Ejection of Quasi-Free-Electron Pairs from the Helium-Atom Ground State by Single-Photon Absorption

We investigate the single-photon double ionization of helium at photon energies of 440 and 800 eV. We observe doubly charged ions with close to zero momentum corresponding to electrons emitted back to back with equal energy. These slow ions are the unique fingerprint of an elusive quasifree photon double ionization mechanism predicted by Amusia et al. nearly four decades ago [J. Phys. B 8, 1248 (1975)]. It results from the nondipole part of the electromagnetic interaction. Our experimental data are supported by calculations performed using the convergent close-coupling and time-dependent close-coupling methods.

Figure 1

(a) Sketch of the dipole structure, different planes and coordinates: light propagation $x$, light polarization $z$, gas jet direction $y$. (b) Slice ($|p_y|<0.73\mathrm{a}.\mathrm{u}.$) of the ion momentum distribution in the $xz$ plane for $\hslash \nu =800\mathrm{eV}$. (c)–(h) Ion momentum distribution perpendicular to the light polarization vector for $\hslash \nu =440\mathrm{eV}$ (left column) and $\hslash \nu =800\mathrm{eV}$ (right column), with different momentum intervals $dp$ parallel to the polarization vector of the doubly charged ion at a given photon energy. (c),(d) $dp=7\%$; (e),(f) $dp=15\%$; (g),(h) $dp=25\%$. The solid circular line represents the maximum possible momentum ($p_{\max }$). The dashed green line in (b)–(d) represents the case when one electron receives all momentum. Panels (i) and (j) show a projection of (g) and (h) to the horizontal $p_x$ axis for a momentum interval of $|p_y/p_{\max }|\le 25\%$ indicated by the yellow bars. The solid light blue line represents the CCC calculation for $\mathrm{\text{dipole}}+\mathrm{\text{quadrupole}}$, while the other lines show TDCC calculations for dipole (dashed and red), quadrupole (dash-dotted green), as well as the coherent sum of dipole and quadrupole (dotted blue). The experimental data have been normalized to Samson et al. [45]; the theory maximum has been normalized to the experiment maximum.

Figure 2

Calculations for 440 (left column) and 800 eV (right column) photon energy with the same geometry as in the experiment [Figs. 1e, 1f, for an interval of $dp=\pm 15\%$]. In panels (a) and (b) the CCC calculations include only the dipole interaction, while (c) and (d) solely display the quadrupole terms and (e) and (f) show the full theory. Panels (g) and (h) show a full TDCC calculation including dipole and quadrupole terms and in contrast to the CCC theory, their interference terms as well.