Photoionization cross sections involving an explicitly correlated initial state: Combination of multiconfigurational linear response and the stieltjes-tchebycheff moment theory

We have performed outer valence photoionization cross section calculations for N₂ and O₂. To do this we have combined several linear response techniques, in particular time-dependent Hartree-Fock (TDHF), multiconfigurational time-dependent Hartree-Fock (MC TDHF), and a modification of MC TDHF (MMC TDHF) with Stieltjes-Tchebycheff moment theory (STMT). To our knowledge, these MC TDHF and MMC TDHF calculations are the first which combine explicitly correlated Green function approaches with STMT. Since, in addition, these calculations are fully coupled, we expect the MC TDHF and in particular the MMC TDHF-STMT results to be highly reliable. For both N₂ and O₂ our MC TDHF-STMT and MMC TDHF-STMT results are in overall agreement with previous static exchange STMT results; however, there are a few significant differences and differences in detail in the partial and total photoionization cross sections. In particular, for example, for N₂ we note that the MMC TDHF-STMT does not give a "hump" resonance in the cross section for the (1π_u^-1)A²Π_u ionic state. In O₂ we note that the (3σ_g^-1) cross section obtained using MMC TDHF-STMT is substantially lower than the static exchange results.