Essentially, a chemical reaction in the gas phase is a reactive collisional scattering between reactants, which is similar to the light scattering processes, except that there are reaction products formed.
Consequences of a reactive collision can be well described by reactive differential cross section (DCS) or product angular distributions that quantify the rate at which the reaction products can be formed at a certain scattering angle in the center of mass frame. Therefore measuring product angular distributions can provide a clear microscopic picture of the fundamental chemical reaction process.
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 so-called state-to-state reactive DCS is an extremely sensitive probe of the transition state and is therefore crucial in understanding the reaction dynamics of a chemical reaction.
Prof. WANG Xing’an of the University of Science and Technique of China and MCAS YANG Xueming of Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences and their co-workers have performed a product quantum state revolved imaging experiment on the H+HD→H2+D reaction.
To achieve highest possible resolution in an imaging experiment, they 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 products 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 by Prof. SUN Zhigang and MCAS ZHANG Donghui of DICP and their co-workers shows that the observed fine structures are mainly caused by a few partial waves near J=28 in the collision, corresponding to an impact parameter of about 2.3 Å.
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 optics. That is the reason that we call these fine angular oscillations observed as “corona oscillations”.
The velocity mapping imaging of the H+HD→H2+D reaction at collision energy of 1.35 eV. (Image by XU Xin etc.)
By capturing the fine angular oscillations, the quantum nature of these oscillations has been clearly revealed. This work also demonstrates that velocity map imaging with (1+1’) near threshold ionization is a very powerful experimental technique to investigate the quantum state resolved reaction dynamics of elementary chemical reactions.
This improved experimental technique drives the molecular reaction dynamics study into a true quantum age, since the simple quasi-classical trajectory theory fails to explain such fine experimental measurements any more.