Third-harmonic generation in Hg vapors, using the two-photon resonance
6S-1(0)-8S-1(0) transition is theoretically and experimentally invest
igated. Calculations of the Hg contribution to the phase mismatch are
presented and the effects of the absorption of the radiation, the popu
lation transfer, the saturation, and the ionization on the efficiency
of the system are considered. It is shown that the population transfer
is the dominant mechanism affecting the efficiency.