Contemporary Advances in Physics, XX, Ionization of Gases by Light
01 April 1930
HE subject of this article is more narrowly restricted than those of m a n y others of the series. It is narrower even than the title might imply; for by " i o n i z a t i o n " I mean for the present only the detachment of the most loosely bound electron of a molecule or an atom, and by " l i g h t " only the waves of the visible spectrum and that adjoining range of wavelengths to which the name of ultraviolet is customarily confined. Either of these limitations is implicit in the other; for though most molecules are fashioned with electrons bound with varying degrees of tightness, and the removal of any one thereof is an act of ionization, it is beyond the power of such light-waves to abstract any except the loosest. Perhaps it will be found instructive if for so definite and circumscribed a problem I relate the methods of experiment, the data of the experiments, the simple theory, and the artifices which have been conceived to reconcile the theory and the data, sometimes with success and at other times in vain. Most of the really valuable data are of recent acquisition, for there are difficulties hampering the attack upon the problem, which the progress of laboratory technique is only gradually clearing away. Consider, for instance, the question of providing the light. It is desirable to be able to illuminate the gas with monochromatic light of any wavelength, photons of any energy. W h e n ionization by electrons is being studied, one varies the energy and the wavelength at will by varying the voltage impressed on the electrons.