Hammami, K.
Lique, F.
Jaidane, N.
Ben Lakhdar, Z.
Spielfiedel, A.
Feautrier, N.
Context. It has been shown that the HOCO+ ion is present in interstellar space. As a large number of HOCO+ lines can be observed in the millimeter and submillimeter wavelengths, this molecule is a useful tracer for both the temperature and the density structure of the clouds. Modeling of the spectra will require accurate radiative and collisional rates of species of astrophysical interest. Aims. The paper focuses on the calculation of rotational excitation rate coefficients of HOCO+ by He, useful for studies of low-temperature environments. Methods. Cross sections are calculated using the quantum Coupled States approach for a total energy range 0-200 cm(-1). These calculations are based on a new ab initio CEPA (coupled electron pair approach) potential energy surface. It was assumed that the HOCO+ ion was fixed at its theoretical equilibrium geometry. Results. Thermally averaged rate coefficients were calculated from the cross sections, at kinetic temperatures up to 30 K.
Feautrier, N.
Balanca, C.
Dayou, F.
Spielfiedel, A.
Cressiot-Vincent, L.
Faure, A.
Wiesenfeld, L.
Senent, M. -L.
The SO2 molecule is detected in a large variety of objects, notably cold dark clouds and star-forming regions. An accurate modeling of the observations requires a very good knowledge of the collisional excitation rates with H-2 due to competition between collisional and radiative processes that excite and quench the different rotational levels of the molecule. The results of our recent collisional calculations are summarized. Pierre was associated to all steps of this collaborative work that was a key project in the Molecular Universe European FP6 network.
Context. Over the next few years, ALMA and Herschel missions will perform high spatial and spectral resolution studies at infrared and sub-millimeter wavelengths. This will provide much greater detail about the composition and evolution of molecules in space. Modeling of the spectra will require accurate radiative and collisional rates for species of astrophysical interest. Aims. We calculate ro-vibrational excitation rate coefficients of SO by He, useful for studies of high-temperature environments. Methods. A new accurate three dimensional (3D) potential energy surface was calculated for the SO-He system which explicitly takes into account the r-dependence of the SO vibration as well as the R-distance and theta angle which describe the relative position of the collision partners. The dynamics calculations were performed according to the VCC-IOS approximation. Results. The new rate coefficients between the ro-vibrational levels are calculated for temperatures from 300 K to 800 K.
Belyaev, A. K.
Yakovleva, S. A.
Guitou, M.
Mitrushchenkov, A. O.
Spielfiedel, A.
Feautrier, N.
Aims. Inelastic processes in low-energy Ca + H and Ca+ + H- collisions are treated for the states from the ground state up to the ionic state with the aim to provide rate coefficients needed for non-LTE modeling of Ca in cool stellar atmospheres. Methods. The electronic molecular structure was determined using a recently proposed model approach that is based on an asymptotic method. Nonadiabatic nuclear dynamics were treated by means of multichannel formulas, based on the Landau-Zener model for nonadiabatic transition probabilities. Results. The cross sections and rate coefficients for inelastic processes in Ca + H and Ca+ + H- collisions were calculated for all transitions between 17 low-lying covalent states plus the ionic state. It is shown that the highest rate coefficient values correspond to the excitation, de-excitation, ion-pair formation, and mutual neutralization processes involving the Ca(4s5s S-1,S- 3) and the ionic Ca+ + H- states. The next group with the second highest rate coefficients includes the processes involving the Ca(4s5p P-1,P- 3), Ca(4s4d (1,3D)), and Ca(4s4p P-1) states. The processes from these two groups are likely to be important for non-LTE modeling.
Mitrushchenkov, A.
Guitou, M.
Belyaev, A. K.
Yakovleva, S. A.
Spielfiedel, A.
Feautrier, N.
The accurate highly correlated ab initio calculations for ten low lying covalent (2)Sigma(+) states of CaH molecule, and one ionic Ca+H- state, are performed using large active space and extended basis set, with special attention to the long-range (6-20 angstrom) region where a number of avoided crossings between ionic and covalent states occur. These states are further transformed to a diabatic representation using a numerical diabatization scheme based on the minimization of derivative coupling. This results in a smooth diabatic Hamiltonian which can be easily fit to an analytic form. The diagonal elements of the diabatic potentials were then empirically corrected to reproduce experimental dissociation energies. Though the emphasis is on the asymptotic region, the obtained spectroscopic constants are in good agreement with available experimental and theoretical data. The resulting analytical Hamiltonian, after back transformation to adiabatic representation, is used to obtain cross sections for different inelastic processes using both the multichannel and the branching probability current approaches. It is shown that while for most intense transitions both approaches provide very close results, the multichannel approach underestimates the cross sections of weak transitions, as a consequence of the short-range avoided crossings that are accounted for only in the branching probability current method. Published by AIP Publishing.
