Nonadiabatic dissociation dynamics in H ₂ O: Competition between rotationally and nonrotationally mediated pathways

Abstract

The photochemistry of H(2)O in the VUV region is important in interstellar chemistry. Whereas previous studies of the photodissociation used excitation via unbound states, we have used a tunable VUV photolysis source to excite individual levels of the rotationally structured C state near 124 nm. The ensuing OH product state distributions were recorded by using the H-atom Rydberg tagging technique. Experimental results indicate a dramatic variation in the OH product state distributions and its stereodynamics for different resonant states. Photodissociation of H(2)O(C) in rotational states with k'(a) = 0 occurs exclusively through a newly discovered homogeneous coupling to the A state, leading to OH products that are vibrationally hot (up to v = 13), but rotationally cold. In contrast, for H(2)O in rotationally excited states with k'(a) > 0, an additional pathway opens through Coriolis-type coupling to the B state surface. This yields extremely rotationally hot and vibrationally cold ground state OH(X) and electronically excited OH(A) products, through 2 different mechanisms. In the case of excitation via the 1(10) <-- 0(00) transition the H atoms for these 2 product channels are ejected in completely different directions. Quantum dynamical models for the C-state photodissociation clearly support this remarkable dynamical picture, providing a uniquely detailed illustration of nonadiabatic dynamics involving at least 4 electronic surfaces.