Ionisation potentials of mercury, cadmium, and zinc, and the single- and many-lined spectra of these elements

Abstract

Abstract In a paper by Frank and Hertz in the ‘Physikalische Zeitschrift,’ these investigators have shown that the minimum energy required to ionise an atom of mercury is that acquired by an electron in passing through a fall of potential of 4·9 volts. These writers have also shown in a later communication that when heated mercury vapour is traversed by electrons possessing energy slightly above this amount the vapour is stimulated to the emission of the single spectral line λ = 2536·72 Å. U. This result constitutes a new and most interesting application of the quantum theory, for it will be seen that in the relation Ve = hv, where h = 6·6 x 10-27 erg sec. 4·9 volts is the potential fall which corresponds to the frequency v of the line λ = 2636·72 Å. U. If the relation just pointed out be applicable generally to all the elements it follows that if the vapour of an element can be shown to be capable of exhibiting a single-line spectrum, the frequency of this single spectral line may be used to deduce the minimum amount of energy required to ionise the atoms of that element. With the object of establishing such a generalisation, if possible, some experiments were recently made by the writers, and it has been found that the vapours of cadmium and zinc as well as that of mercury can be stimulated to the emission of single-line spectra when traversed by electrons possessing the requisite amount of energy. With cadmium vapour the wave-length of the line constituting this single-line spectrum is λ = 3260·17 Å. U., while that of the single-line spectrum of zinc vapour is λ = 3075·99 Å. U. By the quantum theory it follows then that the minimum ionising potential for cadmium and zinc vapours are respectively 3·74 volts and 3·96 volts.