Correlated double electron capture from metal surfaces

Pair Emission - Fig.8
Fig. 1: Schematic view of the double electron capture from the surface to the outer shell of a He2+ ion. This leads to the emission of a single electron and an excited He+ ion which in a subsequent step can capture another electron.

Electrons in matter are not independent of each other but affect each other via the repulsive Coulomb interaction and the Pauli principle. The emergence of ferromagnetism is a macroscopic manifestation of the microscopic interaction. Electron pair emission from surfaces is a unique tool to study the electron-electron correlation.

Besides primary electron or photon excitation it is conceivable to use the neutralization of an incoming He2+ ion to initiate the pair emission process.  A possible pathway leading to electron emission is presented in Fig.1. An incoming He2+ ion captures two electrons in an outer shell. This leads to a decay via an emission of single electron which leaves behind an excited He+ ion. The available energy can be transferred to a surface electron in Auger-type process leading to the emission of second electron.

Pair Emission - Fig. 9
Fig. 2: 2D-Energy distribution from Ir(100) upon 10 eV He2+ impact. The dashed red line marks the maximum energy sum the emitted pair can have.

The double ionization energy of He is 79 eV and becomes available for electrons in the surface region.  Due to the fact that more than 90% of the incoming ions are neutralized the probability of electron pair emission is expected to be high. We studied the electron pair emission from a Ir(100) surface upon impact of 10 eV He2+ ions. In Fig.2 we present the 2D-energy distribution. In the neutralization process and subsequent pair emission 4 surface electrons are involved. Two are required to neutralize the ion while the other two constitute the detected electron pair. Therefore the maximum sum energy of the emitted pair is the ionization potential minus four times the work function. The value for the Ir(100) surface is 57 eV and is included as red dashed line in Fig.2. Usually the neutralization process is believed to proceed via subsequent steps. A consequence of this picture is that the energy of the second emitted electron has to be lower than 15 eV. From the 2D-energy spectrum we see clear evidence of coincidence events where the electron energies are larger. Hence, we can prove that the proper description requires a single step theory.