Above-threshold ionization (ATI) is the most fundamental process in strong field atomic physics and relevant investigations have played an important role in understanding the intense laser-matter interaction in the past decades. Precise experimental measured momentum distributions of photoelectron are essential to reveal the underlying ionization mechanism. A peculiar low-energy structure is recently observed in ATI spectra in the tunneling regime in the mid-infrared femtosecond laser pulses. This observation can not be explained by the existing theories and cause much attention. However, limited by the experimental apparatus, there are only two experimental groups reported the peculiar low-energy structure in mid-infrared laser pulses. In addition, there is a disagreement on the very-low-energy structure with energy below 1 eV.

Very recently, Prof. Gong’s group successfully constructs a COLd Target Recoil Ion Momentum Spectroscopy (COLTRIMS) which have strong capabilities of high momentum resolution, full solid angle collection, and coincident measurement. Based on the photoelectron-ion coincident measurement of COLTRIMS, precise experimental data are measured and the controversy is clarified for the very-low-energy photoelectron from above-threshold ionization in tunneling regime. The underlying mechanism is also revealed with the aid of quantum and semi-classical simulations. As acknowledged by referees, the data and their interpretation will be very importantfor the future analysis and understanding of the ATI physics in the very-low-electron-energy regime.This work has been published by the prestigious journal of Phys. Rev. Lett. (C.Y. Wu, Y. D. Yang, Y. Q. Liu, Q. H. Gong, et al. Characteristic Spectrum of Very Low-Energy Photoelectron from Above-Threshold Ionizationin the Tunneling Regime, Phys. Rev. Lett. 109, 043001 (2012),http://prl.aps.org/abstract/PRL/v109/i4/e043001).