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...

The prototypical F + H2 → HF + H reaction possesses a substantial energetic barrier (~800 K) and might therefore be expected to slow to a negligible rate at low temperatures. It is, however, the only source of interstellar HF, which has been detected in a wide range of cold (10-100 K) environments. In fact, the reaction does take place efficiently at low temperatures due to quantum-mechanical tunnelling. Rate constant measurements at such temperatures have essentially been limited to fast barrierless reactions, such as those between two radicals. Using uniform supersonic hydrogen flows we can now report direct experimental measurements of the rate of this reaction down to a temperature of 11 K, in remarkable agreement with state-of-the-art quantum reactive scattering calculations. The results will allow a stronger link to be made between observations of interstellar HF and the abundance of the most common interstellar molecule, H2, and hence a more accurate estimation of the total mass of astronomical objects.

The reaction between sulfur atoms in the first electronically excited state, S((1)D), and ethene (C(2)H(4)) has been investigated in a complementary fashion in (a) crossed-beam dynamic experiments with mass spectrometric detection and time-of-flight (TOF) analysis at two collision energies (37.0 and 45.0 kJ mol(-1)), (b) low temperature kinetics experiments ranging from 298 K down to 23 K, and (c) electronic structure calculations of stationary points and product energetics on the C(2)H(4)S singlet and triplet potential energy surfaces. The rate coefficients for total loss of S((1)D) are found to be very large (ca. 4 x 10(-10) cm(3) molecule(-1) s(-1)) down to very low temperatures indicating that the overall reaction is barrierless. From laboratory angular and TOF distributions at different product masses, three competing reaction channels leading to H + CH(2)CHS (thiovinoxy), H(2) + CH(2)CS (thioketene), and CH(3) + HCS (thioformyl) have been unambiguously identified and their dynamics characterized. Product branching ratios have also been estimated. Interpretation of the experimental results on the reaction kinetics and dynamics is assisted by high-level theoretical calculations on the C(2)H(4)S singlet potential energy surface. RRKM (Rice-Ramsperger-Kassel-Marcus) estimates of the product branching ratios using the newly developed singlet potential energy surface have also been performed and compared with the experimental determinations.