An experimental cross section for the hydrogen atom, hydrogen molecule exchange reaction as a function of angle and energy.

摘要

Being the simplest bimolecular reactions of neutrals, the hydrogen atom, hydrogen molecule exchange reaction has received detailed experimental and theoretical treatment. We report an experimental study of an isotopic variant of the reaction and compare the results to theoretical calculations. The reaction H + D₂ → HD(ν′ = 3, j ′ = 0) + D is studied at nine different collision energies between 1.39 and 1.85 eV using the PHOTOLOC technique (PHOTOinitiated reaction analyzed with the Law Of Cosines). Lasers are used both to photoinitiate the reaction via photolysis of HBr and detect HD(ν′ = 3, j′ = 0) products via (2 + 1) resonance-enhanced multiphoton ionization (REMPI). Differential cross sections (DCS) show a forward-scattered feature that changes intensity as the collision energy is increased. A peak in the relative ratio of forward to backward scattering is observed at approximately 1.64 eV collision energy. The integral cross section is measured between 1.49 and 1.85 eV. Several experimental modifications are required, including the addition of another laser beamtrain to quantify hydrogen atom generation using (2 + 1) REMPI. Results indicate that the integral cross section changes little over this energy range.; Fully quantum mechanical scattering calculations of Althorpe are presented. These results agree well with the measured differential cross section at all collision energies except 1.54 eV. This discrepancy is not well understood, although the calculated DCS changes wildly with energy near this energy, and it is suspected that these changes are related to the disagreement. Overall good agreement between theory and experiment helps construct an interpretation for the observed scattering. Calculations indicate the presence of two mechanisms; a direct mechanism resulting mostly in backward scattered products, and an indirect mechanism that appears about 15 fs later. Interference between the two mechanisms causes the observed changes in the DCS, including the forward scattering. Comparison between theory and experiment for the integral cross section is quantitative. The mechanism that causes changes is the DCS does not manifest itself as changes in the integral cross section.