Efficient excitation of media for laser isotope separation

A theoretical investigation is made of the photoionisation of a three-level medium intended for laser isotope separation. The results show that the highest ionisation efficiency can be achieved by dividing the process into two stages: coherent population inversion and photoionisation. A study is made of the possibility of inversion of three-level systems for various detunings of the laser frequencies from the radiative transition frequencies. It is shown that there are detunings for which the population of the third level is maximal for any a priori selected parameters of the medium and field. Such detunings lie near a two-photon resonance and depend on the energy density and duration of the laser pulses, and also on the dipole moments of the transitions. The suitability of this method in the case of inhomogeneously broadened optically dense media is considered. It is shown that the method is most efficient in the case of counterpropagating laser pulses. The efficiency of the method is demonstrated for ytterbium.