Vibrationally mediated photolysis dynamics of H2O in the vOH=3 manifold: Far off resonance photodissociation cross sections and OH product state distributions

Abstract

Vibrationally mediated photodissociation dynamics of water on the first excited electronic state surface (Ã) has been studied with slit jet-cooled H2O prepared in the complete polyad of vOH=3 overtone stretch levels (|03+〉, |12+〉, |12−〉, and |03−〉). (Notationally, |n1n2±〉 refers to symmetric/antisymmetric combinations of local mode OH stretch excitation, roughly corresponding to n1 and n2 quanta in the spectator and photolyzed OH bond, respectively.) At 248 nm photolysis wavelength the Condon point for bond cleavage occurs in the classically forbidden region, primarily sampling highly asymmetric H+OH exit valley geometries of the potential energy surface. Rotational, vibrational, spin orbit, and lambda doublet distributions resulting from this “far off resonance” photodissociation process are probed via laser induced fluorescence, exploiting the high efficiency laser excitation and light collection properties of the slit jet expansion geometry. Only vibrationally unexcited OH products are observed for both |12±〉 and |03±〉 initial excitation of H2O, despite different levels of vibration in the spectator OH bond. This is in contrast with “near-resonance” vibrationally mediated photolysis studies by Crim and co-workers in the |04−〉 and |13−〉 manifold, but entirely consistent with theoretical predictions from a simple two-dimensional quantum model. Photolysis out of the rotational ground H2O state (i.e., JKaKc=000) yields OH product state distributions that demonstrate remarkable insensitivity to the initial choice of H2O vibrational stretch state, in good agreement with rotational Franck–Condon models. However, this simple trend is not followed uniformly for rotationally excited H2O precursors, which indicates that these Franck–Condon models are insufficient and suggests that exit channel interactions do play a significant role in photodissociation dynamics of H2O at the fully state-to-state level.