Optimization of the conditions for simultaneous non-selective excitation of plutonium isotopes for isotope ratio measurements in resonance ionization mass spectrometry

Theoretical calculations based on density matrix formalism have been carried out for the non-selective excitation of the isotopes of plutonium for triple resonance excitation scheme with λ1 = 420.77 nm, λ2 = 847.28 nm and λ3 = 767.53 nm. The effects of Doppler broadening, non-Lorentzian lineshape as well as time-dependent laser intensity profiles have all been incorporated. The threshold laser powers and bandwidths for the three excitation steps have been evaluated for non-selective excitation. The saturation behavior of the three excitation steps has been studied. The two-dimensional lineshape contour and its features have been investigated; while the reversal of peak asymmetry of two-step and two-photon excitation peaks under these conditions is discussed. The isotopic bias between the resonant and off-resonant isotope under the optimized conditions is observed to be not more than 3%. Application of RIMS under these conditions for isotope ratio measurements is outlined.