Field Emission through Hydrogen and Helium Layers

Field emission through multilayers of H₂, D₂, HD, and He adsorbed on tungsten field emitters has been studied at 4.2–1.6°K and pressures of 10⁻⁵–10⁻⁷ torr of these gases under ultrahigh‐vacuum conditions. In the case of H₂, HD, or D₂, small work function increases relative to the chemisorbed layer were obtained, indicating a barrier to the solution of free electrons in these layers. In the case of He an apparent decrease in work function was observed for vacuum work functions of 5–3.5 eV (obtained by Ba or O₂ adsorption). The effect is explained in terms of a second‐order resonance transition in which a He electron makes a virtual transition from the ground state to the $\text{2p}$ level, and then tunnels; an electron from the metal then fills the He ground state. In this way horizontal tunneling occurs, but avoids the strong repulsive He potential which would be seen by a metal electron tunneling normally. The mechanism fails when the work function is too low, or when the broadened He $\text{2p}$ level is raised above the Fermi level of the metal. The effect of chemisorbed layers on the physisorption of He on W is shown to be considerable; He physisorption is strongest on oxygen covered, and least on Ba‐covered surfaces.