Collisionally enhanced isotopic selectivity in multiphoton dissociation of vibrationally excited CF_3H

摘要

We have studied infrared multiphoton dissociation of CF_3H pre-excited to the second C-H stretch overtone under collisional conditions in view of developing a laser isotope separation scheme for carbon-13. This single stage process results in a C_2F_4 product that has been enriched in carbon-13 to a level as high as 99% starting from a naturally abundant sample, implying an isotopic selectivity in excess of 9000. While most of the selectivity is gained at the pre-excitation step, it can be increased up to factor of 16 by collisions of the pre-excited ~(13)CF_3H species with room temperature molecules. This collision-induced enhancement in selectivity becomes evidence from the dependence of the isotopic enrichment on both the total sample pressure and the time-delay between the two lasers, and we propose two different models that can account for this behavior. Finally, we evaluate the practical relevance of this two-laser scheme for isotope separation.