Role of Initial Vibrational and Rotational Reactant Excitation for the Reaction Dynamics of H₂(ν₀,J₀) with Si ⁺(²P)

Molecular dynamics simulations with semiempirical quantum chemistry have been used to investigate the reaction dynamics of ground-state silicon ions with molecular hydrogen for translational impact energies between 0.5 and 10 eV. The validity of the employed PM3 method is demonstrated in comparison to high-level ab initio CCSD(T) calculations. Reaction cross sections for both SiH ⁺(ν,J) formation and complete dissociation are determined for different initial vibrational and rotational excitation states of the H ₂(ν₀,J₀) molecules. Heating the commonly used room temperature hydrogen plasma to a maximum possible experimental value of 1000 K only results in a very modest increase of the reactive cross section for SiH⁺ production by about 25%. The use of selective laser excitation of the reactants, however, permits us to increase the reactivity drastically. In this latter case, initial rotational laser excitation enhances the reactivity of our system as much as vibrational excitation, illustrating that only the amount of the initial excitation and not its precise nature influences the reaction dynamics. Furthermore, we show how the initial vibrational, rotational, and translational energies of the H₂ reactants control the final energy distributions of the SiH⁺ products. The mechanisms leading to the observed reaction dynamics are discussed.