Laser-induced fluorescence study of the reactions of F atoms with CH3I and CF3I

Abstract

The internal vibrational and rotational product state distributions of IF formed in the reactions of F+CH₃I→IF+CH₃ and F+CH₃I→IF+CF₃ have been measured with the laser‐induced fluorescence method employing cw dye lasers. The experiments have been performed with crossed molecular beams at a relative collision energy of ∼7 kJ mol⁻¹. The mean fraction of E_tot entering product vibration and rotation has been determined to be 〈f′_V〉=0.15±0.02; 〈f^'_R〉=0.14±0.03 for F+CH₃I; and 〈f′_V〉=0.11±0.02; 〈f′_R〉=0.10±0.02 for F+CF₃I. From the comparison of the fluorescence intensities the ratio of the total average cross sections of the two reactions has been determined to be σ_tot (CF₃I)/σ_tot (CH₃I) =2.2±0.5. The vibrational and rotational product state distributions are essentially statistical indicating that the reactions proceed through a long‐lived complex. Our data support Farrar and Lee’s interpretation of the IF angular and velocity distribution measurement for the reaction F+CH₃I at a similar collision energy [J. Chem. Phys. 63, 3639 (1975)]. From the data of the two independent measurements the average amount of the internal excitation of the CH₃ radical may be estimated to 〈f′_int (CH₃) ≳∼0.4. The smaller amount of IF product excitation in case of the reaction F+CF₃I suggests that even a larger fraction of E_tot is channeled into internal excitation of the CF₃ radical.