Complexity at simple interfaces: Dynamically generated deep trap states at the noble-metal/alkali-halide interface

The trapping of delocalized electrons at ultrathin layers of NaCl on Ag(100) is investigated in real time using two-photon photoemission. Above 79 K, electrons localize to deep trap states assigned to low-coordinated sites on the NaCl surface. Below 79 K, these low-coordinated trap states serve as precursors for a new deep trap state at increased binding energies. The observation that the emerging trap state requires a localized precursor state suggests that a dynamic polarization of the NaCl lattice may be required for its formation. This behavior is consistent with the expectations for the formation of a interfacial (electron) small polaron which is theoretically unstable in bulk NaCl. The alkali-halide/noble-metal interface shows the sequential population of multiple metastable deep trap states in two dimensions in real time.