Abstract
Electron impact ionization is a common way to provide positive ions for mass spectrometry. For most of molecular vapor species, however, experimental values of their electron impact ionization cross-sections are so limited that theoretic estimations become necessary. It is found that classic model such as additive rule or its modified version is no longer valid near the ionization threshold. Knowledge in this area is important for alkaline metals like sodium because their molecules may reach the maximum cross-section just a few eV above the ionization potential and easily crack in higher incident energies. A Binary-Encounter-Bethe (BEB) model developed by NIST was employed to calculate the cross-sections of NaO, NaOH, Na2, and Na2O molecules. The total ionization cross-section is computed from partial cross-sections of molecular orbitals. Gaussian and GAMESS code were employed to calculate the necessary molecular orbital parameters in the ground state. Calculated cross-sections were plotted as a function of electron impact energy. Reliability of the data predicted by BEB model was investigated by measuring corresponding ionization efficiency curves for some of the molecules. It was found that I(E)/Imax is generally equal to σ(E)/σmax within 15% in the testing range from 5 to 30 eV for sodium-containing molecules. It indicates that the BEB predictions seem correct at low electron incident energies.
