PHYSICAL REVIEW A 98, 053418 (2018) Coulomb potential effects in strong-field atomic ionization under elliptical polarization RenPing Sun, 1, 2 XuanYang Lai, 1 Wei Quan, 1 , * ShaoGang Yu, 1, 2 YanLan Wang, 1 SongPo Xu, 1, 2 ZhiLei Xiao, 1, 2 Yu Zhou, 1, 2 MingZheng Wei, 1, 2 Meng Zhao, 1, 2 and XiaoJun Liu 1 , † 1 State Key Laboratory of Magnetic Resonance and Atomic and Molecular Physics and Center for Cold Atom Physics, Wuhan Institute of Physics and Mathematics, Chinese Academy of Sciences, Wuhan 430071, China 2 School of Physics, University of Chinese Academy of Sciences, Beijing 100080, China (Received 9 July 2018; published 15 November 2018) We experimentally and theoretically investigate strong-field ionization of noble-gas atoms in elliptically polarized laser fields at 2000 nm. The ellipticity dependence of the ion yields has been measured at a series of intensities and compared to various theoretical model predictions [i.e., the Ammosov-Delone-Krainov (ADK), strong-field approximation (SFA), and Coulomb-Volkov distorted-wave approximation (CVA) models]. The SFA and ADK calculations match each other, which alludes to the negligible contribution of nonadiabatic effect. In the meantime, they both deviate significantly from the measurements. On the other hand, the good agreement between CVA calculations and measurements reveals the salient role played by the Coulomb potential in atomic ionization. The Coulomb potential effect is further confirmed by comparison of CVA calculations and measurements at 800 nm. Our work demonstrates the significant Coulomb potential effect on the ionization yields, the experimental evidence of which has rarely been documented. DOI: 10.1103/PhysRevA.98.053418 I. INTRODUCTION When an intense laser field (with an intensity I  10 13 W/cm 2 ) interacts with an atom, the fundamental pro- cess to occur is ionization. In the limit of long wavelength and high intensity, the quasistatic approximation [1] can be applied to comprehend the ionization dynamics. Within this approximation, the ionization process can be understood with the well-established static-field ionization theory, where the valence electron tunnels through the barrier formed by the static field and the Coulomb potential [2]. An extension of this approach to arbitrary atoms and atomic ions has been achieved by Ammosov, Delone, and Krainov (ADK) in 1986 [3]. After tunneling, the electron could directly go to the detector, i.e., without any significant interaction with the atomic core, which is usually termed as direct ionization. Alternatively, especially in the linearly polarized laser field, the ionized electron may return to the parent core and collide or combine with the core to induce various highly nonlinear phenomena [4], such as high-order above-threshold ionization (HATI) [5], high- order harmonic generation (HHG) [6], nonsequential double ionization (NSDI) [7], etc. Despite an inevitable role played in the tunneling process, the effect of Coulomb potential on the ultrafast strong-field ionization process has often been underestimated in a lot of documented works (see, e.g., [8]). Recently, it has been recognized that the influence of Coulomb potential is indispensable for a variety of exper- imental observations. For atomic ionization in linearly po- larized laser field, the cusplike structure in transverse mo- mentum distributions [9,10], the prominent structure in the * charlywing@wipm.ac.cn † xjliu@wipm.ac.cn low-energy [11,12] and near-zero energy part [13,14] of the photoelectron kinetic energy spectra or momentum distribu- tion can all be attributed to the Coulomb potential effect. In the nonsequential double-ionization process, Coulomb potential is critical for the enhanced double-ionization yields [15] and the fingerlike structure [16,17] in the photoelectron momen- tum correlation distribution. On the other hand, earlier works demonstrate that the Coulomb potential effect could be also significant in the atomic ionization in elliptically polarized laser fields [18], where the nonvanishing electric-field component perpendic- ular to the major polarization axis can effectively eliminate the rescattering process. The typical signature of the Coulomb potential effect for above-threshold ionization (ATI) in ellip- tically polarized field has been shown experimentally [19,20], manifested in fourfold asymmetry structure of photoelectron angular distributions with respect to the main axes of po- larization. In contrary, a symmetry structure is predicted by the PPT theory (Perelomov, Popov, and Terentev) [21] and strong-field approximation [22], where the Coulomb potential is totally ignored. By using a dedicated semiclassical model, Li et al. [23] disentangle the contribution of direct ionization and multiple forward scattering to Coulomb asymmetry in elliptical field. In fact, the study of Coulomb potential effect in the ellip- tically polarized field could be intriguing and the ionization behavior of atoms in elliptically polarized laser fields has attracted increasing attention in the strong-field physics com- munity recently [19,24–29]. The investigations of attosecond angular streaking [30,31] have achieved great successes in extracting the tunneling delay. Technically, it is accomplished by mapping the instant of ionization to the final angle of the momentum vector in the polarization plane. It is shown theoretically that the Coulomb potential of the atomic ion in 2469-9926/2018/98(5)/053418(6) 053418-1 ©2018 American Physical Society