Michel Costes

Partial wave resonances predicted to occur in bimolecular collision processes have proven challenging to observe experimentally. Here, we report crossed-beam experiments and quantum-scattering calculations on inelastic collisions between ground-state O₂ and H₂ molecules that provide state-to-state cross sections for rotational excitation of O₂ (rotational state N = 1, j = 0) to O₂ (N = 1, j = 1) in the vicinity of the thermodynamic threshold at 3.96 centimeter⁻¹. The close agreement between experimental and theoretical results confirms the classically forbidden character of this collision-induced transition, which occurs exclusively in a purely quantum mechanical regime via shape and Feshbach resonances arising from partial waves with total angular momentum (J) = 2 to 4.

We review experimental studies of bimolecular exchange reactions between neutral species that can occur in dense molecular clouds: kinetics experiments performed with the so-called CRESU technique which measure global reaction rates and dynamics measurements performed with crossed molecular beam apparatuses which bring information on the products and branching ratios. Special attention is devoted to reactions of the carbon atom which may contribute to the synthesis of long polyyne or polyene chains. Results on the reactions of C(³P_J) atoms with methylacetylene, allene, ethylene and acetylene, with CRESU (in terms of rate coefficients) and crossed-beam experiments (in terms of integral and differential reaction cross sections) are discussed. Emphasis is placed on the interest to use these complementary methods in conjunction to obtain detailed rate coefficients. It is also shown that crossed-beam experiments can furnish sensitive tests to the state-to-state cross sections and rates for rotational energy transfer by inelastic collisions obtained by theoretical methods, as exemplified with CO + H₂ collisions.

We report integral cross sections for the S(1D2)+HD(j=0)$\rightarrow${}DS+H and HS+D reaction channels obtained through crossed-beam experiments reaching collision energies as low as 0.46~meV and from adiabatic time-independent quantum-mechanical calculations. While good overall agreement with experiment at energies above 10~meV is observed, neither the product channel branching ratio nor the low-energy resonancelike features in the HS+D channel can be theoretically reproduced. A nonadiabatic treatment employing highly accurate singlet and triplet potential energy surfaces is clearly needed to resolve the complex nature of the reaction dynamics.

We report combined studies on the prototypical S(D21)+H2 insertion reaction. Kinetics and crossed-beam experiments are performed in experimental conditions approaching the cold energy regime, yielding absolute rate coefficients down to 5.8~K and relative integral cross sections to collision energies as low as 0.68~meV. They are supported by quantum calculations on a potential energy surface treating long-range interactions accurately. All results are consistent and the excitation function behavior is explained in terms of the cumulative contribution of various partial waves.

We report combined studies on the prototypical S(^1D_2) + H2 insertion reaction. Kinetics and crossed-beam experiments are performed in experimental conditions approaching the cold energy regime, yielding absolute rate coefficients down to 5.8 K and relative integral cross sections to collision energies as low as 0.68 meV. They are supported by quantum calculations on a potential energy surface treating long range interactions accurately. All results are consistent and the excitation function behavior is explained in terms of the cumulative contribution of various partial waves.

The C(³P_J)+ NO(X ²Π_r)→ CN(X ²Σ⁺)+ O(³P_J) reaction is studied using combined pulsed lasers and pulsed, crossed, supersonic molecular beams. The relative reactive cross-section is found to decrease slowly in the relative translational energy domain sampled, ε_tr= 0.038 – 0.163 eV (1 eV ≈ 1.602 18 × 10^–19 J), with a dependence that can be fitted to a (ε_tr)^–1/3 functionality.