Atomic structure of the surface alloy formed by a room-temperature-deposited Mn monolayer on Ag(001)

The determination of the atomic structure of a Mn monolayer deposited at room temperature on Ag(001) was achieved by using surface extended x-ray absorption fine structure. Keeping in mind the previous results of x-ray photoelectron diffraction, several structural models involving the two topmost layers of the Ag substrate were tested in ab initio polarization-dependent XAFS calculations. Among these models, the most consistent one with the experimental data is made of a mixture of two different local environments that correspond to two different structures: first an inverted layer of Mn atoms substituting Ag ones underneath the topmost plane in fcc Ag; second a ${\mathrm{Ag}}_{0.5}{\mathrm{Mn}}_{0.5}$ surface alloy confined to the two topmost layers of Ag substrate. This latter arrangement corresponds to the observed $c(2\ifmmode\times\else\texttimes\fi{}2)$ low-energy electron diffraction long-range order. Accurate values for the Mn-Ag and Mn-Mn first interatomic distances were derived from standard analysis and found to be $2.86\ifmmode\pm\else\textpm\fi{}0.02$ and $2.88\ifmmode\pm\else\textpm\fi{}0.02\AA{},$ respectively. This definitely establishes that, in this surface alloy, Mn adopts an unusually large atomic volume, essentially the same as Ag. This finding is consistent with a high spin state of the Mn similar to the one in dilute Ag-based alloys or in the atomic ${}^{6}{S}_{5/2}$ ground state.