Molecular Beam Kinetics: Reactions of H and D Atoms with Diatomic Alkali Molecules

Abstract

Crossed-beam studies have been made of the reactions of H and D atoms with K2, Rb2, and Cs2. The hydrogen atoms were generated by thermal dissociation of molecules in a tungsten oven at ∼ 2200–2900°K, the alkali dimers by association of atoms in a supersonic beam expanded from a nozzle at ∼ 900°K. Surface ionization detectors were unable to distinguish among alkali atoms, dimers, and alkali hydrides. However, a kinematic separation of reactive and nonreactive scattering proved to be feasible, owing to the disparity in reactant masses and parity in product masses. The angular distributions of reactive scattering peak at right angles to the initial relative velocity vector. A kinematic analysis indicates that only a small fraction of the available energy appears in product translational motion. This implies that either the vibrational and/or rotational excitation of the product alkali hydride is very high (∼ 30–40 kcal/mole), comparable to or possibly even larger than its dissociation energy, or the product alkali atom is electronically excited. Total reaction cross sections were estimated by normalizing the observed intensity of wide-angle atom–atom elastic scattering to the hard-sphere value. The nominal cross section obtained (no better than a factor of 2) is <̃ 50 Å2 for the H reactions. The D atom reactions show very similar product distributions, with total reaction cross sections larger by a factor of ∼ 2. Variation of the H and D beam temperature indicated the activation energy is negligibly small. The H atoms approach very rapidly compared with the sluggish vibrational and rotational motions of M atoms within M2. From this it is argued that the observed scattering implies an anisotropic reaction probability, favoring configurations with the M2 axis perpendicular to the direction of approach of the H atom. Qualitative comparisons are also made with analogous dissociative electron attachment and photodissociation processes, and with an electron-jump mechanism which involves formation of an intermediate H−+M2+ ion-pair.