Physicists achieve high-precision imaging of complex molecules using highly charged ions

A new study, published in Physical Review Letters and led by researchers from the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS), has demonstrated that a Coulomb explosion induced by highly charged ions is a unique tool for precisely imaging the structures of complex molecules.

Coulomb explosion imaging (CEI) is a powerful method that enables the imaging of molecular structures with atomic-level resolution. Most CEI studies have been performed using intense laser pulses or X-ray pulses, which have been limited to small molecules containing no more than five atoms.

Recently, this limitation was overcome using ultra-strong pulses of X-rays or femtosecond lasers. However, precise measurement of bond lengths, particularly when hydrogen atoms are involved, has remained a challenge.

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

"Using highly charged ions, the complex molecules were exposed to the extremely short and strong pulse of an electronic field, and more than 10 electrons were 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 the study.

Researchers used reaction microscopy to simultaneously record ionic fragments of H⁺, C²⁺, C⁺, and N⁺. By reconstructing the momentum of the fragments, they achieved high-precision imaging of all constituent atoms, including both hydrogen atoms and heavy atoms.

The findings of this study indicate that the accelerator-based CEI approach can precisely image structures of molecules containing 10 atoms. Also, researchers proposed that the angular correlation between fragments can serve as the fingerprint for distinguishing isomeric structures.

"Highly charged ions are unique for CEI of complex molecules. We expect that this study could promote the application of accelerators in the field of single complex molecule imaging," said Prof. MA Xinwen, another corresponding author of the study.