Effect of H atoms and O impurities on the adsorption stability and work function of the cesiated Mo(0 0 1) surface: A study about negative hydrogen ion sources for neutral beam injection systems

The impact of H atoms and O impurities on the adsorption stability and work function of cesiated Mo (001) surface are investigated in this paper through DFT calculations. The 53 surface structures with different surface coverages are constructed in this work. Two types of co-deposition structures are considered: Cs-O (or Cs-H) bonds (1) leaning towards and (2) perpendicular to the surface. Our results demonstrate that the minimum work function corresponds to a 0.5 ML coverage of Cs for any surface coverage of H (or O) when the bonds are tilted. Compared to pure Cs deposition, the work function of Cs and H (or O) co-deposition surface increases at low or saturated Cs coverage, while decreases at 0.5 ML. Analysis of the surface dipole moment revealed that the work function changed inversely with the surface dipole moment density. Comparing charge transfer and polarization, it is evident that charge polarization has a significant impact on dipole moment changes. The presence of O impurities significantly enhances the adsorption stability of Cs, with higher coverage of O impurities leading to greater adsorption stability. The average adsorption energy of Cs atoms increases by ∼0.36 eV when co-deposits with O (0.5 θ). The effect of the H atoms on the adsorption stability of Cs atoms is limited. We also find that the vertical co-deposition case did not offer an advantage in terms of adsorption stability and low work function.