CROSS SECTION OF THE THIRD HARMONIC GENERATION ON FLUORINE NEGATIVE ION V.K.Ivanov, A.V.Korol and M.Yu.Kuchiev St.Petersburg State Technical University, ST.Petersburg, Russia; Russian Maritime Technical University, St.Petersburg, Russia; University of New South Wales, Sydney, Australia The process of third-order harmonic generation on negative F- ion is considered within the lowest order of perturbation theory. The calculations are performed both within the Hartree-Fock approximation and within the method combining the corrected 2p ground state and the Hartree-Fock excited state wavefunctions. Results are presented for photon energies below the two-photon-ionisation threshold.