Selective Excitation of Branched Vibrational Ladder in Uranium Hexafluoride Laser Isotope Separation

抄録

Theoretical investigation was made on the dynamics of initial excitation process in molecular laser isotope separation for uranium hexafluoride (UF₆) based on the generalized N-level density-matrix equation derived by Goodman et al. Branched vibrational model due to anharmonic-splitting components for the ν₃ mode of UF₆ molecule were formalized and phase interferences of density-matrix elements were considered to analyze the selective excitation. Because of a power-broadening effect, no more than 0.05 J/cm² fluence of a laser pulse completely masks high enrichment peaks in broadband (Δν=2 GHz) pumping case. Even when matching the laser frequency with ν₃ band (n=0→4) of ²³⁸UFG₆ the enrichment factor (α≡Rproduct/Rfeed, R≡[²³⁸UF₆]/ [²³⁸UF₆] ; R is the abundance ratio and [ ] means the mole fraction.) does not decrease. A narrow spectral linewidth (Δν=-400 MHz) is shown to be essential to achieve a high concentra-tion ratio in the system with a branched N-level ladder as expected from a general two-level system. The exciting frequency, which does not necessarily accord with the ν₃ band of ²³⁵UF₆, gives the maximum enrichment peak. Both the detuning of the optimum frequency and the sharp enrichment peak suggest a direct excitation due to multiphoton resonance. Spectral stability of the laser pulse is also required to excite only the desired isotope for the system with complicated anharmonic-splittings.