1. Laboratory for Chemical Surface Science, Department of Chemistry, Physical Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden; Institute for Surface Chemistry, P.O. Box 5607, S-114 86 Stockholm, Sweden; Kraft Foods Technology Center, 801 Waukegan Road, Glenview, Illinois 60025; and Experimental Surface Physics, Department of Applied Mathematics, Research School of Physical Sciences, Australian National University, Canberra, A.C.T. 0200, Australia