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

Hyperthermal collisions of ground-state atomic oxygen with H2O have been investigated, with special attention paid to the H-atom elimination reaction, O((3)P) + H(2)O(X (1)A(1)) --> HO(2)((2)A') + H((2)S). 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 approximately 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(2)O --> HO(2) + H) is about 10-25% that of the H-atom abstraction (O + H(2)O --> OH + OH) cross section for collision energies in the 70-160 kcal mol(-1) range.