Cross section for single and double ionization of N2 and O2 molecules by electron impact from threshold up to 170 eV

Abstract

Absolute ionization cross sections have been measured for the processes N2+e→N2++2e, N2+e→N2+++3e, O2+e→O2++2e, and O2+e→O2+++3e as a function of electron energy up to 170 eV in a crossed molecule–electron beam apparatus. The ionization cross section curve for producing the singly charged parent ions N2+ and O2+ is found to have at least one inflection of the type first observed by Lawrence for mercury. Using nth root extrapolation the following minimum ionization potentials were derived from the low energy cross section behavior: N2+(X 2Σg+) =15.6±0.1 eV, N2++(3Πu) =42.9±0.3 eV, O2+(X 2Πg) =12.1±0.1 eV, and O2++(X 1Σg+) =35.6±0.3 eV. From distinct breaks in the single ionization cross section curve the following excited states were observed; N2+(B 2Σ+u) =18.8±0.2 eV, O2+(a 4Πu) =16.0±0.2 eV, and O+2(2Σ−g) =21.0±0.2 eV. In the double ionization cases, a square law was found from threshold up to some 20 eV above threshold. All results are compared with previous measurements if available.