A tantalum ionizer immersed in cesium weapor produces cesium ions. Cesium positive ions are accelerated towards the negatively biased cathode, which then accelerate negative ions always from the cathode. A thin layer of cesium condensed on the cathode surface enhances negative ion production. Not all elements form the stable negative ions, and in these cases it is obvious to use molecular ions.
Molecular beams have two disadvantages in accelerator.
These effects are minimized by choosing the lightest possible molecule, such as hydride. If hydride is not possible, we go for Carbide, Nitride or Oxide.