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 ph otolysis of acetylene has been measured by laser-induced fluorescence in a supersonic jet. CCH fragments in the <(X)over tilde (2) Sigma(+)> State are vibrationally hot, but rotationally cold. Predominant CCH f ragments were observed at levels of the (X) over bar state with large mixing of <(A)over tilde -state> character, particularly levels near t he potential minimum of <(A)over tilde (II)-I-2>. This indicates that a nonadiabatic transition near the exit channels plays an important ro le 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 hy drogen atom by Balko, Zhang, and Lee [J. Chem. Phys. 94, 7958 (1991)] to extract the population distribution of CCH. It is thus determined t hat the majority of CCH radicals are formed in the ground electronic s tate ((X) over tilde). Less than half of the CCH population was detect ed at K=1 levels, and the rest was distributed among K=0, 2, and 3 sta cks. The bond energy of HCC-H is estimated as 131.5+/-0.5 kcal/mol fro m the vibronic energy of the most populated CCH fragments determined i n this work and the translational energy of the recoiled hydrogen atom reported previously by Balko, Zhang, and Lee and Segall, Wen, Lavi, S inger, and Wittig [J.Phys.Chem. 95, 8078 (1991)]. (C) 1996 American In stitute of Physics.