Triple-coincidence experiment to study the correlated electron emission in the collision-induced double electron loss of ${\mathrm{He}}^{-}$

The double electron loss (DEL) from ${\mathrm{He}}^{-}$ has been investigated in collisions with He atoms at 300-keV impact energy and at electron energies that satisfy the cusp condition; i.e., the corresponding electron velocities match the projectile's velocity. In a time-of-flight experiment the energies of both electrons have been determined by detecting triple coincidences between the two electrons and the outgoing ${\mathrm{He}}^{+}$ ion. The coincidence yield shows a peak as a function of the electron energies at the expected cusp position, providing evidence for the existence of the two-electron cusp [L. Sarkadi and A. Orbán, Phys. Rev. Lett. 100, 133201 (2008)] in DEL. The observed energy distribution of the electron pairs is compared with the results of four-body classical trajectory Monte Carlo calculations carried out applying the soft-Coulomb potential approximation for the inclusion of the electron-electron interaction.

Figure 1

(a) The distribution of the measured triple-coincidence events as a function of the time of flight of the two electrons emitted by DEL of ${\mathrm{He}}^{-}$. (b) The contribution of the accidental coincidences reconstructed from the measured ion-electron and electron-electron double-coincidence distributions. The fully random coincidences between the three particles are accounted for by a uniform distribution.

Figure 2

Single electron cusp spectra following DEL of ${\mathrm{He}}^{-}$. The open circles denote the distribution measured by detecting double coincidences between the electrons and the outgoing ${\mathrm{He}}^{+}$ ions. The full circles represent the result of the CTMC calculations obtained for acceptance (half) angle $\mathrm{\Delta }{\vartheta }_{1/2}=1.2^{\circ }$. The peaks are normalized at their maxima. Here and in the following figures the dashed line shows the expected position of the cusp.

Figure 3

The measured relative FDCS for the two-electron emission in DEL of ${\mathrm{He}}^{-}$ as a function of the energies of the electron pairs. In panels (a) and (b) the scale of the FDCS is linear. For the explanation of the dotted line in panel (b) see the main text.

Figure 4

The relative FDCS for the two-electron emission in DEL of ${\mathrm{He}}^{-}$ obtained by the present CTMC calculations.

Figure 5

Energy spectra of the electron $e_1$ emitted from DEL of ${\mathrm{He}}^{-}$ with the condition that the energy of $e_2$ lies in the interval (a) 35.6–40.1 eV, (b) 40.1–41.6 eV and (c) 41.6–46.1 eV. The full circles denote the experimental data. The open circles represent the CTMC calculations. The experimental and theoretical data are normalized at the peak maxima.

Figure 6

Results of the Monte Carlo simulations. Panel (a) represents the correlation pattern expected from Wannier's theory. Panel (b) shows the distribution perturbed by postcollision interaction.

Figure 7

Relative triply differential cross section obtained by integration of the measured relative FDCS over the energy of one of the two electrons ejected in DEL of ${\mathrm{He}}^{-}$.