Thierry Stoecklin

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.

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.

Rate constant calculations for atom–diatom reaction involving an open-shell atom and a molecule in a Σ electronic state Application to the reaction Al(2P1/2,3/2)+O2(X3Σg-)→AlO(X2Σ+) +O(3P2,1,0)

We present a short study of the influence of the open-shell nature of the reactants on the kinetics of the title reaction. We use the adiabatic capture centrifugal sudden approximation (ACCSA) method and a long-range potential comprising the sum of the quadrupole–quadrupole, spin–orbit and dispersion contributions. Matrix elements of this potential are evaluated in the spin–orbit basis set of each reactant for the Al(2P1/2,3/2)+O2(X3Σg-) reaction and used within the ACCSA method to calculate the temperature dependence of the rate constant in the range 15–300 K. In the limit of zero temperature, the reactivities of the two spin–orbit states of the aluminium atom, Al(2P1/2) and Al(2P3/2), towards the O2 molecule are demonstrated to be different and analytical formulae for the rate constants are obtained as a function of temperature.

Potential energy surface of the CO2–N2 van der Waals complex

Four-dimensional potential energy surface (4D-PES) of the atmospherically relevant CO2-N2 van der Waals complex is generated using the explicitly correlated coupled cluster with single, double, and perturbative triple excitation (CCSD(T)-F12) method in conjunction with the augmented correlation consistent triple zeta (aug-cc-pVTZ) basis set. This 4D-PES is mapped along the intermonomer coordinates. An analytic fit of this 4D-PES is performed. Our extensive computations confirm that the most stable form corresponds to a T-shape structure where the nitrogen molecule points towards the carbon atom of CO2. In addition, we located a second isomer and two transition states in the ground state PES of CO2-N2. All of them lay below the CO2 + N2 dissociation limit. This 4D-PES is flat and strongly anisotropic along the intermonomer coordinates. This results in the possibility of the occurrence of large amplitude motions within the complex, such as the inversion of N2, as suggested in the recent spectroscopic experiments. Finally, we show that the experimentally established deviations from the C2v structure at equilibrium for the most stable isomer are due to the zero-point out-of-plane vibration correction.

Cross sections and low temperature rate coefficients for the H + CH+ reaction: a quasiclassical trajectory study

The H + CH(+) reaction is studied by quasiclassical trajectory (QCT) calculations, along with phase space theory (PST) and quantum rigid rotor calculations, employing a global single-valued potential energy surface recently derived by our group. We report QCT total cross sections for each of the three channels, for low collision energies and different reactant rotational quantum numbers. At the lowest collision energies, all cross sections exhibit a capture-like behaviour, as expected from a barrierless reaction. At higher energies, there are important dynamical effects coming from the opening of new channels in the inelastic and reactive exchange collisions. The inelastic cross sections turn out to largely increase, while the reactive abstraction cross sections are declining faster than predicted by the capture theory. A large value of the reactant rotational quantum number tends to suppress these dynamical effects. The QCT rate coefficients are reported for a temperature range from 1-700 K. Below 20 K, the abstraction and exchange QCT rate coefficients are almost constant, as predicted by the capture theory. Above this temperature, the abstraction rate coefficient declines, while the exchange and inelastic rate coefficients are increasing, due to the opening of new channels. A good agreement is observed between the experimental abstraction rate coefficient and the QCT and PST ones. The QCT inelastic results are also compared with those obtained from rigid rotor close coupling (CCRR) calculations in order to check the ability of this approach to provide a reliable estimate of the inelastic rate coefficients for a reactive system without a barrier. The laws of variation as a function of temperature are found to be very similar and the curves are parallel above 20 K. However, reaction is not allowed in the rigid rotor approximation, therefore the CCRR results are about twice as large as their QCT counterparts.

