Electron pair emission from surfaces: Photon versus electron excitation

We have studied the electron pair emission process from a Cu(111) surface excited by a primary electron or photon beam. Although the photon energy was 30 eV the primary electron beam had an energy of 25 eV. We will discuss the energy and momentum distributions. The experiments with the electron beam demonstrate that the contributions from the Shockley surface state can be clearly identified. This is not the case for the work with photons which we ascribe to the low spectral weight of the contribution from two Shockley state electrons compared to two 3d electrons. The momentum distributions reveal a close similarity between the two types of excitation despite the fact that the microscopic origin of the pair emission is very different. There is a distinctive energy dependence of these curves which we describe by an empirical three-parameter polynomial. The numerical values of these parameters are close for both pathways. Additional measurements with different primary electron energies reveals the importance of the initial state on the momentum distributions.