Kinematics of Eley-Rideal Reactions at Hyperthermal Energies

Abstract

Direct or Eley-Rideal reactions between energetic N þ and O þ projectiles and O atoms, adsorbed onto Pt and Pd surfaces, are studied experimentally at incidence energies between 20 and 200 eV. The exit energies of the diatomic molecular products NO and O 2 depend linearly on the incidence energy of the corresponding projectiles. A reaction mechanism is proposed, where the incident projectile collides with a single metal atom on the surface, linked to an adsorbed O atom. At the apsis point, a high-energy transient state is formed between the projectile, substrate, and adsorbate atoms. As the projectile begins to rebound, the transient state decomposes into a diatomic molecule, consisting of the original projectile and the adsorbed O atom, which exits the surface with memory of the incidence energy. Energy and momentum conservation during this single-bounce event (atom in, molecule out) accurately predict the exit energy of the molecular product, thus capturing the kinematics of the direct reaction. DOI: 10.1103/PhysRevLett.116.253202 Gas-surface reactions are often thought to proceed between two extremes: the Langmuir-Hinshelwood (LH) mechanism, which requires all reactants to be adsorbed onto and in thermal equilibrium with the surface, and the Eley-Rideal (ER) mechanism, where an energetic reactant (the "projectile") impinges onto a surface adsorbate and bonds with it directly [1]. The LH mechanism applies to most surface chemical reactions and is well understood [2]. The ER mechanism is still being debated: the rarity of chemical environments where hyperthermal reactive species bombard surfaces, combined with experimental difficulties in detecting energetic reaction products, has impeded progress in understanding such reactions. In fact, even the definition of an ER reaction is outright crude: the notion of a bond forming between an energetic projectile and a surface adsorbate in a head-on collision, followed up by ejection from the surface of an energetic reaction product, violates momentum conservation. This problem does not arise if the projectile collides first with the surface. Hot atom reactions represent an intermediate mechanism, where the projectile undergoes few bounces before reacting with an adsorbate [3]. Owing to multiple collisions, which lead to variable energy losses, these do not qualify as prompt ER reactions.What comprises an ER reaction? The literature consensus suggests that ER reactions should have at least the following attributes: (i) they require a gas-phase projectile with high kinetic or potential energy, impinging onto an adsorbate-covered surface; (ii) they yield a fast-moving molecular product, consisting of the projectile-adsorbate combination, which leaves the surface (no trapping); and (iii) the product translational energy must be directly correlated to the energy of the incident projectile. Secondary attributes include that (iv) the product molecule may be internally excited; and (v) the product angular flux distribution deviates from the cosine law, with subspecular pref...

Dynamic Deuterium Enrichment in Cometary Water via Eley–rideal Reactions

“…The ion exit energy data for H 2 DO + , HDO + , H 2 O + (or OD + ), OH + , and O + , plotted in Figure 3(b), depend linearly on the incidence energy with slopes predictable a priori. The kinematic factor for H 2 DO + , calculated from binary collision theory to be 0.8233, fits the energy data exceptionally well, a strong confirmation of its origin in an ER reaction (Yao & Giapis 2016). The intercept of the linear fit, −5.5 eV, represents inelastic energy loss and relates to the energy required to ionize H 2 DO * .…”

“…H 2 O + , OH + , and O + ion exits may originate from the late dissociation of either H 2 DO * or unreacted beam ions (Figures 2(d), (e)). Energetic H + and D + are also detected, as well as negative ions OD − , OH − , and O − (not shown), formed by resonant electron attachment (Yao & Giapis 2016).…”

“…This dynamic process transfers deuterium locked in cometary materials to normal water, thereby enriching the gas phase in deuterated water. It occurs at hyperthermal collision energies (10-200 eV), but it is independent of surface temperature (Yao & Giapis 2016), as appropriate for cometary conditions.…”

Intramolecular water-splitting reaction in single collisions of water ions with surfaces

“…3A illustrates the dependence of the surviving water–ion exit energy as a function of the incidence energy for all of the three water ions studied. The data are fitted linearly with fixed slopes of 0.8326, 0.8241 and 0.8157 for H 2 O + , HOD + and D 2 O + , respectively, which are the exact kinematic factors predicted by the binary collision theory (BCT) 19,20 for water scattering on Au as a whole molecule. The remarkably good fit (correlation coefficient better than 0.995 for all fits) suggests strongly that the detected signals are due to single collisions with surface atoms.…”

Dynamic molecular oxygen production in cometary comae

“…The latter include Eley–Rideal (ER) reactions, where energetic projectiles collide with surfaces and react with adsorbates to produce projectile-adsorbate molecules without equilibration with the surface11. This dynamic process is driven by the projectile energy, a large fraction of which is carried away by the product molecule.…”