Selective photoionisation of 168Yb in an extended optically dense medium

Mathematical simulation methods are used to optimise of three-stage isotopically selective photoionisation of atoms of the ytterbium-168 isotope. The influence of nonlinear deformation of laser beams is taken into account. It is shown that exact tuning of the laser radiation frequencies to the frequencies of the lines of radiative transitions in the ytterbium-168 atom results in absorption of the laser radiation by atoms of other isotopes and in self-focusing of the laser beams. Introduction of a two-photon detuning reduces considerably the influence of the absorption and self-focusing. A determination is reported of the two-photon detuning, and of the dimensions and the spatial distribution of the power in the laser beam required to ensure that the absorption and nonlinear self-focusing of laser beams do not reduce significantly the ionisation efficiency.