Correlated Two-Electron Momentum Spectra for Strong-Field Nonsequential Double Ionization of He at 800 nm

We report on a kinematically complete experiment on nonsequential double ionization of He by 25 fs 800 nm laser pulses at $1.5\mathrm{PW}/{\mathrm{cm}}^2$. The suppression of the recollision-induced excitation at this high intensity allows us to address in a clean way direct ($e,2e$) ionization by the recolliding electron. In contrast with earlier experimental results, but in agreement with various theoretical predictions, the two-electron momentum distributions along the laser polarization axis exhibit a pronounced V-shaped structure, which can be explained by the role of Coulomb repulsion and typical ($e,2e$) kinematics.

Figure 1

Longitudinal momentum distribution of ${\mathrm{He}}^{2+}$ ions. Arrows indicate the upper limits for the kinematically favorable ion momenta of $\pm 4\sqrt{U_p}$ [25]. Inset: Sum momentum of ${\mathrm{He}}^{2+}$ ion and two electrons detected in coincidence. Broad background below the narrow line is due to false coincidences.

Figure 2

Longitudinal momentum of electron one ($P_{\parallel }^{e1}$) versus that of electron two ($P_{\parallel }^{e2}$). Electrons with momenta $P_{\parallel }<-1.7\mathrm{a}.\mathrm{u}.$ are not detected (see text).

Figure 3

Same as Fig. 1 but for the transverse momentum of each electron restricted to $P_{\perp }^{e1,e2}\le 0.2\mathrm{a}.\mathrm{u}.$ (a) and $P_{\perp }^{e1,e2}\ge 1.2\mathrm{a}.\mathrm{u}.$ (b).