The negative ions of atomic and molecular oxygen

Abstract

The properties, modes of formation and of destruction of the negative ions of atomic and molecular oxygen are examined in detail, using quantal theory to interpret and amplify the somewhat meagre experimental information. A detailed examination of the (Lf)2 (2j)2 (2/>)4 (3s) excited configuration of O - is made in an attempt to decide whether it can give rise to the observed stable excited state in which the attached electron has nearly zero binding energy. This is important in attachment, detachment and electron scattering phenomena as resonance effects will occur if the configuration is on the verge of stability or instability. The Hartree-Fock equations have been solved for the deepest (4P and 2P) terms of this configuration, polarization effects being allowed for by the introduction of a term involving a polarizability p regarded as an adjustable parameter. Stable excited P terms are only found when p is two to four times as large as the polarizability of O deduced from the refractivity of 0 2. This does not completely exclude identification of the excited state as belonging to the configuration considered. To examine the possible resonance effects, radiative attachment and detachment rates are calculated for a variety of values of the polarizability parameter p. The rapid variation of these quantities with p in the region where a real or virtual level of the 3^ electron, with small energy, exists makes it unlikely that definite theoretical values can be given until more information as to the proper value of p is forthcoming. Meanwhile, the parameter p provides a convenient correlation of the probabilities of the two processes with the energy of the 3* electron. The other possible attachment and detachment processes involving O and 0 ~ are also discussed. In order to interpret experiments on attachments of electron swarms in 0 2 and to decide how to extrapolate the results to low pressures, the deep electronic states of O^" are considered in detail, employing the empirical methods commonly used in studying molecular structure. It is found that their distribution is such as to make it most unlikely that Ofl~ ions can be formed with appreciable probability by attachment of slow electrons to Oz at low pressures, by a pressure-independent process other than direct radiative attachment. However, considerable difficulties and uncertainties are found in attempting a detailed interpretation of the experimental results at the higher pressures and more experiments are required. In the final section the formation of pairs of oppositely charged ions from molecules by impact of electrons or light quanta is investigated in terms of the theory of the crossing of molecular potentialenergy curves. The same theory is also applied to obtain information as to the possible magnitude of the cross-section for mutual neutralization of oppositely charged ions by electron transfer on impact. It is shown that a cross-section of between 10~13 and 10-12 cm.2 is quite likely to occur for atomic oxygen ions, but the occurrence of one as high as 1CH1 cm.2 is most unlikely. A detailed summary of results and conclusions is given.