Nonadiabatic dissociation dynamics in H2O: Competition between rotationally and nonrotationally mediated pathways

The photochemistry of H2O 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) over tilde 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 H2O((C) over tilde) in rotational states with k(a)' = 0 occurs exclusively through a newly discovered homogeneous coupling to the (A) over tilde state, leading to OH products that are vibrationally hot (up to v = 13), but rotationally cold. In contrast, for H2O in rotationally excited states with k(a)' > 0, an additional pathway opens through Coriolis-type coupling to the (B) over tilde 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) over tilde -state photodissociation clearly support this remarkable dynamical picture, providing a uniquely detailed illustration of nonadiabatic dynamics involving at least 4 electronic surfaces.