The evolution of Si 2p core-level photoemission during a structural conversion from the Si (111)-root3 x root3-Ag to the Si(111)-root21 x root21-Ag superstructures induced by Ag adatoms adsorption at 140 K was studied using synchrotron radiation. The component from the top-layer Si-trimer atoms on the former surface was found to split into two components in the latter surface. The result is discussed in terms of a relaxation in some of the Si trimers induced by Ag adatoms sitting on the nearby Ag triangles of the root3 x root3-Ag substrate. The intensity ratio between the split components is a key to exclude some structure models proposed so far for the root21 x root21 phases. (C) 2002 Elsevier Science B.V. All rights reserved.