Revealing the Quantum Nature of the Simplest Chemical Reaction
A high-resolution imaging study has unraveled the quantum nature of the fine angular oscillations in the simplest chemical reaction H+HD→H2+D
The paper in Nature Chemistry is here: https://www.nature.com/articles/s41557-018-0032-9
From a microscopic point of view, a chemical reaction in the gas phase is a reactive collisional scattering between reactants. Reaction products are formed and scattered into different directions during a collision. Consequences of the collision can be well described by the reactive differential cross section (DCS) or product angular distribution that quantifies the rate at which the reaction products are formed at a certain scattering angle in the center of mass frame1. Over the past decades, with the fast development of experimental techniques, in particular the crossed molecular beams method, researchers have been capable of measuring the reactive DCS with quantum state resolution. This state-to-state reactive DCS is an extremely sensitive probe of the transition state and is therefore crucial in understanding the dynamics of a chemical reaction. Therefore, measuring the high-resolution product angular distribution of a state-to-state reactive DCS can provide a clear quantum picture of the fundamental chemical reaction process.
While fine oscillatory angular structures have been detected in elastic2 and inelastic3 scattering processes, they have never been experimentally observed in any chemical reaction. In addition, the nature of these structures and their dynamics in chemical reactions are not understood clearly. Therefore, it would be truly interesting to try to experimentally observe these fine angular structures and understand the dynamic origins of these oscillations theoretically.
The H+H2 reaction and its isotope analogues are the simplest chemical reactions in nature. In this work, we have performed a product quantum state revolved imaging experiment on the H+HD→H2+D reaction. To achieve the highest possible resolution in an imaging experiment, we used near threshold ionization through (1+1’) ionization of the D-atom product. This allows us to resolve various quantum states of the molecular hydrogen product in the image. Oscillatory structures in the quantum state resolved DCS are clearly observed in the forward scattering direction at the collision energy of 1.35 eV. In addition, accurate theoretical dynamics analysis shows that the observed fine structures are mainly caused by a few partial waves near total angular momentum J = 28 in the collision, corresponding to an impact parameter of about 2.3 bohr. Intriguingly, the mechanism of these angular oscillations in the forward direction is considered to be very much similar to the corona scattering rings in the atmospheric optics4. This is the reason that we call these fine angular oscillations observed as “corona oscillations”.
By capturing the fine angular oscillations, the quantum nature of these oscillations in the simplest chemical reaction has been clearly revealed. This remarkable work also demonstrates that velocity map imaging with (1+1’) near threshold ionization is a very powerful experimental technique. The collaboration of high-resolution experiments and quantum-mechanical dynamics calculations has proven to be crucial to study the detailed information of reaction dynamics, and will play a key role in understanding the quantum nature of elementary chemical reactions.
Written by Xingan Wang, Zhigang Sun, Dong H. Zhang and Xueming Yang.
1. Tutorials in Molecular Reaction Dynamics, RSC Publishing, 2009.
2. Pirani, F. et al. Beyond the Lennard-Jones model: a simple and accurate potential function probed by high resolution scattering data useful for molecular dynamics simulations. Phys. Chem. Chem. Phys. 2008, 10, 5489-5503
3. von Zastrow, A. et al. State-resolved diffraction oscillations imaged for inelastic collisions of NO radicals with He, Ne and Ar. Nature Chem. 2014, 6, 216-221