Rotationally mediated vector correlations in the photodissociation of H₂O (1,0,0)

The vector correlations following the photodissociation of initially prepared rotational states of the fundamental symmetric stretch of water, H₂O (1,0,0), are studied in detail. The rovibrationally excited water molecules are prepared by stimulated Raman excitation and photodissociated at 193 nm via the first electronically excited state. The OH fragments are probed by both broadband and sub-Doppler polarization spectroscopy. It is shown that the degree of rotational alignment (the μ-J correlation) obtained when the parent molecule is prepared in an in-plane rotational state (3₀₃) is a factor of two higher than in a state containing a mixture of in-plane and out-of-plane rotations (3₂₁ + 3₂₂ + 4₁₄). Also, the μ-ν, ν-J and μ-ν-J correlations following the photodissociation of the 3₀₃ state are close to the limiting values expected for an idealized orientation (with minimal influence of the parent rotation) where the transition dipole moment (μ) of the parent is parallel to the fragment angular momentum (J) and perpendicular to its velocity (ν). In addition, from the degree of misalignment due to the out-of-plane rotation, an estimate of a lifetime of 40 fs of H₂O in the first electronically excited state is obtained. These results demonstrate that experiments which prepare the parent molecule in a particular rotational state, before a second laser dissociates it, provide a powerful means for understanding the directional characteristics of dissociation processes.