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

• Published in: Proceedings of the National Academy of Sciences - PNAS 2008-12, Vol.105 (49), p.19148-19153
• Main Authors: Yuan, Kaijun, Cheng, Yuan, Cheng, Lina, Guo, Qing, Dai, Dongxu, Wang, Xiuyan, Yang, Xueming, Dixon, Richard N
• Format: Article
• Language: English
• Subjects: Anisotropy; Atoms; Contour lines; Cosmic Dust; Energy; Lasers; Molecular rotation; Photochemistry; Photolysis; Physical Sciences; Quantum Theory; Rotation; Rotational spectra; Rotational states; Ultraviolet Rays; Water - chemistry; Wavelengths
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.0807719105

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 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) in rotational states with k'a = 0 occurs exclusively through a newly discovered homogeneous coupling to the à 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 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 1₁₀ [leftward arrow] 0₀₀ 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.