VIBRATIONAL-STATE CONTROL OF BIMOLECULAR REACTIONS

The influence of rotation and vibration on the reactivity and the dyna mics of the reaction X + HCN(nu(1), nu(2), nu(3), J)-->HX + CN(upsilon , J) with X = H, Cl has been studied. The HCN molecule is prepared in a specific rovibrational level by IR/VIS overtone excitation in the wa velength region 6500-18000 cm(-1). The H atoms are generated by laser photolysis of CH3SH at 266 Mn, the Cl atoms are formed in the photodis sociation of Cl-2 at 355 nm. The CN products are probed quantum state specifically by laser-induced fluorescence (LIF). For low rotational s tates of HCN, the reactivity of Cl and H is independent of the initial rotational state. However, an enhancement in reactivity of the Cl + H CN reaction is observed when the time of rotation becomes comparable t o the passing time of the Cl atom. The reaction of Cl as well as of th e H atom with HCN shows strong mode specific behavior, implying a simp le direct reaction mechanism, which is also supported from Rice-Ramspe rger-Kassel-Marcus (RRKM) calculations. An increase in CH stretch vibr ation increases both the reaction rate and the CN product vibration. C hanneling energy in CN stretch vibration has only a minor effect on th e reactivity and the CN product vibration even decreases. Trajectory c alculations of the H + HCN system agree with the experimental results. The dependence of reaction rates on reactant approach geometry is inv estigated by preparing aligned reactants using linear polarized light. The CN signal is markedly influenced by the prepared alignments (ster ic effect). The experimental results suggest that the reaction of hydr ogen and chlorine atoms with vibrationally excited HCN proceeds mainly via a collinear transition state, but the cone of acceptance is large r for chlorine atoms. (C) 1998 American Institute of Physics.