On the nature of the ionisation of a molecule by an α-particle

No experiments have hitherto been made to determine the source of the energy which appears when ionisation takes place. The energy involved may be accounted for in two ways (1), it may be derived from the potential energy of the atom which is set free by a “trigger” action of the ionising agent, or (2), it may be energy directly absorbed from the ionising agent which is transformed into the energy of the ionised molecule. If the energy of the ionising agent is kinetic energy, which is the case with the α-particle, an experiment can be made to decide whether any of the energy of ionisation can be accounted for on the second hypothesis. If an α-particle is losing energy during its motion, its loss of the second hypothesis. If an α-particle is losing energy during its motion, its loss of momentum must be equal to the gain of momentum of the electrons ejected from molecules by α-particles should have on the whole a motion in the same direction as the α-particles. These considerations led the writer to undertake some experiments on the direction of motion of electrons when ionisation is produced by α-particles.