Ionizing collisions of He(${\mathit{n}}^1$P) atoms in a low-Rydberg state with ${\mathrm{SF}}_6$ and ${\mathrm{NO}}_2$ electron-attaching molecules

The ionization of He atoms, prepared in a well-defined (n=14) ${}_{}{}^1{}_{}{}^{}$P Rydberg state, in collisions with electron-attaching molecules like ${\mathrm{SF}}_6$ and ${\mathrm{NO}}_2$, is studied in a crossed-beam experiment at thermal energies. The ionization cross section is observed to be large for ${\mathrm{SF}}_6$, reaching 4.2×${10}^{\mathrm{-}13}$ ${\mathrm{cm}}^2$, and to be smaller for ${\mathrm{NO}}_2$, close to ${10}^{\mathrm{-}15}$ ${\mathrm{cm}}^2$. The dependence of these two cross sections on the relative velocity of the reactants exhibits a characteristic behavior with a saturation at low velocities, a dependence that is completely different from that obtained with polar molecules. The observation of such a dependence for ${\mathrm{NO}}_2$ indicates that, for this molecule, the reaction product is an ion pair, as for ${\mathrm{SF}}_6$, and that ionization is not due to rotational energy transfer even though ${\mathrm{NO}}_2$ has a dipole moment. This dependence cannot be explained by the ‘‘free-electron model’’ usually used in Rydberg-atom–molecule collisions; it clearly shows that the interaction of the molecule with the ion core cannot be neglected.