Dynamics of surface alloying: Determination of diffusion barriers from photoelectron spectra

We show that photoemission observations of the time evolution of the Pd/Ag(100) valence spectrum for different temperatures allow the extraction of diffusion barriers in surface alloy formation. Adatom migration takes place in distinct steps. The first is based on an exchange mechanism, which occurs at about T=135 K with an activation energy of ${\mathit{E}}_{\mathit{d}1}$=0.43(5) eV, while migration from a surface layer to a subsurface layer is observed at T≳360 K with an activation energy ${\mathit{E}}_{\mathit{d}2}$=1.12(4) eV. © 1996 The American Physical Society.