Rh atom ejection from keV ion-bombarded p(2×2)O/Rh{111}: Adsorption site and coverage determination from angle-resolved desorption measurements

Abstract

Angular distributions of Rh atoms desorbed by energetic ion bombardment of an oxygen covered Rh{111} surface are measured accurately using a multiphoton resonance ionization (MPRI) detection technique. The results, in conjunction with molecular dynamics calculations of the ion impact event show that these distributions reflect the near-surface crystal structure. The molecular dynamics calculations were performed using a many-body embedded-atom potential to describe the dynamics of the Rh atoms and a pair-wise additive potential to describe the oxygen–Rh interactions. Several oxygen overlayer structures were considered for molecular dynamics modeling of the desorption process, including p(2×2) overlayers with a coverage of 0.25 monolayer (ML), and p(2×1) overlayers with a coverage of 0.50 ML, both of which are consistent with low energy electron diffraction (LEED) data. Three different adsorption sites were tested: threefold symmetric sites over second layer Rh atoms, threefold symmetric sites over third layer Rh atoms, and atop sites. The calculated azimuthal angular distributions of desorbed Rh atoms for each of these cases are unique, matching the experimental data best in the case of a p(2×1) overlayer with oxygen atoms adsorbed in threefold symmetric sites over third layer Rh atoms. The calculated Rh atom desorption yield (ejected atoms per incident ion) is sensitive to the oxygen coverage in the range 0.25–0.50 ML. These calculations are important in developing a surface bonding site and coverage consistent with LEED and our experiments. The peak in energy distribution of ejected Rh atoms from the oxygen covered surface is at a lower energy value than that of the clean metal. This indicates that collisional energy loss processes contribute to determining the peak position as well as the well known binding energy effect.