Kinetics and Dynamics of the $\mathrm{S}({}^1\mathrm{D}_2)+{\mathrm{H}}_2\to \mathrm{SH}+\mathrm{H}$ Reaction at Very Low Temperatures and Collision Energies

We report combined studies on the prototypical $\mathrm{S}({}^1{\mathrm{D}}_2)+{\mathrm{H}}_2$ 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.

Figure 1

Pseudo-first-order rate coefficients plotted against the concentration of $n\mathrm{\text{−}}{\mathrm{H}}_2$ at 5.8 K. The inset shows the fitted decays of the $\mathrm{S}({}^1\mathrm{D}_2)$ laser-induced fluorescence observed in mixtures containing $2.04\times {10}^{14}$ molecule ${\mathrm{cm}}^{-3}$ $n\mathrm{\text{−}}{\mathrm{H}}_2$.

Figure 2

A comparison of experimental (filled circles) and theoretical (dashed line) rate coefficients for $\mathrm{S}({}^1\mathrm{D}_2)+n\mathrm{\text{−}}{\mathrm{H}}_2$ collisions. The only previous experimental result at 300 K (open circle) is also shown [15].

Figure 3

Integral cross sections (ICSs) of the $\mathrm{S}({}^1\mathrm{D}_2)+{\mathrm{H}}_2$ reaction. Theoretical ICSs computed for (a) ${\mathrm{H}}_2(j=0)$ and (b) ${\mathrm{H}}_2(j=1)$: total ICS (right vertical scale); some of the partial waves (left vertical scale) are also displayed for given values of total angular momentum $J$. The experimental measurements (c), obtained for 3 different sets of beam velocities, which are normalized in the overlapping energy range, are displayed in arbitrary units (left vertical scale) as symbols under the following beam conditions: inverted open triangles (▽): $v_{\mathrm{S}}=v_{\mathrm{H}2}=678\mathrm{m}{\mathrm{s}}^{-1}$; open circles (○): $v_{\mathrm{S}}=v_{\mathrm{H}2}=780\mathrm{m}{\mathrm{s}}^{-1}$; open triangles (▵): $v_{\mathrm{S}}=v_{\mathrm{H}2}=1685\mathrm{m}{\mathrm{s}}^{-1}$. The error bars correspond to 95% confidence limits. The high energy part between $E_T=10\mathrm{meV}$ and $E_T=30\mathrm{meV}$ is not represented for clarity (see Fig. 4). The solid curve refers to the theoretical ICSs calculated as a combination of $3:1$ weighted contributions due to ortho∶para absolute integral cross sections displayed above, convoluted with a Gaussian distribution of collision energies to account for velocity and crossing angle spread in the experiments ($\Delta E_T/E_T\approx 7\%$ half width at $1/e$).

Figure 4

Integral cross sections (ICSs) of the $\mathrm{S}({}^1\mathrm{D}_2)+{\mathrm{H}}_2$ reaction displayed on the whole energy range. Same symbols as in Fig. 3; diamonds (⋄) previous data [17].