UC Berkeley

Angle- and time-resolved two-photon photoemission is used to study the ultrafast electron dynamics at alkali halide-metal interfaces under ultrahigh vacuum conditions. The initial excited states and electron localization at trap states are characterized on the ultrafast timescales.

For NaCl on Ag(100), the well characterized island growth of NaCl on low Miller indices noble metal substrates in the ultrathin coverage limit is studied. The initial excited states observed correspond to the n = 1, n = 2, and n = 3 image potential state (IPS) progression of the NaCl-metal interface. The delocalized n = 1 IPS electrons, evidenced by their light effective mass, m* = 0.7me, are found to localize via population transfer to two localized states on the hundreds of femtosecond timescale. The first localized state, indicated by its flat band dispersion, is located at the bottom of the free state band with a decay lifetime comparable to the free n = 1 electrons of < 100 fs. The second, discrete, localized state observed leads to a binding energy gain of hundreds of meVs and results in a substantial lifetime increase of 0.5-1 ps versus the ca. 100 fs of the initial delocalized n = 1 IPS. The long-lived localized state's lifetime shows an exponential dependence with increasing coverage, indicating the NaCl film acts as a tunneling barrier and that the localized state is located at the surface-vacuum interface, versus inside the film. The long lived localized state is assigned to low coordinated defect sites on the NaCl surface such as step edges, corners, and NaCl pair vacancies. The binding energy and decay lifetime of electron trapping at low coordinated defects shows strong temperature dependence but qualitatively similar behavior for temperatures of 125-350 K.

A new emerging trap state is observed at the NaCl/Ag(100) upon cooling to temperatures << 125 K. This state is only observed after long time delays (> 500 fs) and at increased binding energies of 380 meV. Additionally, the emerging trap state shows an even further enhanced lifetime of a few picoseconds. A transition temperature for observing this state is identified as 81 K or 7 meV of thermal energy. Furthermore, the emerging state is shown to arise from electron population transfer from trapped electrons at low coordinated defect sites. In temperatures > 100 K, the lifetime of electrons trapped at low coordinated defect sites decreases as temperature increases due to thermally activated tunneling back to the metal. However, the reverse behavior is observed for the same state for temperatures < 90 K, where cooling results in a decreased lifetime. At this temperature regime, which overlaps with the observed transition temperature, the emerging trap state opens an additional pathway of decay for electrons trapped at low coordinated defect sites, resulting in the observed decreased lifetime with decreasing temperature. The emerging trap state is assigned to small polaron formation via low coordinated defect intermediates. Polaron formation only becomes stable via defect intermediate at two-dimensional interfaces, whereas the electron small polaron is known to be unstable in a bulk perfect NaCl crystal.

Finally, these results are generalized for other alkali halide systems (KCl, NaF, RbCl) on Ag(100) which show qualitatively similar behavior. The above results are also shown for NaCl on Cu(111) and Ag(111). NaCl on Cu(111) shows a clear image potential state progression through the n = 4/5, providing conclusive evidence the initial excited states predominately correspond to the IPS and not the conduction band of NaCl. On both Cu(111) and Ag(111) substrates, the emerging trap state at low temperatures is observed, revealing that this phenomenon is not system specific. In qualitative agreement with polaron formation observed for NaCl on Ag(100), on Ag(111), and on Cu(111), polaron formation results in binding energy increase of ca. 400 meV, an enhanced lifetime of an order of magnitude greater than electrons trapped at low coordinated defect sites, and is only observed at temperatures < 100 K.