The hydrogen atom and hydrogen molecule reaction in a new light [electronic resource] : scattering dynamics beyond the conical intersection
Keywords: Chemistry.

The H + H2 reaction and its isotopic variants have served for many years as a benchmark reaction system for studying bimolecular reaction dynamics. It is well-established that the minimum energy path for the H + H2 reaction is where the three atoms line up to form a collinear transition state. This path, commonly known as the direct recoil mechanism or Spiral mechanism, is very classical in nature and shows its signature in the angular distribution of the products which is a universal probe to the collision dynamics of a chemical reaction. The signature of the direct recoil mechanism is generally a single backscattered peak for low impact parameter or head-on collisions in the differential cross sections (DCSs) of the product states and a more sideways/forward scattered peak for high impact parameter or glancing collisions. While this behavior has been seen repeatedly in state-of-the-art experiments as well as high dimensional QM and classical theories, there have also been exceptions when the H + H2 system did not behave so classically and provided us with new insights to this reaction and to chemical reactions in general. This dissertation primarily focuses on two studies which bring forward new insights to this simplest chemical reaction: (i) scattering dynamics of H + D2 ⟶HD (v'=1, j') + D reaction at 1.97 eV, and (ii) probing the dynamics of H + D2 reaction beyond the energy of the conical intersection of H3 surface at a collision energy of 3.26 eV. Using the PHOTOLOC (photoinitiated reactions analyzed via law of cosines) technique developed by Zare and coworkers, 4 we have performed state-to-state measurements of the H + D2(v, j=0-2)⟶ HD(v', j') + D reaction at varying collision energies and investigated how energetics play a crucial role in the dynamics of this reaction. What has been observed for the HD product states is that the scattering dynamics do not show signatures of the conventional minimum energy path, vide supra. Instead of showing the conventional backward/sideways scattered single peak, the experimental DCSs showed multiple oscillatory structures. These structures are well reproduced by time-independent quantum mechanical (TIQM) calculations, but, somewhat surprisingly, not by the quasiclassical approach. Analysis of the classical results reveals that for these HD (v', j') states, several classical scattering mechanisms occur simultaneously. However, no clear evidence of these mechanisms is obvious in the quasiclassical state-to-state DCSs because their outcomes overlap in the range of scattering angles where they could be observable. It turns out that, analogous to the observations in a double-slit experiment, quantum interferences between the various mechanisms change and govern the angular distribution of the HD products. Owing to its quantum nature, this effect cannot be described in terms of the classical motion of the nuclei, and the QCT method fails. These mechanisms are characterized by different values of the total angular momentum J, which makes it possible to investigate theoretically the effect of 'closing' or 'opening' one of the slits in the two-slit experiment and determine the origin of the interferences. What is interesting is that while these different classical mechanisms and the interferences arising from them start to show for HD (v'=1, j') at energies (1.97 eV) way below the conical intersection (CI) of the H3 surface (2.74 eV), it is only at collision energies much higher than the CI that the higher vibrational manifolds of HD (v'=3, 4; j') show similar behavior. This can be explained with a total energy perspective and its distribution between the kinetic, internal and potential energies required to cross the barrier. At Ecoll = 1.97 eV, while the HD(v'=1, j') state has enough total energy available for the reaction to go through pathways with higher energy barriers, the relatively higher internal energies for the v' = 3 and 4 manifolds leaves less energy for the reaction and therefore the only feasible pathway for these states is the minimum energy path. At higher energies, for example, at 3.26 eV, there is enough energy available for even the higher vibrational states to undergo multiple mechanisms.