Communication : The origin of rotational enhancement effect for the reaction of H₂O⁺ + H₂ (D₂)

We have measured the absolute integral cross sections (σ's) for H₃O⁺ formed by the reaction of rovibrationally selected H₂O⁺(X²B₁; v₁⁺v₂⁺v₃⁺ = 000; N⁺_Ka⁺_Kc⁺ = 0₀₀, 1₁₁, and 2₁₁) ion with H₂ at the center-of-mass collision energy (E_cm) range of 0.03-10.00 eV. The σ(0₀₀), σ(1₁₁), and σ(2₁₁) values thus obtained reveal rotational enhancements at low E_cm <0.50 eV, in agreement with the observation of the previous study of the H₂O⁺(X²B₁) + D₂ reaction. This Communication presents important progress concerning the high-level ab initio quantum calculation of the potential energy surface for the H₂O⁺(X²B₁) + H₂ (D₂) reactions, which has provided valuable insight into the origin of the rotational enhancement effect. Governed by the charge and dipole-induced-multipole interactions, the calculation shows that H₂ (D₂) approaches the H end of H₂O⁺(X²B₁) in the long range, whereas chemical force in the short range favors the orientation of H₂ (D₂) toward the O side of H₂O⁺. The reorientation of H₂O⁺ reactant ion facilitated by rotational excitation thus promotes the H₂O⁺ + H₂ (D₂) reaction along the minimum energy pathway, rendering the observed rotational enhancement effects. The occurrence of this effect at low E_cm indicates that the long range charge and dipole-induced-multipole interactions of the colliding pair play a significant role in the dynamics of the exothermic H₂O⁺ + H₂ (D₂) reactions. © 2014 AIP Publishing LLC.