X-ray absorption study at the Mg and O K edges of ultrathin MgO epilayers on Ag(001)

We determined the local atomic structure of MgO epilayers on Ag(001) by means of polarization-dependentx-ray absorption spectroscopy measurements at the Mg and O K edges. A quantitative analysis of the data inthe extended energy range has been performed using multiple scattering simulations.We found that, even in theultrathin limit, the local structure of the films is rocksalt and we obtained a quantitative evaluation of theaverage in-plane and out-of-plane film strain at the different thicknesses investigated. An in-plane compressivestrain, due to lattice mismatch with the Ag substrate, is clearly present for the 3 ML film. The out-of-planelattice constant is found to be expanded, in agreement with the expected behavior for a tetragonal distortion ofthe unit cell. This growth-induced strain is gradually released with increasing thickness and it is almostcompletely relaxed at 20 ML. Any significant intermixing with the Ag substrate can be ruled out. An expansionof the interplanar distance at the MgO-Ag interface is detected and its sign and magnitude are found to be inagreement with recent ab initio simulations. This work provides previously unavailable input for modeling thephysical properties of the system and supports the hypothesis that the different electronic properties of MgOfilms on Ag(001) are not related to structural or compositional differences at the ultrathin limit.