Observation of a ring pattern in a field-emission image of Ag

Field-emission images of clean Ag emitters show a ring-like pattern at the (1 1 1) pole, similar to the one previously observed in our field-emission microscopy of Cu and Au emitters [Ultramicroscopy, in Press]. Comparison of field-emission and field-ion images indicates that the ring pattern appears inside the topmost (1 1 1) layer, not on annular terraces as we found on Au and Cu emitters. We interpret the observed ring pattern as due to the quantum confinement of surface-state electrons into a circular top layer. The emission probability is locally enhanced at a peak of a circular standing wave formed by confined electrons, and this leads to a ring pattern in field-emission images. A simple analysis shows that the confinement condition for electrons at the Fermi level can be satisfied on the top layer but not on the second and subsequent narrower terraces.