Barklem, P. S.
Belyaev, A. K.
Spielfiedel, A.
Guitou, M.
Feautrier, N.
Rate coefficients for inelastic Mg+H collisions are calculated for all transitions between the lowest seven levels and the ionic state (charge transfer), namely Mg(3s(2) S-1, 3s3p P-3, 3s3p P-1, 3s4s S-3, 3s4s S-1, 3s3d D-1, 3s4p P-3)+H(1s) and Mg+(3s S-2)+H-. The rate coefficients are based on cross-sections from full quantum scattering calculations, which are themselves based on detailed quantum chemical calculations for the MgH molecule. The data are needed for non-LTE applications in cool astrophysical environments, especially cool stellar atmospheres, and are presented for a temperature range of 500-8000 K. From consideration of the sensitivity of the cross-sections to various uncertainties in the calculations, most importantly input quantum chemical data and the numerical accuracy of the scattering calculations, a measure of the possible uncertainties in the rate coefficients is estimated.
Lique, F.
Tobola, R.
Klos, J.
Feautrier, N.
Spielfiedel, A.
Vincent, L. F. M.
Chalasinski, G.
Alexander, M. H.
Context. Over the next few years, the ALMA and Herschel missions will perform high spatial and spectral resolution studies at infrared and sub-millimeter wavelengths. Modeling of molecular emission requires excitation calculations using radiative, as well as collisional rates, with the most abundant species. In the interstellar medium, the dominant collision partner is H-2, but little data is available for collisions with H-2. If data for collisions with He are available, it has often been proposed to use the more available rate coefficients for collision with He, with the appropriate reduced mass correction, as a first estimate of rate coefficients with H-2(j = 0). The validity of this approximation is not known. Aims. The present paper focuses on the calculation of rate coefficients among the first rotational levels of the SiS molecule in its ground vibrational state in collision with para-H-2 and compares these new data with recently published He ones to investigate the validity of using He rate coefficients to estimate H-2( j = 0) rate coefficients. Methods. A new potential energy surface for the SiS-para-H-2 system was obtained using highly correlated ab initio calculations. Dynamical calculations of pure rotational (de)excitation of SiS by para-H-2 were performed for the first rotational levels within the coupled-states approximation. Results. Collisional cross sections among the 51 first rotational levels of SiS were calculated for kinetic energies up to 2500 cm(-1). State-to-state rate coefficients are calculated for temperatures ranging from 5 K up to 300 K. A propensity rule that favors even Delta j transitions is found and is explained by the near homonuclear symmetry of the SiS-para-H-2 potential energy surface. A detailed comparison with recent SiS-He rate coefficients is also presented. We demonstrate that collision with He is a reasonable model for collisions with para-H-2, although this approximation must be used with caution.
Belyaev, A. K.
Barklem, P. S.
Guitou, M.
Spielfiedel, A.
Feautrier, N.
Vlasov, D. V.
Rodionov, D. S.
Full quantum scattering calculations of cross sections for low-energy near-threshold inelastic Mg + H collisions are reported, such processes being of interest for modelling of Mg spectral lines in stellar atmospheres. Nonadiabatic transions associated with radial couplings at avoided ionic crossings in the (2)Sigma(+) molecular states are found to be the main mechanism for excitation and ion-pair production processes.
Ouesati, I.
Kerkeni, B.
Spielfiedel, A.
Tchang-Brillet, W. -Ue L.
Feautrier, N.
