Nonsequential Double Ionization as a Completely Classical Photoelectric Effect

Abstract

We introduce a unified and simplified theory of atomic double ionization. Our results show that at high laser intensities (I ≥ 10 14 watts/cm 2 ) purely classical correlation is strong enough to account for all of the main features observed in experiments to date.Short-pulse lasers with high peak intensities (10 14 ≤ I ≤ 10 16 , in watts/cm 2 ) now produce multiphoton generation of double ionization, the two-electron photoelectric effect, with surprising results. To summarize briefly, the experimental data show that two atomic (or molecular) outer-shell electrons are highly correlated when photoejected, with a double ionization rate that can be 1-million times higher than uncorrelated sequential theory [1] allows, so the process is called non-sequential double ionization (NSDI). The first laboratory results were reported in 1992 and 1993 [2,3], showing an anomalously high double ionization yield, the principal experimental signature of NSDI. Additional data is being reported from momentum spectroscopy experiments [4,5,6,7,8,9,10,11]. The momentum distribution data, along with the ion-yield data, serve as the benchmarks for various theoretical models.The mechanism that makes NSDI correlation so effective is far from settled, and theoretical exploration has been extensive [12,13,14,15,16,17]. Almost all existing calculations refer to, or are closely guided by, a single few-step rescattering model [18,19], which is based on an imagined picture in which one electron escapes the atom by quantum tunneling through a fieldlowered barrier and is then phase-coherently and classically forced by the laser away from and then back to the core where a quantum collision liberates both electrons at once (consistent with the term non-sequential). However, the patchwork of ad hoc elements typically employed [20] has not been claimed to make a complete, i.e., self-contained, theory. It is the purpose of this note to show that a self-contained theory exists that is compatible with essentially all prominent features of NSDI.Our theory is dynamically classical, and discards all aspects of quantum mechanics including tunneling. It is built on the need for strong electron correlation to explain NSDI, and so must be intrinsically a two-electron theory. We do not advocate such a theory for an electron that does not have the advantage of a strongly correlated partner. It turns out that entirely classical interactions are adequate to generate very strong two-electron correlation, as observed in NSDI, and quantum theory is not needed. Of course atoms are quantum objects but in such strong fields as are used for NSDI it is mainly electron physics rather than atomic physics that determines the experimental outcome. In this sense the early remark of Corkum [19] advocating the adoption of a plasma per- spective was quite appropriate.We note that a new form of energy analysis is very helpful. The graph in Fig. 1 shows the sum of kinetic energy, electron-nucleus binding energy, e-e correlation energy and laser field interaction energy for each of...

Delayed double ionization as a signature of Hamiltonian chaos

“…The parent model is a two active electron atom with soft-Coulomb potentials subjected to an intense and short linearly polarized laser pulse in the dipole approximation [19][20][21]. The corresponding Hamiltonian reads:…”

Complete characterization of single-cycle double ionization of argon from the nonsequential to the sequential ionization regime

“…Strong-field multiple ionization of atoms is widely regarded as a classic example of multielectron dynamics in an external field and has attracted considerable interest of experimentalists [1][2][3][4][5][6] and theorists [7][8][9][10][11] alike. Double ionization in particular is a process characterized by different intensitydependent regimes [12].…”

Breakdown of the independent electron approximation in sequential double ionization

“…These simulations are fully classical, meaning that the concept of tunnelling for the ionization step is avoided. Instead the electrons are treated fully classically from the beginning by using microcanonical initial conditions matched to the actual ionization potentials, as detailed in [23]. The process of ionization is calculated by solving the time-dependent Newton equations for all the involved particles in the presence of the electric field of the laser pulse.…”