect, are identified. This is essentially the conservation of the equilibrium distance of the O-H bond, hardly changed by a close encounter with an oxygen atom. The statistical calculations, which do not take into account the PES in the complex region, yield a high reaction probability, much larger than the probability calculated from the dynamical methods, both classical and quantum. If the statistical cross sections are corrected by a scaling factor, which corresponds actually to scaling the capture probability, then a good agreement is observed between dynamical and statistical calculations of the product state distributions. The differential cross sections calculated with all the methods show a backward-forward symmetry, with sharp polarization peaks. The complex lifetime is divided into two parts by the bottleneck. During the first part, the system remains trapped in a small region of the phase space and has a high probability to dissociate back toward the reactants. This is a nonstatistical effect due to the PES shape. During the second part, fast intramolecular vibrational energy redistribution takes place, leading to a statistical distribution of energy on the rovibrational states of the products. These findings indicate that the O + OH reaction has mixed dynamics, both with statistical and nonstatistical aspects.

Converged quantum dynamics calculations for the F+H2 reaction on the well‐studied M5 potential‐energy surface

We present converged quantum dynamics calculations for the reaction F+H2(vi=0)→HF(vf=0,1,2,3)+H, where vi and vf denote initial and final vibrational quantum numbers, on potential‐energy surface no. 5 of Muckerman.

A new diabatic representation of the coupled potential energy surfaces for Na(3p 2P)+H2→Na(3s 2S)+H2 or NaH+H

We present a new diabatic representation of the coupled potential energy surfaces for Na(3p 2P)+H2→Na(3s 2S)+H2 or NaH+H. The new representation is designed to yield, upon diagonalization, realistic values for the two lowest energy 2A’ adiabatic states at both asymptotes of the chemical reaction as well as near the conical intersection in the three‐body interaction region. It is economical to evaluate and portable. It is suitable for dynamics calculations on both the quenching process and the electronically nonadiabatic chemical reaction.

Theoretical approach to the reaction C(3P)+NO(X 2Π)

Correlation diagram and extended ab initio calculations are used to obtain the main features of the dynamical aspects of the reaction C(3P)+NO(X 2Π). The lowest surface of 2Π symmetry leading to the ground state via an expected intermediate CNO is found to be 4.2 eV below the reactant energies. There is no potential energy barrier for the collinear approach along the 2Π surface. The first excited state of CNO has 2Σ+ symmetry. The high energy barrier appearing for the collinear C+NO approach yielding this state disappears for a bent configuration. A spin orbit avoided crossing between the 2Σ+ and 4Π surfaces is computationally investigated for the collinear approach. It is concluded that any such surface crossing will not play a significant role in the generation of excited CN(A 2Π). Due to the deep wells for the ground and first excited state of CNO, the vibrational populations in the products CN(X 2Σ+) and CN(A 2Π) are expected to be thermalized. However, the rotational populations are expected to be so...

Explicitly correlated treatment of the Ar–NO+ cation

We present an application of the recently developed explicitly correlated coupled cluster method to the generation of the three-dimensional potential energy surface (PES) of the Ar-NO(+) cationic complex. A good overall agreement is found with the standard coupled clusters techniques employing correlation consistent atomic basis sets (aug-cc-pVnZ, n= D, T, Q) of Wright et al. This PES is then used in quantum close-coupling scattering and variational calculations to treat the nuclear motions. The bound states energies of the Ar-NO(+) complex obtained by both approaches are in good agreement with the available experimental results. The analysis of the vibrational wavefunctions shows strong anharmonic resonances between the low frequency modes (intermonomer bending and stretching modes) and the wavefunctions exhibit large amplitude motions.

H2(v = 0,1) + C+(2 P) → H+CH+ STATE-TO-STATE RATE CONSTANTS FOR CHEMICAL PUMPING MODELS IN ASTROPHYSICAL MEDIA

State-to-state rate constants for the title reaction are calculated using the electronic ground state potential energy surface and an accurate quantum wave-packet method. The calculations are performed for H{sub 2} in different rovibrational states, v = 0, 1 and J = 0 and 1. The simulated reaction cross section for v = 0 shows a rather good agreement with the experimental results of Gerlich et al., both with a threshold of 0.36 eV and within the experimental error of 20%. The total reaction rate coefficients simulated for v = 1 are two times smaller than those estimated by Hierl et al. from cross sections measured at different temperatures and neglecting the contribution from v > 1 with an uncertainty factor of two. Thus, part of the disagreement is attributed to the contributions of v > 1. The computed state-to-state rate coefficients are used in our radiative transfer model code applied to the conditions of the Orion Bar photodissociation region, and leads to an increase of the line fluxes of high-J lines of CH{sup +}. This result partially explains the discrepancies previously found with measurements and demonstrates that CH{sup +} excitation is mostly driven by chemical pumping.