Formation of negative ions at metal surfaces

In a previous paper (Massey and Smith 1936) a theoretical treatment of various processes of formation of negative ions in gases has been given. The recent experimental work of Arnot (1936, 1937) has, however, shown the importance of formation taking place at the surface of metal electrodes. The purpose of this paper is to give a theoretical treatment of the formation of negative ions at metal surfaces and to indicate where further experimental research is necessary in order to understand more fully the processes taking place. When a neutral atom captures an electron to form a negative ion, excess energy equal to the sum of the initial kinetic energy of the electron and the electron affinity of the atom must be disposed of. The method of disposal largely determines the probability with which a process will take place. These processes may be divided into two classes, radiative and non-radiative. In the former the excess energy is given off as radiation.