Dynamics of chemical and charge transfer reactions of molecular dications: beam scattering and total cross section data on CF₂D⁺ (CF₂H⁺), CF₂⁺, and CF⁺ formations in CF₂²⁺ + D₂(H₂) collisions

Abstract

Dynamics of formation of the chemical rearrangement product CF₂D⁺, the charge transfer product CF₂⁺, and the dissociative products CF⁺ and CFD⁺ in collisions of the molecular dication CF₂⁺⁺ with D₂ was investigated in crossed beam scattering experiments over the collision energy range 0.3–1.0 eV (center of mass). The scattering data show that coulomb repulsion between two singly charged products, CF₂⁺ + D⁺ and CF₂⁺ + D₂⁺, plays a dominant role in the nondissociative processes. A large fraction of the energy available (about 6 eV in the chemical reaction, about 4 eV in the charge transfer) goes into relative translational energy of the products. Relative total cross sections for formation of the nondissociative and dissociative products in collision of CF₂⁺⁺ with D₂ and H₂ were determined over the collision energy range of 0.2–3.6 eV. The shape of the relative velocity dependence of the cross section for CF₂⁺ formation can be described by a simple model based on the Landau-Zener formalism. The data suggest that the dissociative product CF⁺ is formed prevailingly in a subsequent dissociation of the charge transfer product CF₂⁺. A potential surface model is described which accounts for competition of various processes in dication–neutral collisions.

Introduction

Multiply charged ions are very energy-rich species, highly reactive in collisions with atoms and molecules. This is why they have attracted the increasing attention of experimentalists and theoreticians alike. Compared with the situation of a decade ago [1], much more information is available now, in particular on molecular doubly charged ions (molecular dications) [2]. Electronic states of diatomic or small polyatomic dications lie typically at energies above the asymptote for formation of the respective singly charged fragments. An energy barrier on the potential energy surface prevents them from dissociating and thus the observable molecular dications exist, in fact, in states metastable with respect to dissociation. Their lifetimes are often longer than about 10⁻⁵ s; therefore, molecular dications can be prepared and used as collision species. The energy barrier in many cases supports only the lowest vibrational states, and thus vibrational excitation is usually rather small.

In studies of collision processes of molecular dications, most often charge transfer processes of the type

$$\text{A}{}^{\mathrm{2+}}\text{+BC→A}{}^{\mathrm{+}}\text{+BC}{}^{\mathrm{+}}$$

(A, B, and C are atoms or groups of atoms) with atoms and molecules have been investigated, and large amounts of data have been obtained on the cross section and energy partitioning in the low collision energy region [2], [3], [4].

More recently, chemical (bond forming) reactions of doubly charged ions have been described in collisions of low energy ions with molecules. Chemical reactions of dications can basically be of two types: bond forming reactions between dications and neutrals where a doubly charged ion product and a neutral particle is formed, of the type

$$\text{A}{}^{\mathrm{2+}}\text{+BC→AB}{}^{\mathrm{2+}}\text{+C}$$

or a reaction where two singly charged ions are formed as a result of a bond-rearrangement collision between a dication and a neutral,

$$\text{A}{}^{\mathrm{2+}}\text{+BC→AB}{}^{\mathrm{+}}\text{+C}{}^{\mathrm{+}}$$

The latter type is of a particular interest because of an expected high translational energy release due to coulomb repulsion between the products.

Early reports on chemical reactions of dications came from swarm experiments: their occurrence was briefly mentioned in flow tube studies of Ca²⁺ and Mg²⁺ interactions with simple molecules [5]. Several chemical reactions of transition metal doubly charged ions (Ti²⁺, Nb²⁺, Zr²⁺, Ta²⁺) in collisions with hydrogen and simple hydrocarbons, leading to both doubly and singly charged chemical products, have been reported [6], [7], [8], [9]. More recently, bond-forming chemical reactions of molecular dications have been observed [10], [11], [12], [13] where singly charged ions were formed in both nondissociative and dissociative chemical reactions.

In an earlier paper [11], the authors reported on a crossed beam scattering study of the nondissociative processes in collisions of CF₂²⁺ with D₂, namely of the chemical, bond-forming reaction

$$\text{CF}{}_2{}^{\mathrm{2+}}\text{+D}{}_2\text{→CF}{}_2\text{D}{}^{\mathrm{+}}\text{+D}{}^{\mathrm{+}}$$

and the accompanying charge transfer process

$$\text{CF}{}_2{}^{\mathrm{2+}}\text{+D}{}_2\text{→CF}{}_2{}^{\mathrm{+}}\text{+D}{}_2{}^{\mathrm{+}}$$

The results at the relative collision energy of 0.6 eV showed that the type of scattering of ion products of , was rather similar and governed by coulomb repulsion between the products. The relative translational energy release represented in both cases a larger fraction of the reaction exoergicity, the peak value being about 6 and 4 eV in , , respectively. We formulated a simple model for the chemical reactions of dications based on crossing of the potential energy surfaces of the reactant dication system with coulomb-repulsion surfaces of the cation products both in the reactant and the product valley.

In this article, we report further scattering data on , at relative collision energies of 0.3 and 1.0 eV, results of measurements of total cross sections of the reactions with D₂ and H₂, data on the dissociation product CF⁺, and we extend the discussion of the above mentioned model.