Double photoemission

Pair emission from surfaces - Fig. 4
Fig. 1: The light was incident on the Cu(001) crystal surface at a grazing angle. The axis of the analyser transfer lenses lies in a plane perpendicular to the incident light and at an angle of 90° to each other. The energy dispersing directions are marked E1 and E2, respectively.

The absorption of a single photon by an electronic system and subsequent electron pair emission is called double photoemission (DPE). A finite DPE intensity requires a finite electron-electron interaction. We have demonstrated that it is possible to probe the exchange-correlation hole in a similar way as in the electron excited pair emission [1-3]. A particular DPE process concerns the emission of a core photoelectron and the subsequent decay via Auger electron emission. The Auger effect is usually explained via a two-step process in which a core electron is ejected followed by a de-excitation via Auger electron emission. We provide the first experimental proof that the core-electron and Auger electron emission proceeds via a single step [4].  By using two hemispherical analyzers (Fig. 1), it is possible to map out the energy distribution of correlated electron pairs emitted from the valence band by direct DPE and, simultaniously, of pairs emitted from the processes of core-level photoemission and Auger decay.  Fig. 2 shows the number of electron pairs detected for each combination of individual electron kinetic energies EA and EB. 

Pair emission from surfaces - Fig. 5
Fig. 2: The dashed lines in this 2D-Energy distribution indicate the energetic positions of direct direct and core-resonant DPE.

The onset of direct DPE appears as a continuous line across this plot, which is characteristic of the two electrons sharing their energy continuously to conserve their sum of the energy. Surprisingly, there is similar structure between the most intense points in the distribution which appear at energies characteristic of core-level photoelectrons and the associated Auger electrons. The striking appearance of this linear structure confirms that these electrons also share their energy continuously. Moreover, their individual energies are not constrained to the narrow range they are found to have when measured independently — they are even found with equal energy at which point labelling them as Auger electron or photoelectron becomes meaningless. This is a strong experimental proof that the photoemission and Auger decay proceed in a coherent single-step process and suggests that it should be considered as a special case of DPE resonant with a core-level excitation.