Electronic Bombardment of Metal Surfaces

Secondary electrons from Cu, Ag, Au, W, Pt, Pd, Mg, and Al, produced by electronic bombardment, 0 to 250 volts.—The apparatus used to study the first six metals has been described in a previous ${\mathrm{paper}.\mathrm{}}^1$ The last two metals and especially copper were studied with a modified apparatus constructed out of molybdenum so as to eliminate magnetic effects and permit the study of surfaces condensed from the vapor. (1) Curves are given showing the ratio of secondary (emergent) to primary (incident) electron current as a function of the primary velocity. These curves depend on the previous heat treatment of the metals, but after heating the higher melting point metals red-hot for some time, limiting curves were obtained which are characteristic of the different metals. Each curve (except for Cu) rises sharply from the origin but flattens out at a primary velocity of from 3 to 11 volts then rises more slowly showing perhaps a second tendency to flatten. Au and Ag also show a slight kink at 3 volts. The curve for Cu has two sharp maxima at 3 and 6.5 volts, respectively, with two other less distinct ones at about 14 and 20 volts. The secondary electron currents from Mg and Al were extraordinarily large in comparison with those of the other metals, the ratio to primary current for Al reaching a value of 1.8 at 140 volts. (2) Velocity distribution curves of the secondary electrons show that, in general, for low primary velocities, most of the secondary electrons have velocities nearly equal to the primary velocity. For higher primary velocities the percentage of secondary electrons having low velocities increases with the primary velocity. The primary velocity for which low velocity electrons begin to leave the surface varies somewhat with the metal but lies between 9 and 15 volts for the metals tested. Curves obtained with a special apparatus consisting of a conducting sphere at whose center the target was placed, show slight kinks indicating inelastic collisions in copper at 3 and 6.5 volts velocity, in agreement with the above maxima, but these results are somewhat doubtful. (3) Variation with the previous treatment of the copper surface. The results of various experiments show (a) that the removal of gas from the metal decreases the secondary current but does not change the form of the secondary electron curve, and (b) that the characteristic secondary electron curve with various maxima was obtained only from a crystalline surface after any amorphous layer present had been removed, and, within limits, was little affected by exposure to air. The effect of adding an evaporated film of copper was to decrease the secondary current very considerably and to cause the characteristic maxima to disappear. (4) Effect of simultaneous exposure to radiation from a quartz mercury arc was merely to add the photo-electric effect to that due to the bombardment.