On the basis of a high-precision reflection high-energy electron diffraction (RHEED) investigation details of the growth of Pd on a Cu(1 1 1) single crystal substrate at room temperature are reported. Because of the +7.6% misfit of the Pd lattice spacing as compared to Cu, perfect pseudomorphous growth would result in highly stressed ultra-thin films. RHEED analysis shows that as a function of Pd coverage initially the film starts to grow with the in-plane Cu(1 1 1) lattice parameter. With increasing coverage the lattice parameter rapidly changes to the "natural" lateral lattice parameter of Pd(1 1 1). We propose that a progressive increase of the equilibrium lateral lattice parameter by alloying (Vegards law) releases the stress in the Pd/Cu(1 1 1) system. With a coverage of n > 2 ML (n = number of monolayers, ML) pure Pd layers are formed, since the in-plane lattice parameter equals the expected value for Pd at n = 2 ML. Our conclusion that Pd-Cu surface alloying acts as a relaxation mechanism in this quasi-pseudomorphous growth system, corroborates scanning tunneling microscopy [Surf. Sci. 408 (1998) 43] which also suggests formation of a random surface alloy in the earlier stages of Pd growth on Cu(1 1 1). (C) 2004 Elsevier B.V. All rights reserved.560416992734