Thermal rate constants for chemical reactions using the corrections of zero curvature tunneling (ZCT) and of small curvature tunneling (SCT) methods are reported. The general procedure is implemented and used with high-quality ab initio computations and semiclassical reaction probabilities along the minimum energy path (MEP). The approach is based on a vibrational adiabatic reaction path and is applied to the H + Si(CH3)(4) -> H-2 + Si(CH3)(3)CH2 reaction and its isotopically substituted variants. All of the degrees of freedom are optimized, and harmonic vibrational frequencies and zero-point energies are calculated at the MP2(full) level with the cc-pVTZ basis set. Single-point energies are calculated at a higher level of theory with the same basis set, namely, CCSD(T,full). The influence of the basis set superposition error (BSSE) on the energetics is tested. The method is further exploited to predict primary and secondary kinetic isotope effects (KIEs and SKIEs, respectively). Rate constants computed with the ZCT and SCT methods over a wide temperature range (180-2000 K) show important quantum tunneling effects at low temperatures when compared to rates obtained from the purely classical transition-state theory (TST) and from the canonical variational transition state theory (CVT). For the H + Si(CH3)(4) reaction, they are given by the following expressions: k(TST/ZCT) = 9.47 X 10(-19) X T-2.65 exp(-2455.7/T) and k(CVT/SCT) = 7.81 X 10(-19) X T-2.61 exp[(2704.2/T) (in cm(3) molecule(-1) s(-1)). These calculated rates are in very good agreement with those from available experiments.
Guitou, M.
Belyaev, A. K.
Barklem, P. S.
Spielfiedel, A.
Feautrier, N.
Full quantum scattering calculations of cross sections for low-energy near-threshold inelastic Mg+H collisions are reported, such processes being of interest for modelling of Mg spectral lines in stellar atmospheres. The calculations are made for three transitions between the ground and two lowest excited Mg states, Mg(3s(2) (1)S(0)), Mg(3s3p (3)P) and Mg(3s3p (1)P). The calculations are based on adiabatic potentials and nonadiabatic couplings for the three low-lying (2)Sigma(+) and the first two (2)Pi states, calculated using large active spaces and basis sets. Non-adiabatic regions associated with radial couplings at avoided ionic crossings in the (2)Sigma(+) molecular potentials are found to be the main mechanism for excitation. Cross sections of similar order of magnitude to those obtained in Li+H and Na+H collisions are found. This, together with the fact that the same mechanism is important, suggests that as has been found earlier for Li and Na, processes such as ion pair production may be important in astrophysical modelling of Mg, and motivates continued study of this system including all states up to and including the ionic limit.
An irreducible tensor formalism is applied to isotropic collisions of diatomic molecules with S-1 atoms. Explicit expressions of the generalized spectroscopic relaxation cross sections are given, including pressure broadening cross sections as well as collisional transfer and destruction cross sections of the k-component of the target density matrix. For applications in the high temperature limit, formulae within the infinite order sudden approximation (IOS) are given for (1)Sigma electronic state molecules and (2S+1)Sigma electronic state molecules in the Hund's case (b) limit. Application to collisions of CS by He atoms shows that a good agreement is found between close coupling and IOS results at moderate energies and that multipolar depolarizing rates within a j-level are of the same order of magnitude whatever the considered multipole order k whereas multipolar transfer rates are lower by an order of magnitude. Propensity rules in relation to the CS-He potential energy surface are given.
Spielfiedel, A.
Feautrier, N.
Balanca, C.
Dayou, F.
Lique, F.
Senent, M.-L.
Analysis of light emission from different regions of the interstellar medium and circumstellar environments provides crucial information about the chemical composition and the physical conditions in these regions. Interpretation of the observed spectra requires the knowledge of collisional excitation rates as well as radiative rates participating to the line formation. In the first part, the paper focuses on collisional excitation rates of molecules relevant to the interstellar medium. It discusses currently available data and outlines new work carried out by the authors. Due to the use of accurate ab initio potential energy surfaces, the new rate coefficients differ significantly from previously published ones. In the second part, it is analysed from two examples how the use of the new rate coefficients could lead to important changes in the interpretation of molecular emission emerging from molecular clouds.
Spielfiedel, A.
Senent, M. -L.
Dayou, F.
Balanca, C.
Cressiot-Vincent, L.
Faure, A.
Wiesenfeld, L.
Feautrier, N.
The SO(2) molecule is detected in a large variety of astronomical objects, notably molecular clouds and star-forming regions. An accurate modeling of the observations needs a very good knowledge of the collisional excitation rates with H(2) because of competition between collisional and radiative processes that excite and quench the different rotational levels of SO(2). We report here a five-dimensional, rigid-body, interaction potential for SO(2)-H(2). As a first application, we present rate constants for excitation/de-excitation of the 31 first levels of SO(2) by para-H(2) at low temperatures. Propensity rules are discussed.