Quantum interference (QI), which relies on the wave-particle duality of microscopic particles, significantly affects the dynamics of energy transfer and chemical reactions induced by molecular collisions. Therefore, accurate measurement and description of quantum effects in molecular collisions is the key to understanding molecular dynamics of atoms. 

In a recent perspective titled “A molecular double-slit experiment” published in Science, Prof. WANG Xingan from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS) and Prof. YANG Xueming from Dalian Institute of Chemical Physics of CAS, deeply discussed and prospected the stereodynamics and QI in molecular collisions. 

With the rapid development of experimental techniques like lasers and molecular beams, fine control over the quantum state and spatial orientation of colliding molecules is within reach. Thus, it is possible to study quantum stereodynamics in molecular collisions. The perspective presented detailed description of contemporary achievements published in Science. For instance, U.S. scientists achieved highly effective excitation of the vibrational state of D₂ molecule, and they selectively and accurately prepared two quantum states with different properties.  

One of the two states is called a uniaxial state, which is a spatially oriented vibrational excitation state with an angle between the molecular bond axis orientation and the reference axis of +45 or -45 degrees. Another vibrational excitation state comes from the coherent overlap of two uniaxial states. It is possible that both the molecular bond axis orientation and the reference axis have an angle of +45 and -45 degrees at the same time, and this state is called a biaxial state.  

By measuring the angular distribution of the products of inelastic scattering, the researchers found that molecules in uniaxial state exhibited results similar to a single-slit diffraction, while in biaxial state, the results were analogous to a double-slit interference. The difference between these scattering patterns resulted from QI between different bond axis orientations.  

This is the first time that a "double-slit" similar to Thomas Young's experiment has been prepared by laser in a molecular collision system, which provides an important reference for controlled QI experiments in chemical reaction collisions. 

Besides, the perspective introduced an ideal chemical reaction system for the study of QI and stereodynamics: H+HD→H₂+D and its isotope reaction system. During this reaction, the collision energy of QI presented a conical intersection between two adjacent electronic states in the potential energy surface, and the intersection created two distinctive topological reaction pathways. QI of these two paths could significantly affect the reaction kinetics of the hydrogen exchange system. 

Combining laser quantum state preparation and molecular spatial orientation techniques, the researchers will be able to make more accurate dynamic measurements of hydrogen exchange and other reactions through cross-molecular beam experiments. Besides, through “molecular double-slit experiments”, they will have a better understanding of QI behavior and stereodynamics of elementary reactions. It is expected that three-dimensional quantum dynamics control of elementary chemical reactions will be realized in the near future.