Effect of oxygen or copper impurities on the work function of cesiated surfaces under co-existence with hydrogen: A study about negative hydrogen ion sources for neutral beam injection systems

This study investigates the effect of oxygen and copper impurities on the work function of the Cs/H/Mo (001) surface based on the Density Functional Theory (DFT) method. First, the deposition of 2/16 θ impurities on the Cs_0.5/H_0.5/Mo (001) surface (the subscript 0.5 represents 8/16 θ coverage) is studied. It is found that the deposition of impurities beneath the cesium layer and oxygen impurities as atoms, and in a high-symmetry configuration, both lower the surface system energy. As the impurity coverage increases, it is observed that impurity deposition with a coverage of less than or equal to 8/16 θ effectively reduces the work function of the Cs_0.5/H_0.5/Mo (001) surface. However, as the cesium and hydrogen coverage increases, the impurities are no longer effective at reducing the work function. Moreover, when the cesium coverage reaches 16/16 θ, the surface work function approaches that of a cesium metal surface. Another important discovery is that when oxygen impurities are deposited above the cesium layer on the Cs_0.5/H_0.5/Mo (001) surface, a reduction in the surface work function is observed. This anomalous phenomenon is attributed to significant charge polarization caused by the deposition of oxygen impurities. The migration barriers of impurities traversing the dense cesium layer to the Mo (001) substrate are evaluated using the climbing image nudged elastic band (CI-NEB) method. Oxygen impurities tend to traverse the cesium layer in molecular form, facing a minimum barrier of 0.52 eV, while copper atoms encounter a lower barrier of 0.09 eV when traversing the cesium layer.