MOLECULAR-BEAM KINETICS - REACTIONS OF H-ATOM AND D-ATOM WITH ALKALI-HALIDES

Angular distributions of alkali atoms produced in crossed-beam reactio ns of H and D atoms (at similar to 2800 K) with KF, CsF, KCl, CsCl, an d KBr (at similar to 1100-1200 K) have been measured. For these system s, the reaction exoergicity varies from Delta D-0 similar to 17 kcal/ mol for H + KF to -4 kcal/mol for H + KBr; the thermal distribution of the reactant energy peaks near 12 kcal/mol with E similar to 9 kcal/m ol in relative translation and W similar to 3 kcal/mol in vibration an d rotation of the alkali halide. Kinematic analysis of the data is car ried out by means of a least-squares procedure that includes averaging over the parent beam velocity distributions. The main results are (1) The reaction cross sections Q(r) vary from similar to 1 to 30 Angstro m(2). The magnitude correlates with Delta D-0 and with the identity of the halogen for a given alkali, decreasing as F --> Cl --> Br. For a given halogen, Q(r) is 2-3 times larger for the Cs reaction than for K and about 2 times larger for D than for H. (2) Temperature-dependence measurements for the CsCl case indicate the activation energy (in exc ess of endoergicity) is near zero. (3) The preferred direction of the reactively scattered alkali atoms with respect to the incident alkali halide varies with the halogen, from mainly forward for the KBr reacti on to more sideways for the KF and CsF reactions. (4) The partitioning of energy between product translation E' (M relative to HX) and inter nal excitation W' (vibration and rotation of HX) can only be roughly e stimated; our data are consistent with E' similar to W and W' similar to E + Delta D-0 but a definitive analysis will require product veloci ty distributions or spectra. These results are discussed in terms of a dynamical model akin to Polanyi's DIPR model. This postulates that th e motion of H or D is approximately separable from that of the heavier atoms and the preferred reaction geometry corresponds to a triangular H-X-M complex.