The passage of positive ions through gases

A large number of investigations have been carried out on the motion of charged particles through gases, and the subject has received added impetus from the introduction of quantum mechanics, as this theory is capable of dealing with collision phenomena in a manner that was beyond the reach of Bohr’s quantum theory. In the early days of the wave theory, many experiments were devised with a view to testing the validity of the theory; but now the foundations of quantum mechanics have been so firmly laid that we may use the theory to clarify the more complicated phenomena observed in the passage of charged particles through gases. Both from the experimental and theoretical point of view the most convenient particles to study are electrons; and, as a consequence, the phenomena observed in the collisions of electrons with gas molecules are well known and to a large extent understood. Well-marked diffraction phenomena have been observed and the relative probabilities of elastic and inelastic collisions measured for a number of gases. Although detailed theoretical explanations have not always been given, it is possible in nearly all cases to give a general description of the main processes contributing to the observed effects. For example, the Ramsauer effect is now completely explained.