State-to-state photodissociation of the fundamental symmetric stretch vibration of water prepared by stimulated Raman excitation

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Abstract
The state-to-state photodissociation at 193 nm of the fundamental symmetric stretch vibration of water, H2O (1,0,0), is studied. Stimulated Raman excitation and coherent anti-Stokes Raman scattering are used to prepare and detect, respectively, particular rotational states of H2O (1,0,0). Laser induced fluorescence is used for monitoring the OH species which are formed from particularly selected rotational states of the H2O (1,0,0) and also from photodissociation of all occupied rotational states of the ground vibrational state, H2O (0,0,0), at room temperature. The cross section for photodissociation from a particular rotation of H2O (1,0,0) at 193 nm is found to be ∼550 times greater than that for H2O (0,0,0). The formation of the OH product in different rotational, Λ-doublet and spin–orbit states is analyzed for the photodissociation of H2O (0,0,0) and for the photodissociation of the 101, 110+111, 212+211, and 303 rotational states of H2O (1,0,0). The rotational distribution of the OH resulting from photodissociation of H2O (1,0,0) shows a structured distribution that is dependent on the particular rotation of the vibrationally excited state, while that resulting from photodissociation of H2O (0,0,0) presents a smooth distribution. The Λ-doublet ratio in the two spin–orbit states shows preference of the A″ component for photodissociation from the above rotational states of H2O (1,0,0), while only a small preference at high N is observed for photodissociation from the ground vibrational state. The results are compared to available theoretical calculations based on the Franck–Condon model and show reasonable agreement between experiment and theory.