Criterion for Distinguishing Sequential from Nonsequential Contributions to the Double Ionization of Helium in Ultrashort Extreme-Ultraviolet Pulses

Abstract

We quantify sequential and nonsequential contributions in two-photon double ionization of helium atoms by intense ultrashort extreme-ultraviolet pulses with central photon energies ℏω ctr near the sequential double-ionization threshold. If the spectrum of such pulses overlaps both the sequential (ℏω > 54.4 eV) and nonsequential (ℏω < 54.4 eV) double-ionization regimes, the sequential and nonsequential doubleionization mechanisms are difficult to distinguish. By tracking the double-ionization asymmetry in joint photoelectron angular distributions, we introduce the two-electron forward-backward-emission asymmetry as a measure that allows the distinction of sequential and nonsequential contributions. Specifically, for ℏω ctr ¼ 50 eV pulses with a sine-squared temporal profile, we find that the sequential double-ionization contribution is the largest at a pulse length of 650 as, due to competing temporal and spectral constraints. In addition, we validate a simple heuristic expression for the sequential double-ionization contribution in comparison with ab initio calculations. DOI: 10.1103/PhysRevLett.115.183002 PACS numbers: 32.80.Rm, 33.80.Rv, 42.50.Hz In 1975, the mechanism of nonsequential double ionization was revealed in the photoionization of alkaline earth atoms [1]. It is enabled by strong electronic correlation and thus clearly distinct from the sequential double-ionization mechanism. Nonsequential double ionization was observed for noble gas atoms in 1982 [2] and received rapidly increasing attention from both experimentalists [3-9] and theorists [10][11][12][13].Photoelectron angular distributions of single-photon double ionization were found to owe their structure partly to (dipole) selection rules [11,14,15] and to consist of symmetrical and antisymmetrical contributions (with regard to electron exchange), that each can be written as the product of an angular and a correlation factor [11]. For coplanar emission geometry, where the emitted-electron momenta and polarization axis of the linearly polarized extremeultraviolet (XUV) pulse lie in a plane, and for equal energy sharing (equal asymptotic kinetic energies E 1 and E 2 of the photoelectrons), the angular factor becomes j cos θ 1 þ cos θ 2 j 2 , where θ 1 and θ 2 are photoelectron emission angles relative to the polarization direction of the ionizing light, while the electron correlation factor follows as expf−4 ln 2½ðθ 12 − πÞ=θ 1=2 2 g, with the mutual emission angle θ 12 ¼ jθ 1 − θ 2 j. The adjustable parameter, θ 1=2 , is related to the significance of correlation in the double-ionization process [11]. The antisymmetrical contribution and back-to-back electron emission vanish at equal energy sharing but become progressively more prominent for increasingly nonequal energy sharing, as the antisymmetrical contribution j cos θ 1 − cos θ 2 j 2 gradually appears in joint photoelectron angular distributions [10,11,16].

Experimental evidence for selection rules in multiphoton double ionization of helium and neon

“…In all quantum mechanical calculations, these selection rules are implicitly implemented (see e.g. in direct solutions of the time dependent Schroedinger equation [4,7,20], S-Matrix calculations [21,22] or close coupling methods [23]). Despite their universal character, however, these quantum mechanical selection rules have never been observed in experiments beyond the single photon case.…”

Observing electron-correlation features in two-photon double ionization of helium

“…A similar but differential asymmetry parameter restricted to coplanar emission with fixed angle and energy sharing was introduced for quantifying the degree of (non)-sequentiality [56]. The present asymmetry parameter (Eq.…”

Superelastic rescattering in single ionization of helium in strong laser fields

“…The numerical solution of the time-dependent Schrödinger equation (TDSE) for a helium atom in EUV fields by including two electrons with full dimensions has been carried out for exploring many interesting phenom-ena [25][26][27][28][29][30]. However, simulating two-electron dynamics in full dimensions in long-wavelength laser fields is still a tremendous computational task.…”