Theoretical calculation of two-photon absorption cross sections in atomic oxygen

Two-photon absorption cross sections for transitions from the O(³P) ground electronic state to the 3p ³P, 4p ³P, and the autoionizing O⁺(²D)3p' and O⁺(²P)3p'' states have been calculated by explicit evaluation of the perturbation-theory summation using matrix elements calculated from configuration-interaction wave functions and sum rules to correct for truncation. The role of different contributions in the summation has been analyzed. For the 3p ³P state, a cross section of JJ'{{σ}}₀⁽²⁾(J'←J) of (1.319±0.2)×{10}^{{{-}}35}$ ${cm}⁴$ has been obtained, which is in excellent agreement with the experimental value for the quantity ${J}J'σ₀⁽²⁾(J'{←}2)G⁽²⁾ =(2.66±{}0.80)×{}10^-35 cm⁴, given in the companion paper, if the photon statistical factor, G⁽²⁾, is evaluated in the chaotic field limit (G⁽²⁾=2).