Brome isotope selective control of CF3Br molecule clustering by IR laser radiation in gas-dynamic expansion of CF3Br – Ar mixture

We report the results of research on the experimental control of CF₃Br molecule clustering under gas-dynamic expansion of the CF₃Br – Ar mixture at a nozzle exit by using IR laser radiation. A cw CO₂ laser is used for exciting molecules and clusters in the beam and a time-of-flight mass-spectrometer with laser UV ionisation of particles for their detection. The parameters of the gas above the nozzle are determined (compositions and pressure) at which intensive molecule clustering occurs. It is found that in the case of the CF₃Br gas without carrier when the pressure P₀ above the nozzle does not exceed 4 atm, molecular clusters actually are not generated in the beam. If the gas mixture of CF₃Br with argon is used at a pressure ratio 1 : N, where N ≥ 3, and the total pressure above the nozzle is P₀ ≥ 2 atm, then there occurs molecule clustering. We study the dependences of the efficiency of suppressing the molecule clustering on parameters of the exciting pulse, gas parameters above the nozzle, and on a distance of the molecule irradiation zone from the nozzle exit section. It is shown that in the case of resonant vibrational excitation of gas-dynamically cooled CF₃Br molecules at the nozzle exit one can realise isotope-selective suppression of molecule clustering with respect to bromine isotopes. With the CF₃Br – Ar mixtures having the pressure ratio 1 : 3 and 1 : 15, the enrichment factors obtained with respect to bromine isotopes are k_enr ≈ 1.05 ± 0.005 and k_enr ≈ 1.06 ± 0.007, respectively, under jet irradiation by laser emission in the 9R(30) line (1084.635 cm^-1). The results obtained let us assume that this method can be used to control clustering of molecules comprising heavy element isotopes, which have a small isotopic shift in IR absorption spectra.