Bombardment of Metal Surfaces by Slow-Moving Electrons

Secondary Electrons Produced by Electronic Bombardment of Nickel.—The main features of the apparatus used are: an equi-potential nitrate-coated Pt cathode heated by radiation from a tungsten spiral filament; a series of insulated diaphragms to limit the beam of primary rays and for use in determining their velocity; and a long Faraday cylinder in front of which the nickel target could be alternately interposed and withdrawn. By baking out the tube and using liquid air traps an extremely low vacuum corresponding to less than ${10}^{-7\mathrm{}}$ mm. was attained. The secondary current was determined from the difference between the current to the Faraday cylinder and that to the interposed target. (I) Ratio of the secondary (emergent) to the primary (incident) electron current was found to be independent of the roughness of the surface, but to vary with the treatment. After heating the target red-hot by high frequency induction for some minutes, a limiting curve was reached which probably represents the characteristics of nickel itself, free from surface contamination. In this case the secondary electrons begin when the primary velocity is as low as 0.2 volt; the ratio to primary current then increases rapidly with primary velocity to about 4 volts, remains constant to about 9 volts, then again increases reaching a value of unity for a primary velocity corresponding to about 260 volts. The effect of exposure to air or hydrogen is to increase very considerably the secondary current and to round out the flat part of the curve between 4 and 9 volts. (2) Velocity distribution curves of the secondary electrons indicate that for primary electrons of less than 9 volts velocity, most of the secondary electrons have velocities nearly equal to the primary velocity, while for primary velocities above 9 volts, the percentage of secondary electrons having small velocities increases with the primary velocity, although a small proportion have velocities nearly equal to the primary velocity up to at least 110 volts.