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.

The flowing afterglow technique, coupled with laser induced fluorescence (LIF) and vacuum ultraviolet (vuv) absorption spectroscopy, has been used to determine the fractional H-atom contributions, fH, to the product distributions for the dissociative recombination of a series of protonated ions (N2H+, HCO+, HCO+2, N2OH+, OCSH+, H2CN+, H3O+, H3S+, NH+4, and CH+5 ) with electrons. The measurements were made at 300 K in two separate ways in two laboratories by (i) directly determining the H-atom number density using vuv absorption spectroscopy at the Lα (121.6 nm) wavelength and (ii) converting the H atoms to OH radicals using the reaction H+NO2→OH+NO followed by LIF to determine the OH number density. The agreement between the two techniques is excellent and values of fH varying from ∼0.2 (for OCSH+ ) to 1.2 (for CH+5 ) have been obtained showing that in some of the cases recombination can lead to the ejection of two separate H atoms. Comparison of the oxygen/sulphur analogs, HCO+2/OCSH+ and H3O+/H3S+ showed that the fH values were very different. Possible reasons for these differences are discussed. Comparison is also made with the available theory.

Rate constants were determined for the reactions of $\mathrm{CH}{\mathrm{(X}}^2\text{Π,ν=0)}$ with ${\mathrm{H}}_{\mathrm{2}}$ and ${\mathrm{D}}_{\mathrm{2}}$ and for the relaxation of $\mathrm{CH}{\mathrm{(X}}^2\text{Π,ν=1)}$ by ${\mathrm{H}}_{\mathrm{2}}$ and ${\mathrm{D}}_{\mathrm{2}}.$ The method, employing pulsed laser photolysis to generate CH radicals and laser-induced fluorescence to observe their rate of removal, was implemented between 744 and 86 K in heated and cryogenically cooled cells and from 295 to 13 K in a Cinétique de Réaction en Ecoulement Supersonique Uniforme (CRESU) apparatus. The rate constants for the reaction of $\mathrm{CH(\nu =0)}$ with ${\mathrm{D}}_{\mathrm{2}}$ were determined from 13 to 584 K and those for the removal of $\mathrm{CH(\nu =1)}$ by ${\mathrm{H}}_{\mathrm{2}}$ and ${\mathrm{D}}_{\mathrm{2}}$ from 23 to 584 K. These rate constants show no dependence on total pressure and a mild negative temperature dependence, and they are clearly related closely to the rate of capture to form a strongly bound ${\mathrm{CH}}_{\mathrm{3}}^{\mathrm{*}}$ or ${\mathrm{CHD}}_{\mathrm{2}}^{\mathrm{*}}$ energized collision complex. The rate constants for the reaction of $\mathrm{CH(\nu =0)}$ with ${\mathrm{H}}_{\mathrm{2}}$ were measured from 53 to 744 K. By contrast, their values depend in a complex fashion on temperature and total pressure, the latter effect having been studied from 4 to 400 Torr in heated and cooled cells and from 0.28 to 4.5 Torr at 53 K in the CRESU apparatus. It is clear that reaction proceeds via a ${\mathrm{CH}}_{\mathrm{3}}^{\mathrm{*}}$ complex which, favored by high temperatures and low total pressures, can dissociate to give ${\mathrm{CH}}_{\mathrm{2}}\mathrm{+H},$ $\mathrm{CH}{\mathrm{(X}}^2\mathrm{\Pi )+}{\mathrm{H}}_{\mathrm{2}}{\mathrm{\to CH}}_{\mathrm{2}}\mathrm{+H};$ ${\Delta }_r{H}_{298}^0\text{=+(12.0±5.2)\hspace{0.17em}}{\mathrm{kJ\hspace{0.17em}mol}}^{\mathrm{-1}}$ or, especially at low temperatures and high total pressures, may be stabilized by collisions to yield ${\mathrm{CH}}_{\mathrm{3}}$ radicals $\mathrm{CH}{\mathrm{(X}}^2\mathrm{\Pi )+}{\mathrm{H}}_{\mathrm{2}}{\mathrm{+M\to CH}}_{\mathrm{3}}\mathrm{+M};$ ${\Delta }_r{H}_{298}^0$ ${\text{=−(450.}}_7\text{±2.0)\hspace{0.17em}}{\mathrm{kJ\hspace{0.17em}mol}}^{\mathrm{-1}}.$ Careful analysis of the pressure-dependent fall-off behavior yields the following expressions for the rate constants (in the case of the association reaction, in the limit of low pressure): $k_{\mathrm{(1a)}}{\text{=(3.1±0.7)×10}}^{\text{−10}}\hspace{0.17em}\exp \text{(−(1650±100)/T)\hspace{0.17em}}{\mathrm{cm}}^{\mathrm{-3}}{\mathrm{\hspace{0.17em}molecule}}^{\mathrm{-1}}{\mathrm{s}}^{\mathrm{-1}},$ $k_{\mathrm{(1b)}}^{\mathrm{0}}\mathrm{[Ar]}{\text{=(5.2±1.5)×10}}^{\text{−30}}{\text{(T/298)}}^{\text{−1.6±0.22}}{\mathrm{[Ar]\hspace{0.17em}cm}}^{\mathrm{3}}{\mathrm{\hspace{0.17em}molecule}}^{\mathrm{-1}}{\mathrm{\hspace{0.17em}s}}^{\mathrm{-1}}.$ Comparison of the rate data obtained for the reaction of $\mathrm{CH(\nu =0)}$ with ${\mathrm{D}}_{\mathrm{2}}$ and for the removal of $\mathrm{CH(\nu =1)}$ by ${\mathrm{H}}_{\mathrm{2}}$ and ${\mathrm{D}}_{\mathrm{2}}$ is consistent with all three processes occurring through the formation of a ${\mathrm{CH}}_{\mathrm{3}}^{\mathrm{*}}$ $({\mathrm{CHD}}_{\mathrm{2}}^{\mathrm{*}})$ complex whose rate of formation is independent of the vibrational state of the CH. Consequently the rate constant for the reaction in the limit of high pressure can be estimated from that for the removal of $\mathrm{CH(\nu =1)}$ by ${\mathrm{H}}_{\mathrm{2}}$ yielding: $k_{\mathrm{(1b)}}^{\mathrm{\infty }}{\text{=(1.60±0.11)×10}}^{\text{−10}}{\text{(T/298)}}^{\text{−0.08±0.05}}\hspace{0.17em}{\mathrm{cm}}^{\mathrm{3}}{\mathrm{\hspace{0.17em}molecule}}^{\mathrm{-1}}{\mathrm{\hspace{0.17em}s}}^{\mathrm{-1}}.$