THE NEGATIVE IONS OF ATOMIC AND MOLECULAR OXYGEN

Journal/Book: PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY OF LONDON Series A. Mathematical and Physical Sciences No. 806 Vol. 239 pp. 269-304 2 April 1943 Printed and published for the Royal Society By the Cambridge University Press Bentley House N. W .1;. 1943;

Abstract: University College London SUMMARY OF RESULTS AND CONCLUSIONS The more important of these are as follows 1. Experimental evidence of the existence of a stable excited state of O- of very small binding energy has been obtained by Lozier and by Hagstrum and Tate. 2. The lowest excited state must belong either to a (2s)2 (2p)4 3s or (2s) (2p)6 configuration. 3. It is probable an empirical grounds that the (2s) (2p)6 lies above the (2s)2 (2p)4 3s configuration. 4. According to the Hartree-Fock self consistent field method (including exchange) a value of the polarizability of atomic oxygen two to four times greater than that derived from the observed molar refractivity of O2 is necessary to provide even a stable (2s)2 (2p)4 3s 4P term of O-. 5. In view of the conflict between the theoretical results (3) and (4) and the experimental evidence (1) it is not yet possible to decide what is the sign and magnitude of the binding energy of the 3s electron in O-. 6. The rates of radiative attachment of electrons to atomic oxygen and of photodetachment of electrons from O- are both very sensitive to the value of the binding energy of the 3s electron if it is small in absolute magnitude. 7. In view of (5) and (6) these rates have been calculated for a range of assumed binding energies of the 3s electron these being specified by the introduction of an effective polarizability of atomic oxygen just sufficient to give a 3s level of the particular energy (according to the Hartree-Fock method). 8. To facilitate application mean rates have been derived for a Maxwellian distribution of electrons and for black-body radiation. 9. The rate of attachment of electrons with Maxwellian velocity distribution to O by a dielectronic process analogous to inverse auto-ionization involving a virtual level of the (2s) (2p)6 configuration is only likely to be of comparable importance to that by radiative attachment if the energy of the virtual level is much less than 0.25 eV above that of the ground state of O-. The same conclusion applies to the inverse (detachment) process associated with black-body radiation. 10. Attachment of electrons to O by three-body processes is unlikely to be important compared with radiative attachment at pressures less than 0.1 mm. Hg. 11. The effective cross-section for associative detachment in atomic oxygen viz. O + O- ? O2 + e is likely to be of the order 10-20-10-21 cm2. 12. The experimental evidence an attachment from electron swarms in O2 indicates the existence of two processes one operative at very low energies the other setting in at a mean energy of 1.5 eV. 13. Reasons are given why it is unlikely that slow electrons produced by inelastic collisions are responsible for the observed attachment at the higher mean energies. 14. Difficulties of detail associated with the current Bloch-Bradbury interpretation of the low-energy attachment are discussed. 15. Possible alternative interpretations of the observations in terms of molecular Potential energy curves for O2 are derived and a difficulty common to all is revealed. 16. The electron affinity of O2 is probably about 1 eV. 17. The distribution of Potential-energy curves for O-2 is such that any process of attachment of thermal electrons to oxygen molecules with probability independent of pressure at low pressure apart from direct radiative attachment is unlikely to occur. 18. The relative magnitude of the rate of radiative attachment of electrons of O2 and of photodetachment from O2 can depend very markedly an the relative position of the potential-energy curves for the ground electronic states of O2 and O-2 . 19. The rate of production of pairs of O+ and O- ions by impact of electrons or of quanta an O2 molecules is discussed the excitation energy required being between 19 and 21 eV. 20. Mutual neutralization of O+ and O- ions can occur by transfer of an electron on impact. For mutual neutralization of thermal ions the effective cross-section is probably between 10-13 and 10-12 cm2 but is unlikely to be as high as 10-11 cm.2. ___MH

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