Liberation of Electrons by Positive-Ion Impact on the Cathode of a Pulsed Townsend Discharge Tube

Abstract

the Ta had a gas film on it. As observed in this study, flashing the Pt target removed most of the active gas films so that the work function of the Pt used was not less than the accepted work functions of clean Pt of value 5.3 volts. 26 In consequence if the value of the work function alone determines ji the y* for Ta should be greater than that for Pt. Actually the reverse is true as noted. Again Hagstrum 11 observed that for He 4 " ions on Mo after the background gas had formed a monolayer on the target yi was lowered by 30 percent while cj> had not changed by more than 0.1 volt.A guess as to the nature of this action comes from Hagstrum's observation that the energy distribution of electrons from the gas-covered surface had more slow electrons than that from the clean surface. This indicates that the probability of the excited electron occupying a low level in the range of levels available to it is greater for the gas covered surface than for the clean one. Such a condition would obviously also lower yi. The process active may be crudely envisioned as follows: with a clean surface neutralization involves direct interaction with only one electron of the Fermi band. With a covered surface it could well involve two for the electrons causing neutralization can come from a local state produced by an adsorbed gas atom.In general since such a local level does not lie in the Fermi band it will be refilled after neutralization by a metallic electron. Such a two-step process could well act to lower the energy available to the excited electron.Thus gas coatings not only lower yi by raising <j>, 26 H. B. Michaelson, J. Appl. Phys. 21, 536 (1950).but also lower yi by reducing the available energy for excitation by a more complicated neutralization process. If the lowering of <j>, for instance by H2 treatment, is small the value of yi can still be decreased by H2 films because of reduction of available escape energy. Likewise, clean Pt could yield a higher y* despite its 0.4-volt higher value of <f> relatively to Ta, as Ta has a gas film at its surface.That yi increases with ion energy is not surprising since kinetic liberation superposes on potential for all surfaces. For gas treated surfaces the observed increase of yi with energy is greater than for clean surfaces. This is to be expected for aside from some "cleanup" of the film by high-energy bombardment more of the high-energy ions penetrate the gas film and interact directly with metallic electrons; i.e., the interaction time with the local gas surface states is decreased relative to that with the lattice.The initial decrease in y»-with ion energy, observed only with nitrogen ions, can be accounted for by the very high energy of association of N atoms to form N 2 molecules. Thus in addition to the neutralization energy of N + ions these can react at the surface to yield N2 at 9.6 ev and this energy may be available for electron liberation. Such a reaction may take place for slowly-moving low-energy ions and may decrease as ion energy increases.In conclusion the writ...

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