In this process, photons scatter inelastically with electrons close to the Fermi level via one intermediate ("virtual") state in a single event. Because it requires less momentum change than intraband PL, it could be more effective in producing emissions at longer wavelengths. In addition to the just discussed direct emission mechanisms, sequential pathways involving inelastic Auger-scattering (Figure ) may contribute to metal PL. In such a process, interband excitation initially creates a d -band hole (violet arrow), which is subsequently filled nonradiatively by Auger recombination. That is, two sp-band electrons interact, one being scattered to occupy the d -band vacancy and the other being scattered to an energy above the Fermi level. Due to their partially filled conduction bands, this type of electron-electron scattering is much more likely in metals than in semiconductors. In fact, Auger scattering has been identified as the primary route for the nonradiative recombination of d -band holes and was shown to cause delayed excitation of sp-band electrons above the Fermi level. In nanostructured metals, a fraction of these excited charges might generate PL via intraband recombination (red arrow). As both interband absorption and Auger scattering are momentum-conserved, this Auger-excited intraband emission should be considerably more efficient than the PL following intraband excitation. Only a few investigations have discussed Auger-excited emission so far. Cai et al. suspect Auger-excited emission to cause an unexpected increase in the radiative quantum yield of NIR emission from single gold nanorods excited below the interband threshold. They estimated that at least half of the observed PL could originate from this process but could not provide conclusive evidence for its existence. The extent to which Auger processes contribute to the PL of gold particles thus remains uncertain.