A Study of the Emission of Negative Ions From Hot Metallic Surfaces

Abstract

The interaction of gas molecules with a hot metal surface may result in the abstraction of an electron from the metal and the formation of a negative ion. The transition state theory of Glasstone, Laidler and Eyring is applied to the problem of the calculation of the emission currents with considerable success. The treatment used is simply applicable to the emission of both ions and electrons and is used in the latter case to interpret changes in the electron currents and thermionic electron work functions of metal surfaces exposed to low pressures of gases which can be strongly adsorbed. The formation of negative ions in sparse surface layers is investigated and comparison of theory and experiment is made for ion formation by substituted benzoquinones and cyanocarbon derivatives. The strong adsorption of tetracyanoethylene is investigated in some detail and the ions are shown to be formed in a secondary adsorbed layer located above the primary adsorbed material. The experimental data is used to derive the electron affinities of the capturing molecules. This quantity is shown to be directly proportional to the number and nature of the substituents in the molecule. Charge-~dipole interactions are assumed to be of importance in charge transfer spectra and a simple electrostatic calculation of the interaction energy of the excited state of the complex allows the energy of the charge transfer absorption band to be calculated.