A new study has demonstrated that Coulomb explosion induced by highly charged ions is a unique tool for precisely imaging the structures of complex molecules.

This study, published in Physical Review Letters on November 7, was led by researchers from the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS).

Coulomb explosion imaging (CEI) is a powerful method that enables imaging molecular structures with atomic-level resolution. Most CEI studies have been performed using intense laser pulses or x-ray pulses, which have traditionally been limited to small molecules containing no more than 5 atoms. Recently, such limitation was overcome using ultra-strong pulses of x-ray or femtosecond lasers. However, precise measurement of the bond lengths, particularly when hydrogen atoms are involved, has remained a challenge.

In this study, the research team successfully triggered a Coulomb explosion of C₄H₄N₂ molecules by using the 112.5 eV/u C⁵⁺ ion beam provided by the Heavy Ion Research Facility in Lanzhou (HIRFL). 

"Using highly charged ions, the complex molecules are exposed in an extremely short and strong pulse of electronic field, and more than ten electrons are stripped off in less than one femtosecond. This is the key point for the high-precision imaging of hydrogen atoms," said Prof. XU Shenyue from IMP, one of the corresponding authors of this study.

The researchers recorded ionic fragments of H⁺, C²⁺, C⁺, and N⁺ in coincidence using the reaction microscopy. By reconstructing the fragments momentums, they achieved high-precision imaging of all constituting atoms, including both hydrogen atoms and heavy atoms.

The results indicated that the accelerator-based CEI approach can precisely image structures of molecules containing 10 atoms. And the researchers also proposed that the angular correlation between fragments can serve as the fingerprint for distinguishing isomeric structures. 

"Highly charged ions are unique for Coulomb explosion imaging of complex molecules. We expect that this work could promote the application of accelerators in the field of single complex molecule imaging," said Prof. MA Xinwen, another corresponding author of this study and the head of atomic physics center of IMP.

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

DOI: https://doi.org/10.1103/PhysRevLett.133.193002

Figure. Schematic of the experimental approach (Image by IMP)