Collisions of Slow Polyatomic Ions with Surfaces: Dissociation and Chemical Reactions of CD5+, CD4+•, CD3+, and Their Isotopic Variants on Room-Temperature and Heated Carbon Surfaces

Modeling the intrusion of molecules into graphite: Origin and shape of the barriers

We performed density functional theory calculations to explore the energetic and geometric aspects of the permeation of H2, BeH x , OH x (x = 1, 2) and CH y (y = 1-4) through the central hexagon of coronene. Coronene serves as a cluster model for extended graphene which can be regarded as the first layer of a graphite (0 0 0 1) surface. We compare the energy barriers encountered by these molecular projectiles with the ones that are obtained for atomic H, Be, C and O. The barriers are substantially lower if projectiles possess free valences that can bind to the carbon entity. Furthermore, for some of the species fragmentation is observed. Implications with respect to plasma-surface interaction are discussed.

Formation of HCN+ in heterogeneous reactions of N2(+) and N+ with surface hydrocarbons

A significant increase of the ion yield at m/z 27 in collisions of low-energy ions of N₂⁺ and N⁺ with hydrocarbon-covered room-temperature or heated surfaces of tungsten, carbon-fiber composite, and beryllium, not observed in analogous collisions of Ar⁺, is ascribed to the formation of HCN⁺ in heterogeneous reactions between N₂⁺ or N⁺ and surface hydrocarbons. The formation of HCN⁺ in the reaction with N⁺ indicated an exothermic reaction with no activation barrier, likely to occur even at very low collision energies. In the reaction with N₂⁺, the formation of HCN⁺ was observed to a different degree on these room-temperature and heated (150 and 300 °C) surfaces at incident energies above about 50 eV. This finding suggested an activation barrier or reaction endothermicity of the heterogeneous reaction of about 3–3.5 eV. The main process in N₂⁺ or N⁺ interaction with the surfaces is ion neutralization; the probability of forming the reaction product HCN⁺ was very roughly estimated for both N₂⁺ and N⁺ ions to about one in 10⁴ collisions with the surfaces.

Surface-induced dissociation and chemical reactions of C2D4(+) on stainless steel, carbon (HOPG), and two different diamond surfaces

Surface-induced interactions of the projectile ion C(2)D(4)(+) with room-temperature (hydrocarbon covered) stainless steel, carbon highly oriented pyrolytic graphite (HOPG), and two different types of diamond surfaces (O-terminated and H-terminated) were investigated over the range of incident energies from a few eV up to 50 eV. The relative abundance of the product ions in dependence on the incident energy of the projectile ion [collision-energy resolved mass spectra, (CERMS) curves] was determined. The product ion mass spectra contained ions resulting from direct dissociation of the projectile ions, from chemical reactions with the hydrocarbons on the surface, and (to a small extent) from sputtering of the surface material. Sputtering of the surface layer by low-energy Ar(+) ions (5-400 eV) indicated the presence of hydrocarbons on all studied surfaces. The CERMS curves of the product ions were analyzed to obtain both CERMS curves for the products of direct surface-induced dissociation of the projectile ion and CERMS curves of products of surface reactions. From the former, the fraction of energy converted in the surface collision into the internal excitation of the projectile ion was estimated as 10% of the incident energy. The internal energy of the surface-excited projectile ions was very similar for all studied surfaces. The H-terminated room-temperature diamond surface differed from the other surfaces only in the fraction of product ions formed in H-atom transfer surface reactions (45% of all product ions formed versus 70% on the other surfaces).

Scattering of Low-Energy (5-12 eV) C2D4•+ Ions from Room-Temperature Carbon Surfaces

The scattering of the hydrocarbon radical cation C2D4•+ from room-temperature carbon (highly oriented pyrolytic graphite, HOPG) surface was investigated at low incident energies of 6-12 eV. Mass spectra, angular and translational energy distributions of product ions were measured. From these data, information on processes at surfaces, absolute ion survival probability, and kinematics of the collision was obtained. The projectile ion showed both inelastic, dissociative and reactive scattering, namely the occurrence of H-atom transfer reaction with hydrocarbons present on the room-temperature carbon surface. The absolute survival probability of the ions for the incident angle of 30° (with respect to the surface) decreased from about 1.0% (16 eV) towards zero at incident energies below 10 eV. Estimation of the effective surface mass involved in the collision process led to m(S)eff of about 57 a.m.u. for inelastic non-dissociative collisions of C2D4•+ and of about 115 a.m.u. for fragment ions (C2D3+, C2D2•+) and ions formed in reactive surface collisions (C2D4H+, C2D2H+, contributions to C2D3+ and C2D2•+). This suggested a rather complex interaction between the projectile ion and the hydrocarbon-covered surface during the collision.

