Recently, Liu Xiaojun from the Institute of Precision Measurement Science and Technology Innovation of the Chinese Academy of Sciences and his collaborators proposed a novel molecular attosecond angular fringe (also known as molecular "attosecond") scheme that is completely based on the detection of ion fragments, and used this scheme to precisely measure the electron tunneling time in the strong field ionization process of hydrogen molecules for the first time, giving the upper limit of this time to 10 attoseconds (1 attosecond=10-18 seconds). The related work was published in the physics journal Physical Review Letters.

Quantum tunneling effect refers to the "strange" behavior in the microscopic world where electrons and other microscopic particles can cross potential barriers higher than their own energy. Quantum tunneling effect is a basic quantum property of microscopic particles, which cannot be explained from the perspective of Classical mechanics. Quantum tunneling plays a vital role in understanding many natural phenomena, such as stellar Nuclear fusion, radioactive decay, etc. It is also the physical basis of modern scientific instruments such as Scanning tunneling microscope. However, since the establishment of quantum mechanics, a fundamental issue regarding quantum tunneling, namely whether it takes time to occur, has been controversial.

The atomic molecules in a strong laser field provide a unique "artificial tunneling" system for studying the attosecond time-domain characteristics of quantum tunneling. The outer electrons of atoms and molecules will escape through Tunnel ionization under the action of a strong laser field. By precisely detecting the dynamic behavior of tunneling electrons, we can explore the basic physical problem of whether quantum tunneling requires time (i.e. tunneling time) in the attosecond time scale. Therefore, researchers have proposed an effective "attosecond" scheme in recent years, which converts the tunneling time into the deflection of the tunneling electron emission angle and reads the tunneling time information from the photoelectron spectrum. Interestingly, over the past ten years, different research groups based on the "attosecond" scheme and combined with different atomic systems have reached different conclusions: Tunnel ionization may occur instantaneously, or it may take a hundred attoseconds.

Regarding this controversy in the research field, Liu Xiaojun's team and collaborators have proposed a novel molecular "attosecond" scheme based on ion fragment measurement, which extends tunneling time measurement to molecular systems for the first time. In their scheme, on the one hand, by cleverly utilizing the momentum distribution of fragmented ions accompanying strong field ionization of molecules, the polarization state of the driving laser is measured in situ, avoiding the possible impact of laser polarization direction calibration on the extraction of tunneling time information in traditional schemes; On the other hand, reading the tunneling time information through the deflection of the photoion emission angle avoids the dependence of the traditional "attosecond" scheme on the physical model for reading the tunneling time through the deflection of the electron emission angle. The research team applied this scheme to the study of strong field Tunnel ionization of hydrogen molecules. The photoion emission angular deflection measured in the experiment is in good agreement with the first principle calculation results. The upper limit of the tunneling time obtained based on this measurement scheme is 10 attoseconds, which is consistent with the conclusion that the tunneling instant occurs based on previous studies of Tunnel ionization of hydrogen atoms. The molecular "attosecond" scheme is expected to be extended to other complex molecular systems, and further study the influence of complex molecular properties such as molecular structure, Molecular orbital symmetry, etc. on the strong field Tunnel ionization process, thus deepening the understanding of quantum tunneling time related issues.

In this work, researcher Quanwei from the Precision Measurement Institute led the experimental measurement work, while Dr. Serov from Sartov State University in Russia and Professor Kheifets from Australian National University conducted corresponding theoretical calculations; Kheifets and Liu Xiaojun are the co corresponding authors of this paper. The research work has received funding and support from the National Natural Science Foundation of China, the National Key R&D Program, and the Leading B Program of the Chinese Academy of Sciences.