Negative atomic ions

Abstract

In a previous paper we have applied quantum-theoretical considerations to the passage of positive ions through gases, but in certain circumstances, as for example the electric discharge in iodine vapour, negative ions are of importance. Very little has been done in discussing theoretically the behaviour of such ions, and in fact there has also been little experimental investigation of their properties. It is therefore of interest to discuss, by quantum-mechanical methods, the formation and neutralization of negative ions. Results obtained in this way should indicate the most profitable directions for further experimental research and assist in the interpretation of observations because of the close analogy presented by the attachment of electrons to neutral atoms to the capture of neutrons by atomic nuclei. Throughout the work of this paper we restrict ourselves to the consideration of atomic ions such as Cl - , O - , H - , Before considering the formation of these ions by various processes it is desirable to discuss in general terms how a negative processes it is to be regarded from the quantum standpoint. The existence of a stable negative ion ( i.e ., an atom of positive electron affinity) implies that there is a stationary state of negative energy for the attached electron in the average field of the atomic nucleus and the remaining electrons. This is itself is not remarkable, for a neutral atom exerts an attractive force on an electron and the existence of negative ions might be anticipated. However, the number of stationary states for attached electrons is limited and not infinite, owing to the short range of the effective attractive field in which the electron moves. The operation of this effect combined with the Pauli principle, which further limits the possible states into which an electron can be attached, results in the exclusion of a number of atoms from the list of those which form stable negative ions. For example the ion He - is not known to exist in stable form. The helium effective atomic field is sufficiently strong to give rise to a 1 s electron state of He - but no excited states. Such a state would involve 3 electrons in the same s level, and so would violate the Pauli principle. The third electron is thus left without a vacant state into which to go. Similar considerations apply to the other inert gases. For the halogen atoms, however, a vacant state exists in the incomplete outer shell. Thus an electron can be attached in a 3 p state to form C l , But, since Ne - is unknown, it is very unlikely that a second excited stationary state pf Cl - exists, for an electron in such a state would move in an effective field very similar to that in which an electron in Ne - would move (both in 3 d or 4 s states).