Many physical, biological, as well as chemical processes are influenced by electron motion within atoms or molecules. Understanding and manipulating electron dynamics in atoms is possible only by studying them. The most common technique is pump-probe spectroscopy. A well-known example is the 1999 Nobel Prize in Chemistry, where femtosecond-pumped laser pulses were used to probe the atomic movement involved in chemical reactions. To probe electron motion, however, attosecond pulses are needed because electron motion in atoms and molecules takes place over attoseconds (between 10-18 seconds) instead of femtoseconds (between 10-15 seconds). The attosecond technology has made it possible to use lasers that have pulse durations less than 100 attoseconds. This allows for the manipulation and probing of electron dynamics within atoms and molecules.Strong-field tunneling is another important method to probe electron dynamics. To induce tunneling, a strong femtosecond light is used to induce tunneling ionization. This quantum mechanical phenomenon causes electrons to tunnel through potential barriers and escape from an atom or molecule. This photoelectron-encoded information provides ultrafast electron dynamics. The relationship between the ionization times and the final momentum for the tunneling photoelectron can be used to observe electron dynamics at a subsecond scale.The theoretical relationship between the tunneling photoelectron's final momentum and ionization time has been established using a quantum orbit model. This relationship has been experimentally verified. It is still unknown which quantum orbits are responsible for the photoelectron yield in strong field tunneling ionization. Also, how different orbits correspond to momentum and ionization time has been a mystery. It is crucial to identify the quantum orbits in order to study ultrafast dynamic processes using tunneling Ionization.According to Advanced Photonics, Huazhong University of Science and Technology has proposed a scheme for identifying and weighing quantum orbits during strong-field tunneling Ionization. To perturb tunneling ionization, the scheme introduces a second harmonic frequency (SH). Because the perturbation SH is weaker than that of the fundamental field, it doesn't alter the final momentum for the electron tunneling towards ionization. It can however alter photoelectron yield due to the nonlinear nature tunneling ionization. Different ionization times result in different quantum orbitals having different responses to the SH field. The quantum orbits of tunneling electrons can be identified by changing the phase of SH field relative to its fundamental driving field. Also, it is possible to monitor the photoelectron yield's responses. This scheme allows for the identification of the "long" and the "short" quantum orbits during strong-field tunneling. The relative contributions to each momentum can also be determined. This is an important step in the development of strong-field tunneling Ionization as a method for photoelectron spectrumcopy.The study was a collaborative effort by Jia Tan, a HUST graduate student, and Shengliang Xu, Xu Han, and Professor Qingbin Zhang. It revealed that the hologram produced by the multi-orbit contribution of the photoelectronic spectrum could provide valuable information about the phase of the tunneled element. The wave packet contains valuable information about molecular and atomic electron dynamics. Peixiang Lu (HUST professor, vice-director of the Wuhan National Laboratory for Optoelectronics and senior author of this study), said that the new scheme for weighing and resolving quantum orbits makes it possible to measure electron dynamics at subangstrom and temporal resolutions.###Jia Tan and colleagues published the open access article "Resolving the quantum orbits during strong-field tunneling Ionization," in Adv. Photonics 3(3): 035001 (2021), doi:10.1117/1.AP.3.3.035001.
