Laser Isotope-Selective IR Dissociation of Molecules with a Small Isotopic Shift in Absorption Spectra in Nonequilibrium Thermodynamic Shock Conditions

Abstract

We report on the results of investigation of laser isotope-selective IR dissociation of molecules (CF3Br and CF2HCl as examples) characterized by a small (less than 0.25 cm–1) isotopic shift in the IR absorption spectra in nonequilibrium thermodynamic conditions of a compression shock (shock wave) formed in front of the solid surface under the action of an intense incident pulsed gasdynamically cooled molecular beam. Experiments were made using pure CF3Br and CF2HCl gases as well as the CF3Br/CF2HCl mixture with a pressure ratio of 1/1 for the formation of the molecular flow and a shock wave. It is found that the efficiency of dissociation of molecules in the shock wave and in the beam incident on the surface is much higher than in an unperturbed flow. It is shown (for CF3Br as an example) that the dissociation yield in the case of excitation of molecules in the shock wave and in the beam incident on the surface increases strongly (by 5–10 times) as compared to the dissociation yield in the unperturbed flow, while the dissociation threshold decreases substantially (by 3–5 times). It is also established that in the case of irradiation of molecules in the mixture, a mutual strong increase in the efficiency of their dissociation is observed as compared to the case when molecules are exposed to radiation separately. This makes it possible to induce isotope-selective laser IR dissociation of molecules for low excitation energy densities (Φ ≤ 1.0–1.5 J/cm2), thus improving the selectivity of the process. This is demonstrated for dissociation of these molecules in the shock wave, which is selective in chlorine and bromine isotopes. We obtained enrichment coefficients Kenr(35Cl/37Cl) = 0.90 ± 0.05 in the residual CF2HCl gas and Kenr(79Br/81Br) = 1.20 ± 0.09 in the formed product (Br2) under laser irradiation of the CF2HCl/CF3Br = 1/1 mixture and CF3Br molecules at the 9R(30) CO2-laser line (at a frequency 1084.635 cm–1) for energy density Φ ≈ 1.3 J/cm2. The application of the results for laser isotope separation is considered.