Measurement of boron isotopic ratio with non-gated molecular spectroscopy of femtosecond laser-produced plasma

Laser-induced breakdown spectroscopy can be augmented by concurrent spectroscopic measurements of molecules and molecular ions resulting from recombination of laser-ablated plasmas with the ablated sample and surrounding environment. Molecular emissions can exhibit significantly greater isotopic shifts than atomic emissions, making this approach attractive for isotopic characterization of nuclear materials. We describe a measurement of boron isotopic ratios in four boron-containing samples utilizing the emission from boron monoxide radicals formed in the expanding laser-induced ablation plume. Femtosecond laser excitation is used and the emission in the 532–540 nm region of interest is studied, where a linear superposition of characteristic spectra for two boron isotopes has been applied for reconstruction of the boron isotopic ratio. It is also demonstrated that the use of non-gated measurements of emission spectra suffices for accurate isotopic characterization using this method. The application of multivariate regression to deconvolve the isotopic ratio from the measured emission spectra is discussed in detail, including the limitations and subtleties of this approach.