Transition State Spectroscopy of the OH + H 2 - H 20 + H Reaction Via Photodetachment of H 3 O - and D 30 -

The transition state region of the reaction OH + H2 -. H2O + H is investigated by photoelectron spectroscopy of the H3Oand D30anions. The peaks observed in the spectra are from a combination of vibrational progressions and overlapping anion -. neutral electronic transitions. The photoelectron angular distributions indicate that two processes contribute to the spectra; these are assigned to photodetachment from the H-(H20) and OH-(H2) forms of the anion. A comparison of experiments performed in two different laboratories shows that the two forms of the ion readily interconvert and that the relative populations are determined solely by the temperature of the ions. To interpret the spectra, a two-dimensional ab initio potential energy surface for the anion was constructed, wave functions for the first few vibrational levels were determined, and the photoelectron spectra were simulated using the Walch-Dunning-Schatz-Elgersma surface for the OH + H2 reaction. A comparison of the experimental and simulated spectra showed that photodetachment from the v = 0 level of the anion, which is localized in the H-(H20) well, primarily probes the H + H20 exit valley of the neutral surface. The v = 2 level of the anion is the first with significant amplitude in the OH-(H2) well, and photodetachment from this level probes the OH + H2 transition state region. The simulated spectra are in qualitative agreement with the experimental spectra but do indicate that the neutral reactive surface needs to be modified.