State-to-state dynamics of the H+p-H2→o,p-H2+H reaction: Feshbach resonances and vibrational spectroscopy of the transition state

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Abstract
We describe the results of an extensive series of state‐to‐state dynamics experiments on the hydrogen exchange reaction, H+p‐H2(v=0, j=0,2)→o,p‐H2(v’, j’)+H, for which we have measured the v’, j’ product state distributions and partial reaction cross sections at 22 collision energies between the v’=1 and v’=2 thresholds at 0.7 and 1.1 eV. A tunable ultraviolet laser photodissociates HI to make H atoms and select the collision energy. Coherent anti‐Stokes Raman scattering (CARS) is used to detect the reaction products with state selectivity under single‐collision conditions. We observe sharp oscillations in the v’=1 partial cross sections as the collision energy is varied, with three well‐defined maxima at 0.70, 0.93, and 0.98 eV, and possibly another at 0.83 eV. These are energies at which Feshbach dynamical resonances have been predicted theoretically, and we interpret the peaks we observe in the energy‐dependent yield of v’=1 product to be such resonances. These resonances result from excited vibrational states of the H3 transition state, and their observation amounts to vibrational spectroscopy of the transition state. In particular, the three clear resonance maxima we observe are assigned to the (1,0,0), (1,20,0), and (1,22,0) excited vibrational states of H3. Our experimental observations are compared with the results of recent quantum scattering calculations for this reaction. Although resonance features very similar to those we observe are evident in calculations for individual partial waves and for cross sections summed over small partial waves, fully converged calculations do not show the sharp resonances that we observe. This discrepancy implies that either the experimental or theoretical results are in error, and we discuss possible sources such error in both the experiments and the theory.