Angular dependence of strong field ionization of N2 by time-dependent configuration interaction using density functional theory and the Tamm-Dancoff approximation

Abstract

The ionization of N2 serves as an important test case for computational methods for strong field ionization. Because Koopmans’s theorem fails for Hartree-Fock calculations of N2, corrections for electron correlation are needed to obtain the proper ordering of ionization energies of N2. Lopata and co-workers found that real-time integration of time-dependent Hartree-Fock (rt-TD-HF) gave a ratio for strong field ionization parallel and perpendicular to the molecular axis that was too small compared to experiment, but real-time integration of time-dependent density functional theory (rt-TD-DFT) with an appropriately tuned long-range corrected functional, lc-ωPBE*, was in good agreement with experiment. The present study finds that time-dependent configuration interaction (TDCI) with single excitations based on a Hartree-Fock reference determinant (TD-CIS) has the same problems as rt-TD-HF. These problems can be overcome within the TDCI framework by calculating the excitation energies and transition dipole moments with density functional theory using linear response TD-DFT in the Tamm-Dancoff approximation (TDA) with suitably tuned long-range corrected functionals (TD-TDA). The correct angular dependence of the total ionization rate is obtained with TD-TDA using tuned lc-ωPBE*, lc-BLYP*, and ωB97XD* functionals. Partitioning of the total ionization rate into orbital components confirms that the larger ionization rate perpendicular to the molecular axis found for TD-CIS is due to greater π orbital contributions than those seen in TD-TDA. The use of density functional theory corrects this problem. At higher fields, both the TD-CIS and TD-TDA simulations show an increased ionization rate perpendicular to the molecular axis because of increased ionization from the π orbitals.