Evidence of superoxide-like CsO2 formation on a cesiated model surface

The investigation of light atoms and molecules interaction on cesiated surfaces attracted remarkable attention during the last years, mainly due to its application on tuning the operating conditions of negative ion sources for fusion devices. The commonly suboptimal vacuum level in such facilities makes the process prone to contamination by impurities and, particularly, by oxygen. In the present study, we investigate the interaction of molecular oxygen on a surface slab of molybdenum with medium Cs coverage. Plane Wave (PW) based DFT and ab initio molecular dynamics calculations on this system revealed strong chemisorption of oxygen species toward Cs atoms on the surface. The binding energies for molecular oxygen calculated at DFT level, range between 0.5 eV and 2.0 eV, depending on the impact site on the surface and interaction orientation, thus confirming a strong bound state at surface. The adsorbed O molecule was further stabilized over the slab by constrained and free first-neighbours PW molecular dynamics of the oxygen molecule on the proximity of Cs atoms. As a result, a stable caesium oxide structure was found on the surface, and, from an accurate multi-property analysis, we have found that its geometrical and electronic features strongly resemble those typical of a CsO superoxide compound.