Production of Negative Ions

The most widely used method for producing positive ions of alkali, alkaline-earth, and rare-earth metals is their ionization on a heated tungsten surface. The atoms of these metals have a low ionization potential and readily give up one valence electron to tungsten, which has a high work function (4.55 eV).

The author of the paper under review*) made an attempt to apply the described principle of ionization to the production of negative ions of a number of substances.

It was found out that oxygen easily forms negative ions and halogens do so still more readily, the smaller the work function of the material on whose surface ionization occurs.

The work function of an electron from tungsten is large; nevertheless, when a refractory halogen compound is deposited on its surface, for example ZrOCl₂, upon heating in vacuum one can obtain negative halogen ions. The latter may also be obtained by heating a tungsten ribbon in an atmosphere of a gas or vapor containing halogen compounds (HJ, CCl₄, SnJ, AgBr). However, the productivity of such a source in producing negative ions is very low. This is clear from Fig. 1, where the dependence on the emission time of positive and negative ions from heated tungsten powder to which \(5\cdot 10^{-6}\) g of NaCl has been added is shown.

Fig. 1.          Fig. 2.

For creating an effective source of negative ions it is necessary to use a metal with the minimum possible work function. If the work function of this metal is less than the electron affinity of the atom adsorbed on its surface, formation of a negative ion will readily occur.

Unfortunately, metals possessing a small work function for the most part have a low melting temperature, which prevents their use.

Thorium proved to be a suitable metal (possibly also hafnium). The melting temperature of thorium is \(1800^\circ\), and the electron work function is \(3.29\) eV. It is impossible to make foil from thorium onto which the substance under investigation could be applied. Therefore, as the ion source the author used a briquette pressed from thorium powder with a very small admixture of the substance under investigation (\(0.1\%\) NaCl).

*) Helvetica Physica Acta 24, 307 (1951).

In Fig. 2 are shown the results of measurements carried out by the author with indirect heating of such a briquette. An ion source whose design is based on the principle described may be used, for example, for mass-spectrometric analysis of halogens.

At present the author is conducting work on obtaining negative ions of substances of the sulfur group.

L. L.