Mechanism of isotope scrambling for CO2 laser photolysis of trifluoremethane-h and-d mixtures

The mechanism of isotope scrambling was studied for infrared multiphoton dissociation of trifluoromethane-h (CHF₃) and-d (CDF₃) mixtures by a CO₂ TEA laser. The CO₂ laser was tuned to the R(14) line of the 001–100 transition, where CDF₃ absorbs the light selectively. Although the highly selective decomposition of CDF₃ occurred at low partial pressures of CDF₃, significant decomposition of CHF₃ was also observed with increasing pressure of CDF₃. The selectivity (s) was found to depend only on the pressure of CDF₃, a resonant molecule, but not on that of CHF₃, an off-resonant molecule. The mechanism involving the formation of two kinds of vibrationally excited molecules, unimolecular decomposition (rate constantk ₂ ),V-V energy transfer (k _3b), and collisional deactivation leads to the simple relationship,s=1+(k ₂ /k _3b)/CDF₃], which can explain qualitatively the observed results. The ratiok ₂ /k _3b has values of 20–200 Torr, probably depending on experimental conditions.