Photofragment translational spectroscopy of propargyl radicals at 248 nm

The photodissociation of propargyl radical, C 3 H 3 , and its perdeuterated isotopolog was investigated using photofragment translational spectroscopy. Propargyl radicals were produced by 193 nm photolysis of allene entrained in a molecular beam expansion and then photodissociated at 248 nm . Photofragment time-of-flight spectra were measured at a series of laboratory angles using electron impact ionization coupled to a mass spectrometer. Data for ion masses corresponding to C 3 H 2 + , C 3 H + , C 3 + , and the analogous deuterated species show that both H and H 2 loss occur. The translational energy distributions for these processes have average values ⟨ E T ⟩ = 5.7 and 15.3 kcal ∕ mol , respectively, and are consistent with dissociation on the ground state following internal conversion, with no exit barrier for H loss but a tight transition state for H 2 loss. Our translational energy distribution for H atom loss is similar to that in a previous work on propargyl in which the H atom, rather than the heavy fragment, was detected. The branching ratio for H loss/ H 2 loss was determined to be 97.6 ∕ 2.4 ± 1.2 , in good agreement with previous calculations.