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

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 Etot 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 σtot (CF3I)/σtot (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 Etot is channeled into internal excitation of the CF3 radical.