Vibrational Signatures of Lithium Isotopologues by Free Electron Laser Spectroscopy

Abstract This work demonstrates the significant advantage of a 27 cm–1 isotopic spectral shift in the lithium shuttling vibration for separating lithium isotopes using a free electron laser (FEL), a difference approximately 40 times greater than those typically observed in uranium analogues. Leveraging the FEL’s broad spectral tunability and high photon flux, we identify specific signatures of lithium salt isotopologues through an unambiguous mass selection approach. By resolving the isotope-dependent infrared fingerprints, these results demonstrate a more comprehensive understanding of lithium chemistry. Infrared ion spectroscopy experiments, coupled with quantum chemical calculations, confirm these distinct isotopic shifts in the vibrational spectra of the 6Li(NO3)2– and 7Li(NO3)2– anions.