When a surface is bombarded by an electron beam low energy secondary electrons (LE-SEs) represent the most abundant yield. By means of (reflection) electron energy loss spectroscopy (R-EELS) detailed information on the excitation channels of a solid can be studied. Collective charge excitations – plasmons - can be identified as particularly prominent spectral features in EELS. In the ‘70s, it was already speculated [1,2] that these excitation channels play a major role in the emission of LE-SEs. Even though LE-SE emission is known since over a century, the mechanisms leading to the their ejection are still debated. In fact, by the mere study of the excitation channels it cannot be elucidated to which extent the considered scattering processes contribute to the intensity of the secondary electron yield (SEY) of a material. By the sole study of SEY-spectra - due to their spectral featurelessness - it is intrinsically difficult, to identify the processes, which contribute to their creation and ejection. However, by collecting time-coincident electron pairs instead, one of which is the inelastically backscattered electron, that has transferred part of its energy and momentum to the solid-state electrons, and the other is the SE emitted as a consequence of this interaction, it ibecomes possible to reconstruct by means of energy- and momentum-conservation the complete kinematics and the energy dissipation occurring during the collision. Such an experiment, known as (e,2e) spectroscopy, enables to completely determine the kinematics of an investigated ionisation process and is ideally suited to study electron correlation effects. Owing to its high surface sensitivity, (e,2e) in reflection mode enables the investigation of processes involving LE-SE emission at surfaces, such as plasmon decay. This is done by measuring in coincidence, the energy-loss- together with the SE-spectrum. Al(100)-results show that the ejection of a LE-SEs is predominantly induced by the relaxation of singly excited plasmons. The (e,2e)-cross section changes by a factor of 10 when tuning in and out plasma frequency. This suggests predominance of the plasmon-resonant channel over direct electron-electron scattering. Wecould also observe that doubly excited plasmons exhibit a sequential character rather than a coherent one in their decay process.Our results are compared and discussed along with other (e,2e) experiments [3,4] and with a more recent model .
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Financial support by the FP7 People: Marie-Curie Actions Initial Training Network (ITN) SIMDALEE2 (Grant No. PITN 606988) is gratefully acknowledged.