The excitation of inner electrons in zinc, cadmium and mercury by electron impact

Abstract

Certain characteristic features, which could not be conclusively interpreted, were found in the energy losses of electrons in the vapours of zinc and cadmium by Thorley, in some unpublished work in these laboratories. Zinc, cadmium and mercury form a well-defined triad of metals, occupying analogous positions in Group II of the Periodic Table, and having similar electron configurations. They may therefore be expected to behave similarly in our experiments, and for this reason it was considered desirable to extend the work to mercury vapour. The magnetic spectra of all three elements could then be compared, and additional information might be gained from that of mercury which would lead to a satisfactory explanation of the losses. The experiments were carried out with an improved pattern of magnetic spectrograph of the type described by Whiddington and Roberts (1931). In this method, a beam of electrons from an electron gun is incident on the vapour to be investigated in a field-free collision space. The vapour passes through this collision chamber continuously at a low pressure. Some electrons collide with the vapour atoms, and the now-heterogeneous beam emerges through a slit system into a continuously evacuated camera, where a magnetic field, perpendicular to the beam direction, bends the electrons into semicircular paths whose radii depend on the electron energy. The different beams impinge on a specially treated photographic film, and the magnetic spectrum thus obtained is calibrated by reference to lines of known energy (see figure 1). For the work on zinc and cadmium an oven arrangement was necessary to supply the vapour. Such an oven was designed and constructed specially for use with the apparatus by Robinson (1935); it was made from pyrex and heated electrically. Vapour was discharged from the oven mouth into the collision space and was subsequently condensed, during the taking of a photograph. This device, however, was unnecessary in the mercury investigations since it was found that a small brass tube containing a globule of mercury let into the side of the collision chamber and warmed by the gun filament was sufficient to maintain a working pressure.