The excitation function of helium

There are two methods of investigating inelastic collisions between atoms and electrons. By measuring the energies of the scattered electrons we obtain at once the probabilities of excitation to the various levels. Small energy differences, however, cannot be resolved electrically and so only the probabilities of excitation to the lower, widely separated levels can be obtained in this way. In the method described in this paper the intensities of the spectral lines emitted by the atoms after collision are measured. It is possible by this means to estimate the absolute probability of excitation to a given state by collision with an electron of known velocity. The probability of excitation to a given state plotted as a function of the velocity of the exciting electrons is known as the excitation function of that excited state.