Density functional theory calculations for the enrichment of calcium and titanium

Within PRISMAP, we are investigating innovative isotope enrichment techniques that could lead to a new European supply chain. We investigate the technique of laser-enhanced isotopically selective condensation, that has been demonstrated by the team of Hubert van den Bergh from Ecole Polytechnique de Lausanne in the 1980s. It consists in the nucleation and clusterisation of argon atoms to a volatile molecule as the mixture of argon and titanium- or calcium-containing molecules is expanding through the nozzle of a gas cell. During the gas expansion, dramatic cooling of the gas mixture from room temperature (~300K) down to ~15K occurs, enabling this process. However, by applying infrared (IR) excitation at a frequency specific to an isotopomer (molecule containing a specific isotope), that molecule can be heated up hereby preventing the nucleation and thus the cluster formation. The clusters and the free molecules experience then different drag forces in the jet that allows their physical separation.

In our recent investigation at KU Leuven, we have explored many different titanium- or calcium-containing molecules with density functional theory (DFT) calculations. Those have allowed to identify different IR transitions in those molecules, as well as which ones are sensitive to the mass of the titanium or calcium isotopes. Simple calculations were first performed on TiFx [x=1,2,3,4] molecules to benchmark the approach and challenge the DFT calculations against experimental IR spectra. We then proceeded on exploring the binding with argon atoms as TiFxArn at room temperature and 15K, which demonstrated the lack of cluster formation at room temperature while more than 20 atoms can cluster around a cold molecule.

Finally, we performed our calculations on more complex molecules that might be more practical to synthesize and manipulate. From this investigation, it was concluded that the IR region where the sensitive transitions were identified do not vary substantially between molecules and are all located around 12µm to 13µm, corresponding to those transitions arising from a substantial motion of the titanium or calcium atom, while transitions in more accessible IR regions are originating from the motion of other parts of the molecule, which are thus not sensitive to the different isotopes of titanium or calcium.

Research into the efficient production of laser light in that region, as well as the experimental validation of those DFT spectra will now be investigated at CERN.

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