Collisions of slow ions C3Hn+ and C3Dn+ (n = 2-8) with room temperature carbon surfaces: mass spectra of product ions and the ion survival probability

Collisions of C3Hn+ (n = 2-8) ions and some of their per- deuterated analogs with room temperature carbon (HOPG) surfaces (hydrocarbon-covered) were investigated over the incident energy range 13-45 eV in beam scattering experiments. The mass spectra of product ions were measured and main fragmentation paths of the incident projectile ions, energized in the surface collision, were determined. The extent of fragmentation increased with increasing incident energy. Mass spectra of even-electron ions C3H7+ and C3H5+ showed only fragmentations, mass spectra of radical cations C3H8*+ and C3H6*+ showed both simple fragmentations of the projectile ion and formation of products of its surface chemical reaction (H-atom transfer between the projectile ion and hydrocarbons on the surface). No carbon-chain build-up reaction (formation of C4 hydrocarbons) was detected. The survival probability of the incident ions, S(a), was usually found to be about 1-2% for the radical cation projectile ions C3H8*+, C3H6*+, C3H4*+ and C3H2*+ and several percent up to about 20% for the even-electron projectile ions C3H7+, C3H5+, C3H3+. A plot of S(a) values of C1, C2, C3, some C7 hydrocarbon ions, Ar+ and CO2+ on hydrocarbon-covered carbon surfaces as a function of the ionization energies (IE) of the projectile species showed a drop from about 10% to about 1% and less at IE 8.5-9.5 eV and further decrease with increasing IE. A strong correlation was found between log S(a) and IE, a linear decrease over the entire range of IE investigated (7-16 eV), described by log S(a) = (3.9 +/- 0.5)-(0.39 +/- 0.04) IE.

Collisions of Slow Polyatomic Ions with Surfaces: Dissociation and Chemical Reactions of C2H2+•, C2H3+, C2H4+•, C2H5+, and Their Deuterated Variants C2D2+• and C2D4+• on Room-Temperature and Heated Carbon Surfaces

Interaction of C2Hn+ (n = 2-5) hydrocarbon ions and some of their isotopic variants with room-temperature and heated (600 degrees C) highly oriented pyrolytic graphite (HOPG) surfaces was investigated over the range of incident energies 11-46 eV and an incident angle of 60 degrees with respect to the surface normal. The work is an extension of our earlier research on surface interactions of CHn+ (n = 3-5) ions. Mass spectra, translational energy distributions, and angular distributions of product ions were measured. Collisions with the HOPG surface heated to 600 degrees C showed only partial or substantial dissociation of the projectile ions; translational energy distributions of the product ions peaked at about 50% of the incident energy. Interactions with the HOPG surface at room temperature showed both surface-induced dissociation of the projectiles and, in the case of radical cation projectiles C2H2+* and C2H4+*, chemical reactions with the hydrocarbons on the surface. These reactions were (i) H-atom transfer to the projectile, formation of protonated projectiles, and their subsequent fragmentation and (ii) formation of a carbon chain build-up product in reactions of the projectile ion with a terminal CH3-group of the surface hydrocarbons and subsequent fragmentation of the product ion to C3H3+. The product ions were formed in inelastic collisions in which the translational energy of the surface-excited projectile peaked at about 32% of the incident energy. Angular distributions of reaction products showed peaking at subspecular angles close to 68 degrees (heated surfaces) and 72 degrees (room-temperature surfaces). The absolute survival probability at the incident angle of 60 degrees was about 0.1% for C2H2+*, close to 1% for C2H4+* and C2H5+, and about 3-6% for C2H3+.

Collisions of slow polyatomic ions with surfaces: the scattering method and results

Surface-induced dissociation (SID) and reactions following impact of well-defined ion beams of polyatomic cations C2H5OH+, CH4+, and CH5+ (and its deuterated variants) at several incident angles and energies with self-assembled monolayers (SAM), carbon surfaces, and hydrocarbon covered stainless steel were investigated by the scattering method. Energy transfer and partitioning of the incident projectile energy into internal excitation of the projectile, translational energy of products, and energy transferred into the surface were deduced from the mass spectra and the translational energy and angular distributions of the product ions. Conversion of ion impact energy into internal energy of the recoiling ions peaked at about 17% of the incident energy for the perfluoro-hydrocarbon SAM, and at about 6% for the other surfaces investigated. Ion survival probability is about 30-50 times higher for closed-shell ions than for open-shell radical cations (e.g., 12% for CD5+ versus 0.3% for CD4+, at the incident angle of 60 degrees with respect to the surface normal). Contour velocity plots for inelastic scattering of CD5+ from hydrocarbon-coated and hydrocarbon-free highly oriented pyrolytic graphite (HOPG) surfaces gave effective masses of the surface involved in the scattering event, approximately matching that of an ethyl group (or two methyl groups) and four to five carbon atoms, respectively. Internal energy effects in impacting ions on SID were investigated by comparing collision energy resolved mass spectra (CERMS) of methane ions generated in a low pressure Nier-type electron impact source versus those generated in a Colutron source in which ions undergo many collisions prior to extraction and are essentially vibrationally relaxed. This comparison supports the hypothesis that internal energy of incident projectile ions is fully available to drive their dissociation following surface impact.