Ionization cross-sections and ionization efficiency curves from polarizability volumes and ionization potentials

It has been shown by simple electrostatic considerations and confirmed using literature experimental data that a strong correlation exists between the maximum electron impact ionization cross-section and the square root of the ratio of the atomic or molecular polarizability volume to the ionization potential. The observed shape of the ionization efficiency curves for a wide range of molecules may be rationalized in terms of the interaction of an incoming electron wave with the molecule, which has an effective diameter determined from the polarizability volume. The peak in the curve occurs when the electron wavelength matches this diameter. An expression for the ionization cross-section at any incident electron energy, which is a function only of the molecular polarizability and ionization potential, has been deduced. The results of this treatment are in good agreement with experiment and the predictions are generally at least as good as those of more sophisticated theoretical treatments of the electron impact ionization process. (C) 1997 Elsevier Science B.V.