A comparative study of the reactivity of the silicon atom Si(3PJ) towards O2 and NO molecules at very low temperature

We present both an experimental and a theoretical study of the reactions Si(3PJ) + O2 and Si(3PJ) + NO. The CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) experimental technique is used to measure the rate constants in the temperature range 15–300 K. The laws of variation of the rate constants as a function of temperature are found to exhibit a maximum around 30 K for both reactions and are experimentally described using the following expressions: kSi+O2(T) = (1.72 ± 0.17) × 10−10 (T/300 K)−(0.53±0.10)exp−(17±4)K/T cm3 molecule−1 s−1 and kSi+NO(T) = (0.90 ± 0.10) × 10−10(T/300 K)−(0.96±0.10)exp−(28±3)K/T cm3 molecule−1 s−1. Adiabatic capture calculations are also performed and the temperature dependences obtained for the rate constants present a maximum in good agreement with the experimental results. A perturbative approach is eventually used in the limit of zero temperature to differentiate the spin–orbit reactivity of the silicon atom. These results are also compared with an earlier similar study concerning the reactivity of carbon atoms in the ground state, C(3PJ), with the same molecules.

Ro-vibrational relaxation of HCN in collisions with He: Rigid bender treatment of the bending-rotation interaction

We present a new theoretical method to treat atom-rigid bender inelastic collisions at the Close Coupling (RB-CC) level in the space fixed frame. The coupling between rotation and bending is treated exactly within the rigid bender approximation and we obtain the cross section for the rotational transition between levels belonging to different bending levels. The results of this approach are compared with those obtained when using the rigid bender averaged approximation (RBAA) introduced in our previous work dedicated to this system. We discuss the validity of this approximation and of the previous studies based on rigid linear HCN. We find that l-type transitions cross sections have to be calculated at the RB-CC level for the He-HCN collision while pure rotational transitions cross sections may be calculated accurately at the RBAA level.

Rotational excitation and de-excitation of CH+ molecules by 4He atoms

We present a quantum mechanical investigation of rotational energy transfer in cold collisions of CH + with 4 He atoms. We use a global 3D potential energy surface obtained using the reproducing Kernel Hilbert Space (RKHS) method. Rotational deactivation transition cross-sections are performed for collision energy ranging from 10 −6 to 3000 cm −1 and the corresponding rotational deactivation and excitation rate coefficients are evaluated for the transitions of levels up to j = 5 and temperatures up to 500 K. We also discuss the validity of the rigid rotor approximation for this collision.

Low temperature quantum rate coefficient of the H + CH+ reaction.

In this paper we report the first theoretical study of the title reaction. A global, single-valued model of the ground-state potential energy surface has been obtained by fitting to an extensive set of high-level ab initio calculations. The surface is found to be attractive apart from linear geometries where energy barriers appear due to conical intersections. This model was then used to calculate the reactive reactant state selected cross sections for collision energies ranging from threshold up to 4000 cm(-1). These calculations were performed using our version of the Baers approach of the RIOSA-NIP method which is based on the use of a negative imaginary potential. We find that the reaction probability is extremely oscillatory as a function of kinetic energy as it is a case for insertion reactions with a low exoergicity. The resulting reaction rate coefficient is found to first increase slowly as a function of temperature up to a broad maximum around 20 K and then to decrease slowly when temperature keeps increasing.

On the use of explicitly correlated treatment methods for the generation of accurate polyatomic –He/H2 interaction potential energy surfaces: The case of C3–He complex and generalization

Through the study of the C3(X1Σg (+)) (1)Σg (+)) + He((1)S) astrophysical relevant system using standard (CCSD(T)) and explicitly correlated (CCSD(T)-F12) coupled cluster approaches, we show that the CCSD(T)-F12/aug-cc-pVTZ level represents a good compromise between accuracy and low computational cost for the generation of multi-dimensional potential energy surfaces (PESs) over both intra- and inter-monomer degrees of freedom. Indeed, the CCSD(T)-F12/aug-cc-pVTZ 2D-PES for linear C3 and the CCSD(T)-F12/aug-cc-pVTZ 4D-PES for bent C3 configurations gently approach those mapped at the CCSD(T)/aug-cc-pVXZ (X = T,Q) + bond functions level, whereas a strong reduction of computational effort is observed. After exact dynamical computations, the pattern of the rovibrational levels of the intermediate C3-He complex and the rotational and rovibrational (de-) excitation of C3 by He derived using both sets of PESs agree quite well. Since C3 shows a floppy character, the interaction PES is defined in four dimensions to obtain realistic collisional parameters. The C-C-C bending mode, which fundamental lies at 63 cm(-1) and can be excited at very low temperatures is explicitly considered as independent coordinate. Our work suggests hence that CCSD(T)-F12/aug-cc-pVTZ methodology is the key method for the generation of accurate polyatomic - He/H2 multi-dimensional PESs.