Invited
Young double-slit experiments in chemical reactions: Quantum interference in reactive scattering

The Young`s double-slit experiment applied to interferences of single electrons has been deemed one of the most intriguing experiments in physics [1] and has become a standard example in quantum mechanics (QM) textbooks for illustrating wave-particle duality. In the experiments, electrons, one at a time, are shot against a screen that contains two small slits, and the detection of the transmitted electrons results in an interference pattern similar to that obtained with light. Moreover, experiments with heavier particles [2] demonstrate the quantum nature of large and presumably more classical molecules. Does such interference effect occur in chemical reactions and, if so, how do they affect the reaction observables?

In scattering processes interference is a genuine quantum phenomenon that appears whenever two seemingly distinct classical trajectories lead to the same outcome. They are common in elastic and inelastic scattering but are seldom observable in chemical reactions since their observation requires fully initial and final state selection and sufficiently narrow collision energy distributions. It has been recently shown that for certain initial and final states interference produces a characteristic oscillation pattern that governs the angular distributions for the H + D₂ exchange reaction [3-5]. In this talk, we will discuss the interpretation of recent experimental results for the H+D₂ reaction [3-5]. It was found that the angular distributions for certain states display characteristic oscillation pattern in the backward region. The comparison between quasiclassical trajectory and QM calculations evinces that the oscillation pattern arises when groups of partial waves than span different ranges of the total angular momentum give rise to scattering at the same deflection angles. The various groups of partial waves can be assigned to different classical mechanisms that take place under certain conditions. Moreover, the analysis of the quantum interferences serves to identify those mechanisms. The phenomenon is analogous to that found in the double (and multiple)-slit experiment wherein the analysis of the diffraction pattern allows us to determine the double-slit characteristics.

We will show that the conditions under which interference determines the shape of the angular distribution depend on the shape of the potential energy surface, the collision energy [4,5], and the initial and final states. In particular, it will be also shown how interferences depend on the initial rotational state j of the D₂(v =0, j) reagent and diminish in strength with increasing rotation [4]. We will present a detailed explanation for this behavior in terms of the dependence on the projection of the initial rotational angular momentum j of the D₂ reagent on the approach direction. Each of them corresponds to different internuclear axis distributions, with the consequence that the dependence on W reveals the preference of the different mechanisms as a function of approach direction.

[1] R. P. Feynman, R. B. Leighton and M. Sands, Lectures on Physics, Vol 3, Chp. 3. Addison-Wesley 1964. Jim Baggot, The Quantum Story: A history in 40 moments. Oxford University Press. 2015.

[2] M. Arndt, O. Nairz, J. Vos-Andreae, C. Keller, G. van der Zouw and A. Zeilinger, Nature 401, 680-682 (1999). O. Nairz, M. Arndt and A. Zeilinger, Am. J. Phys. 71, 319-325 (2003). S. Gerlich, S. Eibenberger, M. Tomandl, S. Nimmrichter, K. Hornberger, P. J. Fagan, J. Tuxen, M. Mayor and M. Arndt, Nat. Commun., 2011, 2, 263.

[3] P. G. Jambrina, D. Herráez-Aguilar, F. J. Aoiz, M. Sneha, J. Jankunas and R. N. Zare, Nat. Chem. 7, 661-665 (2015)

[4] P. G. Jambrina, J. Aldegunde, F. J. Aoiz, M. Sneha, and R. N. Zare. Chem. Sci. 7, 642-649 (2016).

[5] Mahima Sneha, Hong Gao, Richard N. Zare, P. G. Jambrina, M. Menéndez, and F. J. Aoiz, J. Chem. Phys. 145, 024308 (2016)