Chinese scientists made important progress in the research area of ultrafast molecular imaging
Schematic view of the self-tomographic imaging method. The outmost electron in a molecule is tunnel ionized by an intense femtosecond laser field. When the laser electric field direction is reversed, a portion of the ionized electron wave packet is driven back to the parent ion and interacts with the diatomic center of the molecule and form a photoelectron diffraction pattern. The molecular internuclear separation can be directly extracted by analyzing the photoelectron spectra under different molecular alignment angles.
Supported by the National Natural Science Foundation of China (No. 11834015，11874392，11804374，11847243，11774387，11527807), the research group led by Prof. Xiaojun Liu at Wuhan Institute of Physics and Mathematics (WIPM) of Chinese Academy of Sciences proposed a novel tomographic method for imaging molecular structures based on the interaction of the intense femtosecond laser fields with the gaseous molecules and demonstrated successfully the validity of this method in experiment. The relevant work was published in Physical Review Letters in May 2019 and was selected as “Editors' Suggestion”. (Link: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.122.193202).
Imaging molecular structure plays an essential role in physics, chemistry and biology. In the conventional X-ray and electron diffraction methods, the image of molecular structure can be obtained by external photons or electrons impinging on the molecular targets. With the rapid development of the ultrafast laser technique, a self-imaging method has been developed recently, in which the coherent electron bursts are liberated by a laser field in the target molecules that are being imaged. By analyzing the photoelectron diffraction pattern caused by the interaction of the electron wave packets with the parent ion, the information of the molecular structure and its ultrafast dynamical evolution can be extracted. In comparison with the conventional X-ray and electron diffraction methods, this self-imaging approach can reach sub-angstrom and sub-femtosecond precision. However, currently, the extraction of the molecular internuclear separation with the self-imaging methods requires a priori knowledge of the atomic differential cross-section (DCS) and a very complicated computational procedure.
Recently, Liu’s group proposed and realized an improved molecular self-imaging scheme, which avoids the difficulties caused by the influence of atomic DCS and the complex computational procedure in extracting the molecular structure information. Different from the previous self-imaging methods, in their proposed scheme, the detected photoelectron momentum is fixed, while the molecular axis is rotated with respect to the detection direction. The molecular structure information can thus be directly extracted from the photoelectron yields as a function of the molecular alignment angle. This scheme is essentially a tomographic method and may be termed as a self-tomographic imaging. Compared to previous approaches, this method exhibits several benefits, e.g., the DCSs of the constituent atoms have no influence on the extraction of the molecular structure and the relatively complicated derivation in the extraction procedure is not necessary. By combining the developed molecular alignment technique, Liu’s group carried out the relevant experiment with the diatomic molecules and successfully extracted the molecular internuclear separation from the photoelectron spectrum of molecular nitrogen, demonstrating for the first time the validity of the self-tomographic imaging approach. This approach may be applied to the more complex molecules, e.g., the polyatomic molecules to retrieve the information of the molecular structure.