Ionization of N 2 and CO by 10 keV electrons as a function of the energy loss : I. Valence electrons

Abstract

The small-angle inelastic scattering of 10 keV electrons in N 2 and CO has been measured in coincidence with the ions and fragments formed. Continuous oscillator-strength spectra were obtained for the production of N + 2, N +(N ++ 2), CO +, C +, O + and CO ++, via outer-shell excitations. Besides the onset of excitation to the X, A and B electronic states of N + 2 and CO + (which is observed as an increase in the oscillator strength), the N + 2 spectrum shows that a considerable fraction of the oscillator strength goes to double electron transitions, i.e. to auto-ionizing Rydberg states converging to the C state. The formation of N + ions at the lowest threshold for dissociation is discussed with regard to earlier suggestions in literature. In the case of N 2, the mass-14 spectrum ( i.e. N + and N ++ 2) clearly shows the contributions of N ++ 2 formation at energy losses above 42 eV, while the structure in this range correponds to onset of ionization to N ++ 2 levels known from Auger-electron spectroscopy. The measurements of CO show that most of the doubly charged ions dissociate to C + and O + with lifetimes shorter than about 10 −8s. Comparison with optical measurements and electron-impact data shows the contribution to the total oscillator strength of fragment ions with excess energy of formation.