A Theory of Electron Impact on Some Polyatomic Molecules

A method by which we can get mass spectral pattern of a polyatomic molecule from the data of appearance potentials of various ions and those of optical measurement has been obtained. It is theoretically expected that the total molecular ionization cross section and relative yields of ions do not so much depend on the incident energy, except the low incident energy region, and that the total cross section does not always rise linearly with the increase of incident energy at a threshold. Applications to CH 4 and CD 4 have been carried out and the results are in plausible agreement with mass-spectroscopic data. In the course of the application, we have taken two ionized states ( 2 A 1 , 2 T 2 ) into account and have assumed that the potential energy surfaces of ionized states are not so different from that of the ground state of the parent molecule with respect to almost all normal modes except one or two. It is also theoretically expected that we have a larger abundance of parent ion in the CD 4 case than in the CH 4 case by isotope effect. The experimental data shows the tendency predicted by the theory.