A new method for studying ionising potentials

It is evident that an exact knowledge of the energy necessary to remove the electrons from a molecule is of great value in formulating definite ideas of atomic structure and in testing the correctness of any quantitative theory. Not only is it desirable to know the work required to detach the outermost valence electron but also the further energy steps corresponding to the removal of more closely bound electrons. Moreover, in the case of polyatomic molecules, it would be most interesting to learn what the actual products of ionisation are, whether it is necessarily accompanied by dissociation and, if so, what is the state of the dissociated components. In ionisation of a gas by the impact of slowly moving electrons we have a method of disrupting molecules under known energy conditions, and therefore turn to experiments of this type for the solution of the problems just propounded. The first important quantitative work was that of Franck and Hertz in 1913 using a method originally due to Lenard. Modifications and improvements in this general mode of attack have enabled subsequent investigators to contribute greatly to knowledge of atomic structure. For a detailed discussion of methods and results reference can be made to Hughes’ “Report on Photo-electricity” or to a recent series of papers by Franck, but for the purpose of the present paper it will be sufficient to state the principal results as briefly as possible.