Electron scattering in mercury vapour

When a beam of homogeneous electrons passes through a gas some of the electrons will collide with gas molecules, and be scattered out of the beam. The collision may be elastic, in which case the electron suffers only a change of direction (the loss of energy is inappreciable); or it may be inelastic, in which case the electron loses some of its energy besides being deflected. Thus if we examines the velocity distribution of the elecrons scattered at a particular angle, θ, two main groups of electrons will be found, the elastic group and the inelastic group. Except in the case of ionisation, the electron can only loss an appreciable amount of energy in discrete amounts corresponding to the excitation potentials of the gas. The inelastic group may therefore the subdivided into a number of groups corresponding to the different excitation potentials of the gas. The relative sixes of these subsidiary groups will depend upon the relative probabilities of excitation of the different states of the gas molecule. the probability of a collision resulting in a high state of excitation of the atom is usually small compared to the probability of a resonance or ionisation collision. Thus the inelastic group resolves itself into two main subsidiary groups; one group consists of the electrons which have ionised the atom, and the other group is composed of electrons which have caused resonance transitions in the atom.