The Simpleman’s model lays the foundation of our understanding on fundamental processes involved in interaction of atoms and molecules with intense low-frequency laser fields. Within this model, the effect of Coulomb potential has been ignored. Recent experimental observations have provided strong evidences of the breakdown of the Simpleman’s model in low-energy regions of ATI and HHG spectra. However, for the high-energy electron recollision, unambiguous experimental evidences of Coulomb effects are still lacking.

 Researchers at State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences (SIOM, CAS) present experimental and theoretical evidences on breakdown of Simpleman’s model in the high energy recollision region by examining both energy resolved angular distribution (AD) of photoelectron in ATI and spectrum of HHG in elliptically polarized driver fields. As an example, the Fig.1 (a) and (b) below show the experimentally measured yields of 71st and H91st harmonics as a function of ellipticity of driver pulses for both short- and long-trajectories respectively. The calculated results without the Coulomb potential are shown in Fig.1 (c) and (d). The normalized ellipticity dependence of H71/H91 ratio for both long- and short-trajectories are ploted, the corresponding experimental and SFA-model calculation results are shown in Fig.1(e) and 1(f). Also the researchers take the semi-classic model to quantitatively evaluate the H71/H91 ratios. The calculated results, with and without the Coulomb potential, are displayed in Figs. 2(a) and 2(b), respectively. It can be seen that the result in Fig. 2(a) shows a good agreement with the experimental observation in Fig. 1(e). In contrast, when the Coulomb field is switched off, the result in Fig. 2(b) resembles to a great extent to the SFA result (Fig.1(f)). The comparisons clearly manifest the important role of Coulomb potential. 

The result of this work will lead to a more complete understanding of the Coulomb influence on electron wavepacket dynamics in strong laser field across the entire energy spectrum. These findings will have important implication for both attosecond metrology and molecular imaging.

This work entitled " Unexpected breakdown of the simple man’s model for strong-field photoionization in the high-energy recollision region " has been published in Phys. Rev. A85, 021403 (2012), http://pra.aps.org/abstract/PRA/v85/i2/e021403.

Fig 1. (a) and (b): The experimentally  yields of 71st and H91st harmonics; (c) and (d): the SFA calculated results without the Coulomb potential; (e) and (f): experimental and SFA-model calculated H71/H91 ratios. (Image by SIOM)

Fig 2. Semi-classic model (a) with and (b) without the Coulomb potential　(Image by SIOM)