Angular dependence of strong field ionization of N2 bytime-dependent configuration interaction using density functional theory and theTamm-Dancoff approximation

• Published in: The Journal of chemical physics 2019-08, Vol.151 (5)
• Main Authors: Hoerner, Paul, Lee, Mi Kyung, Bernhard, Schlegel H
• Format: Article
• Language: English
• Subjects: Approximation; Computer simulation; Configuration interaction; Density functional theory; Dipole moments; Excitation; Field ionization; Mathematical analysis; Real time; Time dependence; Time integration
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.5108846

The ionization of N2 serves as an important test case for computationalmethods for strong field ionization. Because Koopmans’s theorem fails for Hartree-Fockcalculations of N2, corrections for electron correlation are needed to obtainthe proper ordering of ionization energies of N2. Lopata and co-workers foundthat real-time integration of time-dependent Hartree-Fock (rt-TD-HF) gave a ratio forstrong field ionization parallel and perpendicular to the molecular axis that was toosmall compared to experiment, but real-time integration of time-dependent densityfunctional theory (rt-TD-DFT) with an appropriately tuned long-range corrected functional,lc-ωPBE*, was in good agreement with experiment. The present study finds thattime-dependent configuration interaction (TDCI) with single excitations based on aHartree-Fock reference determinant (TD-CIS) has the same problems as rt-TD-HF. Theseproblems can be overcome within the TDCI framework by calculating the excitation energiesand transition dipole moments with density functional theory using linear response TD-DFTin the Tamm-Dancoff approximation (TDA) with suitably tuned long-range correctedfunctionals (TD-TDA). The correct angular dependence of the total ionization rate isobtained with TD-TDA using tuned lc-ωPBE*, lc-BLYP*, andωB97XD* functionals. Partitioning of the total ionization rate into orbitalcomponents confirms that the larger ionization rate perpendicular to the molecular axisfound for TD-CIS is due to greater π orbital contributions than those seen in TD-TDA. Theuse of density functional theory corrects this problem. At higher fields, both the TD-CISand TD-TDA simulations show an increased ionization rate perpendicular to the molecularaxis because of increased ionization from the π orbitals.