In this paper two semi-empirical models are presented. By the one model the electron impact ionization cross sections of polyatomic molecules can be estimated for electron energies in the range from 15 to 150 eV. Using the other model the photoionization cross section of polyatomic molecules is determined at photon energies of 11.8 and 16.7 eV. In the case of electron impact, the model reproduces the molecular ionization cross sections within 20% at electron energies above 50 eV. The photoionization cross sections calculated with the model deviate not more than 20% from the experimentally determined values for all polyatomic molecules tested.

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Journal Chem. Phys.
Bobeldijk, M, van der Zande, W. J, & Kistemaker, P. G. (1994). Simple models for the calculation of photoionization and electron impact ionization cross sections of polyatomic molecules. Chem. Phys., 179, 125–130.