PHYSICAL REVIEW A 98, 023410 (2018) Coulomb effect in laser-induced recollision excitation T. Shaaran, * K. Z. Hatsagortsyan, and C. H. Keitel Max-Planck-Institute für Kernphysik, Saupfercheckweg 1, D-69117 Heidelberg, Germany (Received 16 January 2018; published 13 August 2018) Our investigation considers the amended strong-field approximation (SFA) ionization of rare-gas atoms in a strong laser field accompanied by excitation of the atom due to laser-induced recollision. This process can be viewed as inelastic above-threshold ionization and corresponds to the recollision-excitation step of the laser- induced nonsequential double ionization via RESI (recollision excitation with subsequent ionization) mechanism. Within the SFA framework, up to now, Bornapproximation has been used to calculate the recollision-excitation process during RESI, which is not a good approximation at low-energy recollisions. In this work, we improve this model by employing Coulomb-Born approximation, where the recolliding electron is described with a Coulomb continuum wave function. We calculate the photoelectron momentum distributions (PMD) of the rescattered electron for helium and argon atoms by considering different excitation states. The Coulomb effect is shown to play a significant role for shaping the recollision-excitation probability, visible as in the fully differential PMD, as well as in the PMD integrated over transverse momenta. DOI: 10.1103/PhysRevA.98.023410 I. INTRODUCTION Electron correlations play an important role in the dynamics of atoms and molecules subjected to a strong laser field [1–4]. In particular, it has been employed for developing ultrafast molecular imaging [5–11]. Today, there is a general consensus that for linearly polarized laser field with infrared frequencies and high intensities, within 10 13 –10 15 W/cm 2 , the strongly correlated electron motion in laser atom interaction is effectuated by laser-induced rescattering [12–14]. The rescattering is initiated by the tunneling of electron from atom. It gains energy during excursion in the laser field, is subsequently driven back by the laser field toward its parent ion, recollides with it, and as a result brings about the correlated dynamics of atomic electrons. A particular consequence of the recollision in strong-field ionization dynamics is the initiation of nonsequential double ionization (NSDI), when the recolliding electron induces the ionization of the second electron of the atom [15,16]. The existence NSDI was demonstrated for the first time in the early 1980s, when the cross section of multiphoton ionization of xenon atoms was investigated [17,18], to measure the rate of ionization against the intensity of an external laser field. Subsequently this phenomenon has been observed for other noble gas atoms [19–28] and for molecules [29–31]. In the recollision process, the second electron may be dislodged by direct impact ionization, a (e, 2e )-like double ion- ization. Alternatively, it is first promoted to an excited bound state, which is ionized by the laser field afterwards. The latter mechanism consists of two steps: (1) recollision-induced exci- tation (RE) and (2) subsequent ionization (SI), together termed as recollision-induced excitation with subsequent ionization (RESI). The key features of RESI mechanism are pronounced at lower rescattering momenta. In this case the kinematically * Corresponding author: t.shaaran@yahoo.com allowed region of the momentum components, parallel to the laser polarization axis of asymptotic photoelectrons, is located around p 1‖ = 2  U p and p 2‖ = 0, respectively, where U p is the ponderomotive potential (average oscillatory energy) of the electron in the laser field. The (e, 2e )-like double ionization is a well studied mechanism and considerably easier to model in the context of semianalytical approaches [32–35], while RESI treatment is more elaborate. It is required to account for several effects, such as different excitation channels, multielectron effect of the bound state, depletion [36,37], quantum interference between all the available channels [38], and the possibility of creating a superposition of excited states at the time of rescattering. Moreover, the focal averaging and Coulomb focusing significantly modify photoelectron momentum distribution (PMD) for RESI. In addition, for multicycle laser pulses at near-infrared wavelength PMD of the correlated electrons in (e, 2e ) and RESI channels coexist and overlap with each other, which limits the detailed experimental investigation of the underlying processes. There are several approaches for the RESI treatment. RESI has been calculated classically via classical trajectory Monte Carlo simulations [39–42], quasiclassically using S -matrix formalism based on strong-field approximation (SFA) [15,16], and by the so-called quantitative rescattering (QRS) method [36,37,43,44]. In the classical method it is not possible to include excitation in a rigorous way. This is accomplished by considering an ad hoc time lag between the rescattering of the first electron and ionization of the second electron. With this assumption both electrons leave with opposite momenta, yielding distributions mainly located in the second and fourth quadrants of PMD, rather than in all quadrants [40–42,45–50]. How the time delay may arise during excitation is not addressed in this treatment. As it was shown in [15], the electron- impact ionization with a time delay also can populate the low regions of electron momentum distributions. On the other hand, the RESI description based on SFA [51–55] and on QRS lead to electron momentum distributions in all 2469-9926/2018/98(2)/023410(10) 023410-1 ©2018 American Physical Society