Recoil-Ion Momentum Distributions for Two-Photon Double Ionization of He and Ne by 44 eV Free-Electron Laser Radiation

Recoil-ion momentum distributions for two-photon double ionization of He and Ne ($\hslash \omega =44\mathrm{eV}$) have been recorded with a reaction microscope at FLASH (the free-electron laser at Hamburg) at an intensity of $\sim 1\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ exploring the dynamics of the two fundamental two-photon–two-electron reaction pathways, namely, sequential and direct (or nonsequential) absorption of the photons. We find strong differences in the recoil-ion momentum patterns for the two mechanisms pointing to the significantly different two-electron emission dynamics and thus provide serious constraints for theoretical models.

Figure 1

Density plot of the measured recoil-ion momentum distributions for (a) single and (b) double ionization of He by 44 eV FLASH photons. The arrow in (a) indicates the direction of the FLASH polarization. Inner and outer circles in (b) mark the maximum ${\mathrm{He}}^{2+}$ momentum for the cases where one electron would have taken all the excess energy and for the equal energy sharing with the emission of both electrons in the same direction, respectively. (c) Projections of the 2D distribution of panel (b) onto the axis parallel (solid squares) and perpendicular (open circles) to the polarization directions. Arrows indicate the positions of the circles shown in panel (b).

Figure 2

Theoretical density plot of the recoil-ion momentum distributions for two-photon double ionization of He at (a) 44 eV and (b) 60 eV photon energies.

Figure 3

The same as Fig. 1 but for Ne.