On the Extraction of Electrons from a Metal Surface by Ions and Metastable Atoms

An elementary theory is developed for the process by which a helium ion or a 2³S metastable helium atom may extract an electron from a metal surface. The helium ion after collision may become a neutral atom although sometimes an excited or even a metastable atom. In the case of the metastable atom the collision involves an exchange of electrons between the metal and the atom, the excess energy being carried away by the ejected L shell electron. The average distance of transition s̄ for He⁺ and for the 2³S metastable helium atom is calculated for different velocities. For v=10⁵ cm/sec., s̄ₘₑₜ=2a₀ and s̄ᵢₒₙ=11.5a₀ where a₀=0.528×0^-8 cm. For v=3×10⁷ cm/sec., s̄ₘₑₜ comes out less than 0.1a₀ and s̄ᵢₒₙ=6.0a₀. These values are different from the values obtained by Massey in a similar computation. The theory accounts for some but not all of the experimental evidence. The probable shape of the potential that the metal surface offers to a metastable atom is found.