Since the first X-ray image of a comet was reported using an X-ray telescope in 1996, the investigation of charge exchange in collisions between highly charged ions and atoms or molecules has emerged as a hot research topic. Astrophysicists require more atomic data to model observed X-ray spectra. Traditionally, the charge exchange is assumed to follow statistical rules regarding the total spin quantum number. These assumptions of pure spin statistics are of fundamental importance across various fields.

However, a new study published in Physical Review Letters on October 22 has challenged the assumptions by providing direct evidence of the breakdown of spin statistics in ion-atom charge exchange collisions. This study was led by scientists from the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS).

The experiment was performed at the low-energy setups of the Heavy Ion Research Facility in Lanzhou, employing the high-resolution reaction microscope, which is characterized by high precision, sensitivity and detection efficiency. In the experiment, the researchers used the neutral helium as a target in collisions with C³⁺ions. 

"The C³⁺ ion is a good candidate for this study because it has no long-lived excited states and is always in its ground state in the collision region. Furthermore, using the reaction microscope, we can easily determine the atomic states at the moment of electron captured in collisions, overcoming the difficulties encountered in previous experiments. Thus, it is relatively easier to accurately analyze the underlying mechanisms in the current study," said Prof. ZHU Xiaolong from IMP, the first author of this paper.

Through experimental and theoretical approaches, the researchers directly measured spin-resolved cross sections ratio, as a probe of spin statistics, which demonstrated the breakdown of spin statistics assumptions at high impact energies where they are traditionally expected to be valid.

"These novel findings raise intriguing questions both in understanding the electronic dynamics during such fast collisional processes and in exploring quantum manipulation of atomic and molecular reactivity," said Prof. MA Xinwen from IMP, one of the corresponding authors of this paper.

This work was supported by the National Key Research and Development Program of China, the Strategic Priority Research Program of CAS, and the National Natural Science Foundation of China.

DOI: https://doi.org/10.1103/PhysRevLett.133.173002 

Figure. The reaction microscope at IMP. (Image from IMP)