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 97.62.4+/-1.2, in good agreement with previous calculations.