Crossed-beams and theoretical studies of the O( ³P) + H ₂O → HO ₂ + H reaction excitation function

Hyperthermal collisions of ground-state atomic oxygen with H ₂O have been investigated, with special attention paid to the H-atom elimination reaction, O( ³P) + H ₂O(X ¹A ₁) → HO ₂( ²A′) + H( ²S). This reaction was observed in a crossed-beams experiment, and the relative excitation function in the region around its energy threshold (50-80 kcal mol ^-1) was measured. Direct dynamics calculations were also performed at two levels of theory, B3LYP/6-31G(d,p) and MP2/6-31G(d,p). The shape of the B3LYP excitation function closely matches that of the experiment. The calculations provided a detailed description of the dynamics and revealed a striking dependence of the reaction mechanism on collision energy, where the cross section rises from a threshold near 60 kcal mol ^-1 to a peak at ∼115 kcal mol ^-1 and then decreases at higher energies as secondary dissociation of the internally excited HO ₂ product becomes dominant. The calculations show that the cross section for H-atom elimination (O + H ₂O → HO ₂ + H) is about 10-25% that of the H-atom abstraction (O + H ₂O → OH + OH) cross section for collision energies in the 70-160 kcal mol ^-1 range.