Cryogenic Vibrationally Resolved Photoelectron Spectroscopy of OH - (H 2 O): Confirmation of Multidimensional Franck-Condon Simulation Results for the Transition State of the OH + H 2 O Reaction.

We present a transition state spectroscopic study of the OH + H ₂ O reaction using the experimental technique of cryogenic negative ion photoelectron spectroscopy (NIPES). The recorded NIPE spectrum at 193 nm exhibits multiple vibrational progressions that include excitations to the shared H atom antisymmetric stretching mode with an interval of 0.32 eV as well as other progressions, mainly involving the H bending and O···O symmetric stretching modes. The vertical detachment energy (VDE) was measured at 3.53 eV, whereas an upper limit for the adiabatic detachment energy (ADE) was estimated at 2.90 eV. These values are in excellent agreement with the theoretically computed values of 3.51 and 2.87 eV, respectively, obtained at the CCSD(T)/aug-cc-pV5Z level of theory. The recorded NIPE spectrum is in very good agreement when compared to the one recently reported from four-dimensional Franck-Condon simulations, in which a similar spectral profile was predicted. Besides observing the ground state, we identified a charge-transfer excited state in the form of [OH ^- (H ₂ O) ⁺ ] with a relative energy of 1.39 eV, well matching the previous prediction of 1.36 eV.