Study of Negative Ion Formation by Electron Impact II. C- and O- Ion Formations from Carbon Monoxide

Abstract

The negative ions C- and O- produced in the dissociative resonance electron capture(DREC)and ionpair formation(IPF)from carbon monoxide have been measured by means of the RPD method using a single focusing mass spectrometer. In the DREC, the cross section of the C- ion formation is2×10-22 cm2 at the peak maximum, which is about 10-3 times for that of theO- ion formation. Within the resolution of this method, the electron energy dependence of the cross section of the C- ion formation near the threshold, as well as that of theO- ion, shows probably vertical onset, and the threshold energy of the C- ion is0.34±0.05eV higher than that of theO- ion. The shapes of the energy dependence suggest that the potential energy curves of the compound states cross to the respective dissociation limit in the Franck-Condon region, therefore, these ions have no kinetic energy at their thresholds. Consquently, the difference of the threshold energies means the difference of the electron affinities. From this point of view, the electron affinity of the C atom is estimated to be 1.22±0.05eV, taking1.465±0.005eV for the electron affinity of the oxygen atom. The potential energy curves of the compound molecular-negative ion states of CO are also discussed. The energy dependence of the cross sections of the C- and O- ion formations in the IPF processes has also been measured. The step functional curves involved one and two excited levels were observed for C- and O- ions, respectively. The results were also confirmed by using CO containing 56% of 13C and the same results were obtained, and any isotope effect neither for cross section nor for threshold energy has been observed within the experimental accuracy.