Molecular beam kinetics: reactions of H and D atoms with alkali halides

Abstract

Angular distributions of alkali atoms produced in crossed-beam reactions of H and D atoms (at ∼2800 K) with KF, CsF, KCl, CsCl, and KBr (at ∼ 1100–1200 K) have been measured. For these systems, the reaction exoergicity varies from ΔD0 ∼ 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 ∼ 9 kcal/mol in relative translation and W ∼ 3 kcal/mol in vibration and rotation of the alkali halide. Kinematic analysis of the data is carried 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 Qr vary from ∼1 to 30 Å2. The magnitude correlates with ΔD0 and with the identity of the halogen for a given alkali, decreasing as F → Cl → Br. For a given halogen, Qr 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 excess 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 reaction to more sideways for the KF and CsF reactions. (4) The partitioning of energy between product translation E′ (M relative to HX) and internal excitation W′ (vibration and rotation of HX) can only be roughly estimated; our data are consistent with E′ ∼ W and W′ ∼ E + ΔD0 but a definitive analysis will require product velocity distributions or spectra. These results are discussed in terms of a dynamical model akin to Polanyi's DIPR model. This postulates that the 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.