Nonadiabatic dissociation dynamics in H2O: 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.

SUBMITTER: Yuan K 

PROVIDER: S-EPMC2614730 | biostudies-literature | 2008 Dec

REPOSITORIES: biostudies-literature