Semiclassical Approach to Computing Vibrationally Resolved Ionization Cross Sections for Molecular Nitrogen

A semiclassical model based on the Gryzinski theory is used to compute vibrationally resolved electron-impact ionization cross sections for molecular nitrogen. This model extends the approach used by Wünderlich for molecular hydrogen and its isotopomeres. The multireference configuration interaction (MRCI) method in Molpro is used with complete active space self-consistent field reference wave functions to compute potential energy curves (PECs) and electronic wave functions for several states of interest. Nuclear wave functions and vibrational energy levels are computed from the MRCI PECs using the Fourier grid Hamiltonian method. The target orbital electron kinetic energies, Franck-Condon factors, and transition energies are parameters for the semiclassical model and are calculated directly from the computed electronic and nuclear wave functions and vibrational energy levels. The target electron kinetic energies are computed as the expectation value of the one-electron kinetic energy operator for the product of the appropriate nuclear vibrational wave function and the MRCI natural orbital for a particular state-to-state ionization process. From the fully vibrationally resolved ionization cross sections, lumped and total cross sections are calculated by summing the partial cross sections over the closure relationship of the Franck-Condon theory.