193.3 nm photodissociation of acetylene: Nascent state distribution of CCH radical studied by laser-induced fluorescence

The nascent rovibronic distribution of CCH radicals in the 193.3 nm photolysis of acetylene has been measured by laser-induced fluorescence in a supersonic jet. CCH fragments in the X̃ ²∑⁺ state are vibrationally hot, but rotationally cold. Predominant CCH fragments were observed at levels of the X̃ state with large mixing of Ã-state character, particularly levels near the potential minimum of à ²Π. This indicates that a nonadiabatic transition near the exit channels plays an important role in the 193.3 nm photodissociation of acetylene. Some, but not all, of the K=1 levels have distinctively bimodal rotational distributions. The relative vibrational energy distributions obtained from this work were used to simulate the translational energy distribution of the hydrogen atom by Balko, Zhang, and Lee [J. Chem. Phys. 94, 7958 (1991)] to extract the population distribution of CCH. It is thus determined that the majority of CCH radicals are formed in the ground electronic state (X̃). Less than half of the CCH population was detected at K=1 levels, and the rest was distributed among K=0, 2, and 3 stacks. The bond energy of HCC-H is estimated as 131.5±0.5 kcal/mol from the vibronic energy of the most populated CCH fragments determined in this work and the translational energy of the recoiled hydrogen atom reported previously by Balko, Zhang, and Lee and Segall, Wen, Lavi, Singer, and Wittig [J. Phys. Chem. 95, 8078 (1991)].