  We propose a multielectron approach to calculate superexchange interaction in
magnetic Mott- Hubbard insulator La2CuO4(further La214) that allows to obtain
the effect of optical pumping on the superexchange interaction. We use the cell
perturbation theory with exact diagonalization of the multiband pd Hamiltonian
inside each CuO6 unit cell and treating the intercell hopping as perturbation.
To incorporate effect of optical pumping we include in this work the excited
singlehole local states as well as all two-hole singlets and triplets. By
projecting out the interband intercell electron hopping we have obtained the
effective Heisenberg-like Hamiltonian with the local spin at site Ri being a
superposition of the ground and excited single-hole states. We found that
antiferromagnetic contribution to the exchange energy in La214 will increase in
accordance to ~ 4*10_{-3} eV /(%) at the resonance light occupation of the
excited single hole in-gap state.
