Oscillator Strengths of the Ultraviolet Bands of Hot and Relaxed Methallyl, Allyl, and Methyl Radicals

Abstract

Absorption spectra of methallyl, allyl, and methyl radicals were measured in the ArF laser flash photolysis of halogenated methanes and olefins, and 2-methyl-1-butene. Broad absorption spectra were detected immediately after excitation, indicating that the radicals carried high internal energy (called “hot” radicals here). The spectra were sharpened with time due to collisions with foreign gases and finally reached equilibrium (spectra in relaxed states). The oscillator strengths (f) of methallyl and allyl radicals were found to remain about the same for hot and relaxed states. The values (f) in the relaxed states were determined with errors of ±10% to be 0.14 for the methallyl radical, 0.26 for the allyl radical, and 1.57 × 10⁻² (including the partition function) for the methyl radical. The molar extinction coefficients of the allylic radicals were 1.4–1.8 times as great as ones previously reported. Rate constants of collisional relaxation by nitrogen were discussed for seven hot radicals and molecules. Larger species gave greater rate constants. The hot methyl radical relaxed with a rate constant of 1.0 × 10⁷s⁻¹ in the presence of 760 Torr of nitrogen, while for the trimethylallyl radical it was 20 times as great as that of the methyl radical.