Step contrast reversal in LEEM during Pb deposition on W(110).

Anomalous step contrast in low energy electron microscopy (LEEM) images observed during Pb deposition on a W(110) surface is discussed. The steps are dark on the clean surface, and become bright by Pb deposition at about 200 °C. The contrast reversal is related to the presence of a two-dimensional (2D) Pb gas on the surface and its atomic density distribution. Upon further deposition the steps become dark again and show an anomalous intensity profile. This change is attributed to the 2D crystallization process.