Reactive scattering of alkali dimers: K₂+ Br₂, ICl, IBr, BrCN

Abstract

Angular distribution measurements of KX or KCN reactive scattering of a potassium dimer Ks beam by cross beam molecules Br2, IC1, IBr and BrCN are reported. The magnitude of the total reaction cross-sections indicates that the entrance valley is dominated by an electron jump. Forward peaking of the centre of mass differential cross sections suggests that the Ks + ion dissociates more slowly than the XY-ion in large impact parameter collisions. The predominant reaction path yields a potassium halide (or cyanide) and potassium atom, KX+ K+Y. These features are rendered plausible in terms of the electronic structure of the potential surface. 1. INTRODUCTION Considerable insight has been gained into the dynamics of chemical reactions through the techniques of molecular beam kinetics. Direct mechanisms with rebound [1], intermediate [2] and stripping [3-5] angular distributions have been exhibited by the reactions of alkali atoms with alkyl iodide, polyhalide and halogen molecules. Long-lived complex mechanisms have been observed in the reactions of alkali atoms with alkali halides [6] ; short-lived complexes in halogen atomhalogen molecule [7, 8] and alkali atom-thallium halide reactions [9]. However, even this prodigious versatility represents only the reactions of atoms with diatomic molecules, i.e. three centre metathetical reactions. wThe next stage of mechanistic complexity arises with four centre reactions. The reactions of alkali atoms with triatomic molecules [10], proceeding by a stripping mechanism do demonstrate some effects of the internal motion of the triatomic molecule. However, the full dyamical complexity of four centre reactions is to be anticipated in diatomic molecule-diatomic molecule reactions. In this category only the alkali halide-alkali halide exchange reaction [11] has been studied. Even here the limited reaction exothermicity results in a long-lived complex mechanism, so that potential dynamical complexities are ' washed out ' Thus, considerable interest attaches to studies of diatomic-diatomic four centre reactions proceeding by a direct mechanism. With this end in view, we have studied the reactions of alkali dimers with a range of cross beam molecules. A preliminary account [12] of the reactions of potassium dimers with halogen molecules has already been reported.

Reactive scattering of alkali dimers : K2+ C3H5I, C3H5Br, CH3I, C2H5I, C6H12

“…This difference may arise [10] from the lack of spherical symmetry in the reactive molecules ; the C6HI~ molecule approximates more closely to spherical symmetry. However, as in previous dimer work [1][2][3], reactively scattered K atoms or K alkyls (see below) may contribute to the Pt/W intensity in addition to elastically scattered K s dimers. AI wide angles, 0 > 30 ~ the two branches of the K 2 + C6H~ fail to agree ; the principle branch shows a notable hump.…”

“…The bond energy of KR is taken nominally as ~15 kcal mole-l~ in all cases. Since potassium alkyl and allyl molecules KR will be ionized by the Pt/W filament in just the same way as K atoms these reaction paths introduce no ambiguities into the interpretation of the reactive E AD01 ADo ~ scattering, in contrast to previous reactions [1][2][3]. One KX molecule only is formed from each reacting K~ dimer and measured as the difference between W and Pt/W relative intensity.…”

“…A stochastic method [11] has been employed to determine the angular distribution in centre of mass coordinates. As previously [1,3] an angular function of the form (1) and a velocity function…”

“…Velocity analysis measurements [6,7] show that more than half the reaction exoergicity is disposed into product translational energy. Thus, reaction occurs only in collisions at small impact parameters and the reaction potential energy surface involves strong repulsion in the exit valley, Thus the K 2 dimer reactions with alkyl iodides give an opportunity to study the dynamics on a potential surface very different from those involved previously [1][2][3]. The reactions of alkali atoms with allyl halides have been studied at thermal energies [8].…”

“…The reactive scattering of potassium dimers K 2 with halogen molecules [1,2] and polyhalide molecules [3] has recently been observed. The understanding gained of K~ dimer reaction dynamics may be tested and augmented by extending to reactions with alkyl and allyl halide molecules.…”