REAL-SPACE IMAGING OF ELECTRON-SCATTERING PHENOMENA AT METAL-SURFACES

Real space studies of the interaction of the two-dimensional electron gas provided by metal surface states with localized scatterers are pre sented. The results involve electron scattering by steps and point def ects (adsorbates) at Au(111) and Ag(111) surfaces. These scattering ev ents lead, through interference, to an oscillatory local density of st ates (LDOS), which is imaged in maps of (dI/dV)/(I/V). (I/V). Analysis of the LDOS oscillations provides insights into the scattering phenom ena involved. We show that the decay of the amplitude of the oscillati ons as a function of distance from the scatterer can be accounted for by a model that describes the loss of coherence as a result of the wav e number (k(parallel-to)) spread of the states probed by the STM. This model also explains the energy dependence of the amplitude of the osc illations and provides a basis for comparing results from different me tal surfaces. Analysis of the properties of the oscillations shows tha t at low k(parallel-to), steps act very much like hard walls isolating the surface states of different terraces. At high k(parallel-to), how ever, the barriers become permeable. The circular LDOS waves that surr ound individual adsorbates on the terraces can be used to obtain infor mation on their nature and charge state. Finally, we show that when th e size of a terrace is decreased to a few coherence lengths, as in the case of small metal islands, quantum size effects appear. State densi ties of confined (''particle in a box'') states are revealed in the sp ectroscopic images.