Inelastic excitation and charge transfer processes for oxygen in collision with H atoms.

Potential energy functions of the OH molecule are investigated from small to large inter-atomic distances R. The electronic problem is treated using an efficient Full Configuration Interaction (Full CI) approach that avoids orbital jumps found usually in multi-configuration self-consistent-field followed by multi-reference configuration interaction calculations of excited states. The calculations are performed for all the doublet, quartet, and sextet OH molecular states, up to the O(2p34s 3S) + H(1s 2S) asymptote, and for the lowest O- + H+ and O+ + H- ionic states. Inter-atomic distances, ranging from 0.5 Å to 20 Å, are spanned with a very small step in order to describe accurately the avoided crossings between the adiabatic potential energy functions. The accuracy of the potentials at small and large R values is analyzed. These Full CI calculations provide for the first time a global description of the 40 lowest molecular states of OH, well suited for dynamical calculations. The resulting potentials are used to obtain first estimates of cross sections and rate coefficients for different inelastic processes through the multichannel approach. This method, based on a Landau-Zener formalism taking into account the ionic-covalent avoided crossings at large distances, gives reliable results for the most intense transitions. It is shown that the largest rate coefficients correspond to mutual neutralization and ion-pair production processes.

Calcium-hydrogen interactions for collisional excitation and charge transfer.

The accurate highly correlated ab initio calculations for ten low lying covalent Σ+2 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 Å) 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.

Ab initio investigation of the abstraction reactions by H and D from tetramethylsilane and its deuterated substitutions.

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 → H2 + Si(CH3)3CH2 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 × 10(-19) × T(2.65) exp(-2455.7/T) and k(CVT/SCT) = 7.81 × 10(-19) × 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.

Communication: Mapping water collisions for interstellar space conditions.

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 (2008)] 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.

A five-dimensional potential-energy surface for the rotational excitation of SO2 by H2 at low temperatures.

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.

Rotationally inelastic collisions of SO(X3Sigma-) with H2: potential energy surface and rate coefficients for excitation by para-H2 at low temperature.

Rotational excitation of the interstellar species SO(X3Sigma-) with H2 is investigated. The authors present a new four-dimensional potential energy surface for the SO-H2 system, calculated at an internuclear SO distance frozen at its experimental minimum energy distance. It was obtained at the RCCSD(T) level using the aug-cc-pVTZ basis sets for the four atoms. Bond functions were placed at mid-distance between the SO center of mass and the center of mass of H2 for a better description of the van der Waals interaction. Close coupling calculations of the collisional excitation cross sections between the fine structure levels of SO by collisions with para-H2 are calculated at low energies which yield, after Boltzmann thermal average, rate coefficients up to 50 K. The exact level splitting is taken into account. The propensity rules between fine structure levels are studied. It is shown that F-conserving cross sections are much larger, especially for high-N rotational levels, than F-changing cross sections, as found previously for SO-He collisions and expected from theoretical considerations. The new rate coefficients are compared with previous results obtained for this molecule and they find that important differences exist that can induce important consequences on astrophysical modeling. Comparison with excitation by collision with He shows that the rate coefficients differ by important factors that cannot be only explained by the reduced mass ratio in the thermal average. This may be due to differences between the potential energy surfaces as well as to the contribution of the different reduced masses in the scattering equations.

Rotational excitation of sulfur monoxide by collisions with helium at low temperature.

We present two new two-dimensional potential-energy surfaces for the SO-He system calculated at SO r distance frozen at its experimental minimum-energy distance. Both are obtained at the RCCSD(T) level using two different basis sets (AVTZ and AVQZ) for the three atoms. Bond functions are placed at mid-distance between the SO center of mass and He for a better description of the van der Waals well. Close-coupling calculations of the collisional excitation cross sections of the fine-structure levels of SO by He are calculated at low energies. The exact level splitting is taken into account. It is found that the results obtained from the two surfaces are very similar, except for some small differences observed in the region of resonances at low energies. The propensity rules between fine-structure levels are studied, it is shown that F-conserving cross sections are much larger for high-N rotational levels than cross sections between F-changing levels, as expected from theoretical considerations. The use of infinite order sudden recoupling techniques from spin-free cross sections is investigated. Excitation rate coefficients among fine-structure levels are calculated at low temperatures.

New ab initio potential energy surface for the (HOCO+-He) van der Waals complex.

A three-dimensional potential energy surface has been calculated for the ground electronic state of the HOCO+-He system. The calculations were performed at the coupled electron pair approximation level with an extended basis set which ensures a balance between accuracy and feasability. The validity of the method and of the basis set was tested through calculations of the polarizability of the He atom and of the spectroscopic constants of the HOCO+ ion. The calculated potential energy surface has been fitted to a spherical harmonic expansion to facilitate calculations of rotational excitation of HOCO+ by collisions with He.