A SCANNING-TUNNELING-MICROSCOPY INVESTIGATION OF THE REACTION OF HYDROGEN-ATOMS WITH AG SI(111)-(ROOT-X-ROOT-3/

We report the results of a scanning tunneling microscopy investigation of the reaction of the Ag/Si(111)-(square-root 3 x square-root 3) sur face with atomic hydrogen. The uniform Ag(square-root 3 X square-root 3) surface undergoes substantial rearrangement in the presence of hydr ogen atoms resulting in the formation of small nuclei of Ag. The nucle i have distinct crystallography and are oriented with respect to the u nderlying surface. In addition we observe that the nuclei form only in regions where the Ag(square-root 3 x square-root 3) structure was ori ginally present. The size of the nuclei increases as the initial cover age of Ag(square-root 3 x square-root 3) increases.