Abstract
The H3 system has served as a prototype for geometric phase (GP) effects in bimolecular chemical reactions for over three decades. Despite a large number of theoretical and experimental efforts, no conclusive evidence of GP effects in the integral cross section or reaction rate has been presented until recently [B. Kendrick et al., Phys. Rev. Lett. 115, 153201 (2015)]. Here we report a more detailed account of GP effects in the H + H2(v = 4, j = 0) → H + H2(v', j') (para-para) reaction rate coefficients for temperatures between 1 μK (8.6 × 10-11 eV) and 100 K (8.6 × 10-3 eV). The GP effect is found to persist in both vibrationally resolved and total rate coefficients for collision energies up to about 10 K. The GP effect also appears in rotationally resolved differential cross sections leading to a very different oscillatory structure in both energy and scattering angle. It is shown to suppress a prominent shape resonance near 1 K and enhance a shape resonance near 8 K, providing new experimentally verifiable signatures of the GP effect in the fundamental hydrogen exchange reaction. The GP effect in the D + D2 and T + T2 reactions is also examined in the ultracold limit and its sensitivity to the potential energy surface is explored.
Highlights
(constructive) interference which can dramatically alter the reactivity
All of the PES sensitivity studies indicate that the large geometric phase (GP) effects reported in this work and in our previous studies for ultracold collision energies are due to a new quantum interference mechanism8 and are not a numerical artifact associated with a particular potential energy surface
The rotationally resolved rates add constructively leading to large GP effects (≈10×) in both the vibrationally resolved and total rates
Summary
(constructive) interference which can dramatically alter the reactivity. Including the additional sign change (or π phase shift) associated with the GP reverses the nature of the interference and significantly alters the theoretically predicted reaction rate coefficient. The sensitivity of the results on the PES was checked by performing an identical set of calculations for J = 0 using the surface of Mielke et al. which contains improvements to the long range anisotropic behavior. Both PESs give similar ultracold rate coefficients and predict that the GP enhances the ultracold reactivity by a full order of magnitude (see Fig. 2 in Ref. 5). All of the PES sensitivity studies indicate that the large GP effects reported in this work and in our previous studies for ultracold collision energies are due to a new quantum interference mechanism and are not a numerical artifact associated with a particular potential energy surface. The calculations were performed using a time-independent coupled-channel formalism based on the Adiabatically adjusting Principal axis Hyperspherical (APH) approach of Pack and Parker. The methodology is numerically exact for a given BornOppenheimer PES (i.e., no dynamical approximations are used) and has been validated against other quantum reactive scattering codes and high resolution crossed molecular beam experiments. We refer the interested reader to previous publications which give a detailed description of the Hamiltonian, computational methodology and parameters.
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