Evidence for double resonant Raman decay from a Ag surface

We have investigated the electron pair emission from a Ag(100) surface upon photon absorption which leads to the emission of a $4p$ photoelectron. The subsequent Auger decay proceeds in a single-step process as shown previously. Key finding was the emergence of a diagonal intensity feature in a two-dimensional kinetic energy distribution of the electron pair whose width reveals a characteristic time of 30 as for the dynamics of the core-hole decay process. We utilize this observation as a tool to identify an unexpected pathway of electron pair emission. Once the photon energy is at the threshold of a double $4p$ core hole creation we notice a sizeable broadening of the energy sharing distribution. We associate this process to the excitation of two $4p$ electrons between the Fermi and vacuum level followed by radiation less double electron excitation. This feature is characterized by an energy sum of the pair which disperses linearly with the photon energy. Therefore, we regard it as a double resonant Raman decay.