Electron-Impact Ionization Cross Sections of Molecules Containing Heavy Elements (Z > 10).

The binary-encounter-Bethe (BEB) theory has been successful for computing electron-impact ionization cross sections of many molecules. For molecules that contain heavy atoms (defined here as atoms with valence principal quantum number n > 2), there are two alternative BEB procedures in the literature. The first involves a kinetic-energy correction for molecular orbitals that are dominated by atomic orbitals with n > 2. The second alternative is to use effective core potentials (ECPs), which were developed for other purposes but yield valence pseudo-orbitals with reduced kinetic energies. In the present study, the results of these two approaches are compared with experimental cross sections for several molecules containing heavy elements. Although both procedures perform well, the ECP results agree somewhat better with experimental measurements. Cross sections are presented for C2Cl6, C2HCl5, C2Cl4, both isomers of C2H2Cl4, CCl4, TiCl4, CBr4, CHBr3, CH2Br2, P2, P4, As2, As4, GaCl, CS2, H2S, CH3I, Al(CH3)3, Ga(CH3)3, hexamethyldisiloxane, and Zn(C2H5)2. Incorrect BEB calculations have been reported in the literature for several of these molecules. As an ancillary result, the dipole polarizability of Zn(C2H5)2 is predicted to be 12.1 Å(3).