Photochemistry of the Water Molecule: Adiabatic versus Nonadiabatic Dynamics

Water and light are two common constituents of both the earth's atmosphere and interstellar space. Consequently, water photodissoclation is a central component of the chemistry of these environments. Electronically excited molecules can dissociate adiabatically (on a single potential energy surface, or PES) or nonadiabatically (with transfer between PESs), and water serves as a prototype for understanding these two processes in unimolecular dissodation. In recent years, extensive experimental and theoretical studies have been focused on water photolysis, particularly on the primary product of the dissociation, the OH radical. The use of the high-resolution H-atom Rydberg tagging technique, in combination with various vacuum ultra-violet (VUV) sources, has spurred significant advances in water photochemistry. As the excitation energy increases, different excited electronic states of water can be reached, and the mutual interactions between these states increase significantly. In this Account, we present the most recent developments in water photodissociation that have been derived from the study of the four lowest electronic excited states. The (A) over tilde (1)B(1) state photodissociation of H(2)O has been studied at 157.6 nm and was found to be a fast and direct dissociation 4 process on a single repulsive surface, with only vibrational excitation of the OH(X(2)Pi) product. In contrast, the dissociation of the (B) over tilde (1)A(1) state was found to proceed via two main routes: one adiabatic pathway leading to OH(A(2)Sigma(+)) + H, and one nonadiabatic pathway to OH(X(2)Pi) + H through conical intersections between the (B) over tilde state and the ground state (X) over tilde (1)A(1). An interesting quantum A interference between two conical intersection pathways has also been observed. In addition, photodissodation of H(2)O between 128 and 133 nm has been studied with tunable VUV radiation. Experimental results illustrate that excitation to the different unstable resonances of the state has very different effects on the OH(X(2)Pi) and OH(A(2)Sigma(+)) product channels. The (C) over tilde (1)B(1) state of H(2)O is a predissodative Rydberg state with fully resolved rotational structures. A striking variation in the OH product state distribution and its stereodynamics has been observed for different rotational states. There are two kinds of nonadiabatic dissociation routes on the (C) over tilde state. The first involves Renner-Teller (electronic Coriolis) coupling to the (B) over tilde state, leading to rotationally hot and vibrationally cold OH products. The second goes through a newly discovered homogeneous nonadiabatic coupling to the (A) over tilde state, leading to rotationally cold and vibrationally hot OH products. But the (D) over tilde (1)A(1), state shows no rotational structure and leads to a fast, homogeneous, purely electronic predissociation to the (B) over tilde state. These studies demonstrate the truly fascinating nature of water photochemistry, which is extremely variable because of the different electronic states and their interactions. The results also provide a rather complete picture of water photochemistry and should be helpful in the modeling of interstellar chemistry, with its abundant VUV radiation.