Core-level subsurface shifted component in a 4d transition metal: Ru(10(1)over-bar-0)

Three components have been found in the Ru(10 (1) over bar 0) 3d(5/2) core- level spectra. Based on a photoelectron diffraction experiment they have be en assigned to emission from the bulk and the first two surface layers. The first layer line is shifted by -480 meV from the bulk peak position and th e second layer line by -240 meV. The relative magnitude of the observed shi fts is in good agreement with a simple tight-binding model. The same model would, however, predict clearly observable shifts for other open 4d transit ion-metal surfaces, where only one shifted component has been reported. Pos sible reasons for the seemingly anomalous behavior of Ru are discussed.