We report combined studies on the prototypical S( 1 D 2 ) + H 2 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. , with kinetics and crossed-beam experiments approaching the cold (T < 1 K) regime, supported by state-of-the-art quantum calculations [4].Rate coefficients for the total removal of S( 1 D 2 ) by collisions with H 2 were measured using a CRESU apparatus [5,6] from T = 298 K down to T = 5.8 K, which is the lowest temperature ever achieved in kinetics experiments where laser cooling schemes are not applicable. Briefly, this experimental technique uses the supersonic expansion of a buffer gas through convergent-divergent Laval nozzles, each of them working under specific pressure conditions to cool the gas to a given temperature. A special double-walled nozzle was manufactured allowing pre-cooling to 77 K by liquid nitrogen, along with the reservoir upstream of the nozzle, to obtain a temperature of 5.8 K in the uniform supersonic flow.S( 1 D 2 ) atoms were produced in the supersonic flow by the laser photolysis of CS 2 at 193 nm. were fitted by single exponential functions to extract the pseudo-first order rate coefficientsThe values of k 1st were plotted as a function of the n-H 2 density to yield a straight line whose gradient corresponds to the second order rate coefficient (Fig. 1). The measured rate coefficients are given in Table I Intersystem crossing between the singlet and triplet PESs is ignored and the relaxation process is not considered. A previous theoretical trajectory surface-hopping study at higher energies indicated that the electronic quenching process may play a significant role in the total removal of S( 1 D 2 ) [16]. Nevertheless, the current theory accounts well for the total number of complexes formed in the collision, which finally decompose to give either products via reaction or ground-state reactants via electronic quenching. This explains the agreement of the theoretical rates with the experimental total removal rates, where both possibilities exist.Concerning the observed inversion between the experimental and theoretical rate coefficients at 5.8 K, the theoretical result at such a low temperature becomes highly sensitive to even small inaccuracies in the 1A' PES and also on the effect of non-adiabatic couplings between this PES and the 2A' and 3A' PESs which have been neglected in the present work. undulations originating from the highly structured reaction probability [18] are observed in the theoretical excitation functions. This is more pronounced for H 2 (j = 0) (Fig. 3a) than for H 2 (j = 1) (Fig. 3...

Context. Within shocked regions of the interstellar medium and circumstellar environment of AGB stars the proper modelling of SiO line emission through non-LTE radiative transfer calculations requires accurate values of collisional rate coefficients. Aims. The present study focuses on the transitions among the rotational levels of the SiO molecule in its ground vibrational state induced by collision with He. The H 2 molecule being the main colliding partner for the astrophysical regions of interest, the collisional process between SiO and para-H 2 ( j = 0) is also investigated in an approximated way. Methods. A new 2D SiO-He potential energy surface is computed by means of highly correlated ab initio calculations. Collisional rate coefficients corresponding to the pure rotational (de)excitation of SiO by collision with He are obtained from close-coupling quantum scattering calculations of inelastic cross sections. The SiO-He potential energy surface is also employed to compute rate coefficients for the rotational (de)excitation of SiO by collision with para-H 2 ( j = 0). Results. Rate coefficients for rotational levels up to j = 26 and kinetic temperatures in the range 10-300 K are obtained for the SiO-He colliding system. The large asymmetry of the SiO-He potential energy surface induces a propensity rule that favours odd ∆ j transitions over even ∆ j. The estimated values of the SiO-para-H 2 ( j = 0) rate coefficients are compared with those of Turner et al. (1992) for the twenty first rotational levels. As a result of significant differences between the SiO-He interaction potentials employed in the two studies, the rate coefficients are found to differ by a factor 2.5-5 for the main rotational transitions, whatever the temperature range.