Competing reaction pathways from Y+C2H2 collisions

The crossed molecular beams method with 193 and 157 nm photoionization dete ction was used to study the competing reaction pathways resulting from coll isions of ground state Y atoms with acetylene (C2H2). Three channels, corre sponding to nonreactive decay of collision complexes, H-2 elimination, and H atom elimination, were studied as a function of collision energy (< E-col l>=6-25 kcal/mol). Production of YC2+H-2 and decay of long-lived complexes back to reactants were observed at all collision energies studied. Product translational energy distributions for the H-2 elimination channel demonstr ate that a substantial fraction of excess energy available to the YC2+H-2 p roducts is channeled into relative translational energy. Analogous H-2 elim ination channels were studied in reactions of Zr and Nb with C2H2 at < E-co ll>=6.0 kcal/mol. For these reactions, the H-2 elimination product translat ional energy distributions were found to peak near zero kinetic energy, in contrast to the behavior observed for the YC2+H-2 products. This suggests t hat a significant potential energy barrier exists in the exit channel of th e YC2+H-2 elimination step, whereas no exit channel barrier exists in formi ng ZrC2+H-2 and NbC2+H-2. The reformation of Y + C2H2 reactants following d ecay of long-lived collision complexes was found to transfer 40%-50% of the initial relative translational energy into C2H2 internal excitation. The Y C2H+H product channel was only observed to occur above a collision energy t hreshold of 21.5 +/- 2.0 kcal/mol. Since YC2H+H production is fully spin-al lowed and involves simple Y-H bond fission in the intermediate HYC2H comple x, it is unlikely that any significant potential energy barrier is present in excess of the reaction endoergicity. Additional studies of Y+C2D2 reacti ons confirm that the observed collision energy threshold for the H or D ato m loss channel corresponds to the energetic threshold for reaction, allowin g determination of D-0(Y-CCH)=110.2 +/- 2.0 kcal/mol. (C) 1999 American Ins titute of Physics. [S0021-9606(99)00933-2].