Imaging and Controlling Multielectron Dynamics by Laser-induced Tunnel Ionization

Using sequential strong-field double ionization in a pump-probe scheme we show through calculations how electronic dynamics can be prepared and imaged. Electronic dynamics arise whenever multiple states of the ion are accessed in the ionization step. The dynamics in the cation influence the rate of the second ionization step and the momentum distribution of the ejected electron, allowing their detailed characterization. We show how the probe step is controlled through spatial propensities of the ionizing orbitals and the energy level structure of the dication. Both the final electronic state of the dication and the spin state of the ejected electron pair can be controlled through the time delay between the two pulses. We discuss how our results will extend to the preparation and measurement of attosecond electron dynamics. Imaging and Controlling Multielectron Dynamics by Laser-induced Tunnel Ionization 2 Understanding and controlling the collective dynamics of electrons in molecules and solids is one of the central challenges of modern ultrafast science. Recent efforts have concentrated on the development of methods for the measurement of the electronic dynamics in the valence shell of atoms, molecules and solids [1, 2, 3]. This insight is expected to have important consequences for our understanding of all physical phenomena determined by the interactions between these electrons, ranging from the basic electronic structure of matter to coherent phenomena in large molecules and cooperative phenomena in solids, like supraconductivity. An attractive approach to studying the properties of a complex static system consists in removing it from equilibrium and investigating the induced dynamics. In the case of a multielectron system such a possibility is offered by temporally confined ionization [4]. The temporal confinement makes a broad bandwidth of ionic states accessible. The ensuing dynamics allows the investigation of the properties of the initial wave function and of the interactions of its constituents. Strong-field ionization (SFI) in the tunneling limit represents an attractive approach to studying and controlling the dynamics of multielectron systems. SFI usually affects multiple electronic shells implying that the ion can be left in different electronic states [5, 6]. The high nonlinearity of the strong-field interaction generates a photoelectron with a very large bandwidth, spanning tens of electron volts under typical experimental conditions. Therefore, strong-field ionization may leave the ion in a partially coherent nonstationary state [5, 7]. Strong-field ionization therefore typically induces collective electron dynamics. In molecules, strong-field ionization also prepares vibrational wave packets whenever the potential surfaces of the neutral and ionic states differ, a phenomenon that has been studied in some detail [8, 9, 10, 11]. In this communication, we show by calculations how an electronic wave packet can be imaged using strong-field ionization and used to control the products of a subsequent ionization step. SFI is known to be highly sensitive to the electronic structure [12, 13, 14] because of its pronounced sensitivity to the shape of the ionizing orbitals. We transfer these concepts from static systems to probing electronic dynamics in atoms and molecules. As an example we use the neon atom, in which strong-field ionization prepares a spinorbit wave packet [15, 7]. The ground state of the neon cation has two fine-structure components that are separated by 0.1 eV due to spin-orbit interaction. In this case, spinorbit interaction drives the electronic dynamics in the cation. In virtually all polyatomic molecules, the relaxation of the orbitals upon ionization induces attosecond electronic dynamics driven by electron correlation [16, 17] which may be probed using a second ionization step, in analogy to the case discussed in this article. As a second aspect, we show how the proposed pump-probe experiment enables a high degree of control over the accessed state of the dication and the ejected electron pair. The dication can be prepared preferentially in either the triplet ground state or in the low-lying singlet states. Through the conservation of total spin, the ionized pair of electrons can therefore be prepared preferentially in the singlet or triplet state. Although we show model calculations for the rare gas atoms only, the underlying principles are directly applicable to all atoms and molecules possessing a fully occupied degenerate highest occupied orbital (e.g. halogen dimers, hydrogen halides, carbon dioxide, allene, Imaging and Controlling Multielectron Dynamics by Laser-induced Tunnel Ionization 3 benzene etc.). The ionization of these neutral species prepares a spin-orbit wave packet in the cation that can be imaged through a second ionization step. We consider the sequential double ionization of a rare gas atom by a pair of intense (10-10 W/cm) femtosecond infrared laser pulses as illustrated in Fig. 1. The atom (Ne, Ar, Kr or Xe) is initially in its ground electronic state S0 of configuration (...)(ns)(np). The first pulse ionizes the neutral atom almost exclusively from the p orbital aligned along the polarization of the laser pulse because the SFI rate Γ`,m is highly sensitive to the angular momentum projection quantum number m of the ionizing orbital [18]. As an example, the relative ionization rates of the orbital aligned along the direction of the laser field m = 0 to that of the perpendicular orbitals m = ±1 amounts to Γ`=1,m=0/Γ`=1,m=±1 ≈ 30 for a neon atom in a laser field of 5·10 W/cm. Consequently, SFI prepares the rare gas cation in an electronic state of configuration (...)(ns)(np0p 2 −1p 2 +1), i.e. it generates an electron ”hole” that is aligned along the laser field [19, 15, 20]. The corresponding wave function density is represented in Fig. 1 for ∆t = 0. However, this electronic configuration does not correspond to an eigenstate of the cation, but rather to a linear combination of its two lowest spin-orbit levels φ(t = 0) = Φ1,0,+1/2 = √ 2