The results of the various investigations which have been carried out during the last few years on the critical potentials for the excitation of soft X-rays, and for the production of secondary electrons, from solids, have shown that the effects occurring at solid surfaces under electronic bombardment
in vacuo
are more complex than was anticipated when this line of investigation was begun, and that they cannot be interpreted in any simple way in terms of the displacements of electrons within the atoms of the target. The work of various investigators* on the distribution of velocities among the electrons leaving a surface subjected to bombardment by primary electrons of known energy, has shown that a certain number of the electrons leaving the bombarded surface have energies practically equal to that of the primary stream, suggesting that a readily detectable proportion of the primary electrons is scattered or reflected at the target surface without appreciable loss of energy. The proportion of such electrons is greatest for small bombarding energies,
e.g
., about 10 volts, and decreases as the voltage accelerating the primary electrons increases. The other marked feature in the velocity distribution curves, for bombarding voltages up to about 1000, is a group having a sharp maximum at about 10 volts. Apart from these features the distribution is a more or less continuous one, the number of electrons having a given velocity increasing as that velocity increases, except that after achieving a small maximum at about 25 volts less than the primary voltage, the curve falls to a minimum before rising to the very sharp peak indicating true reflection There are no indications of maxima for electron energies differing from the primary by amounts corresponding to those required to effect characteristic electron transitions within the atoms of the target. Moreover, there appears to be nothing in the velocity distribution curves for the secondary emission to correspond to the discontinuities which have been found by various investigators to occur in the current-voltage curves of the secondary electron current from a bombarded surface, or in the current-voltage curves of the photoelectric effect of the soft X-radiation excited by the bombardment. As regards the latter effect an explanation is to hand on the view that the proportion of the primary electrons whose energy is converted, in part, to photoelectrically active radiation is so small that indications of the various different energy transfers suggested by the critical potential curves are swamped in the velocity distribution curves of the secondary electrons. It is, however, more difficult to reconcile the absence of any correlation between the discontinuities which have been observed in the current-voltage curves for secondary electron emission, and the velocity distribution of the latter.
The efficiency of secondary electron emission
