Recently, Prof. Jing Chen and Prof. Xiantu He of Center for Applied Physics and Technology (CAPT) made important progress in their studies on strong-field physics, through cooperation with research groups of Physics Department of Peking University, Wuhan Institute of Physics and Mathematics and Shanxi University. They further studied the dynamic process of low-energy photoelectron in the atomic above-threshold ionization (ATI) process, and found a universal peak structure with electron energy well below 1 eV in the photoelectron spectrum, corresponding to the double-hump structure in the longitudinal (direction of polarization of the laser field) momentum distribution, for all atomic species. Through quantum and semiclassical analysis, they reveal that multiple rescattering of low-energy photoelectron upon the ionic Coulomb potential is responsible for the production of this very low-energy structure observed in tunneling ionization process in intense infrared laser field. Their work has been published in Physical Review Letters (108，223001(2012)).

Ionization dynamics of atoms and molecules in intense laser field is the key problem and hot topic of research in strong-field physics. As the most basic physical process in interaction between intense laser field and atoms/molecules, above-threshold ionization (ATI) has been intensively studied since its first observation in 1979 and yielded several surprises, e. g., plateau and cutoff structure in high-energy ATI photoelectron spectrum, resonance-like enhancement in plateau of ATI spectrum etc.

In 2009, collaborating with some domestic research groups, He and Chen found an abnormal hump structure in the low-energy region of the ATI energy spectrum in mid-infrared intense laser field and revealed the underlying physical mechanism of this new structure (Physical Review Letters 103，093001(2009)) (P. Agostini and L. F. Dimauro group in Ohio University of USA also independently reported this phenomenon (Nature Physics 5, 335 (2009)). This work has been considered as a new surprise in strong-field atomic physics and led to intensive investigations on dynamics of low-energy ATI photoelectron in recent years.

The referee of this latest paper highly appraises the work：“It always contained plenty of novel and important information about the low- and very-low-energy structure. The data and their interpretation will be very important for the future analysis and understanding of the ATI physics in the very-low-electron-energy regime……”

This work is supported by NNSFC and the National Basic Research Program of China.

Low energy photoelectron energy spectra and longitudinal momentum distributions of Ne ((a) and (d)), Kr ((b) and (e)) and Xe ((c) and (f)) in infrared laser fields.
 

Low energy photoelectron energy spectrum (a) and longitudinal momentum distribution (b) calculated using numerical solution of TDSE for Xe (black line) and short-range potential (red line). Calculated result using a hydrogen-like potential with the same ground state energy of Xe is also shown (green line).
 

Low energy photoelectron energy spectra and longitudinal momentum distributions of atoms calculated using semiclassical model.