Adsorption and recombination via abstraction of H(D) on graphite (0001) surfaces

1. Adsorption and recombination via abstraction of H(D) on graphite (0001) surfaces Thomas Zecho2 and Jürgen Küppers 1,2 1 Experimentalphysik III, Universität Bayreuth, D-95440 Bayreuth, 2 Institut für Plasmaphysik, D-85748 Garching, EURATOM Association n d sp2 H HD s abs calculations (1/8 ML) by Jackson et al.: d C-H(D) n C-H(D) D D D 1100(760) 2600(1900) Eley-Rideal + steering effect The study was carried out in a UHV system with a base pressure of < 1*10-10 torr, equipped with: H/D source • QMS • (direct product detection) • TDS • (thermal desorption spectroscopy) • AES, EELS, HREELS • (electron spectrosopies) • H/D source The H(D) atoms were produced by dissociation of H2(D2) in a hot tungsten capillary. graphite flake The graphite crystals were clamped via tungsten wires to a precision manipulator. Resistive heating and cryocooling did allow sample temperatures between 80 K and 1600 K. suggested reconstruction: ZYA/H oxidiced in air monomer mixed dimer temperature [K] Workshop: Solid State Astrochemistry of Star Forming Regions 14 – 17 April 2003, Leiden University Experimental setup Surface characterisation (SEM) natural graphite flake Based on recent theoretical and experimental work [1,2,3] it is now established that H chemisorbs on top of a C of the graphite basal plane. The C atom moves out of the plane by about 0.4 Å, causing an activation barrier of about 0.2 eV. The subsequent recombination reaction higly oriented pyrolytic graphites (HOPGs) abstraction (1800 – 2200) K thermal H(D) ZYA quality hydrogen and traces of hydrocarbon desorption at elevated temperatures terrace reconstruction crystalline quality adsorption + kT thermal desorption edge ZYH quality 80 K – 1600 K via abstraction of H(D) is dominated by a strong attractive interaction between the impinging and chemisorbed hydrogen atom. This leads to a steering effect and to a high recombination cross section at low H(D) precoverages [4,5]. oxidiced in air x 10 10 mm Adsorption on terraces Thermal desorption from terraces Abstraction from terraces HOPG-ZYH HOPG-ZYH HOPG-ZYH H(D) uptake kinetics HREELS analysed with first order kinetics: Thermal desorption from edges adsorption: d[CD](t)/dt = σads Φ [C](t) oxidiced in air abstraction: d[D2](t)/dt = σabsΦ [CD](t) only surface with detectable edge desorption solution: [CD](t) = [CD]¥ (1 – exp[ – (σads + σabs ) Φ t ] ) with saturation coverage: [CD]¥ = [C]0 σads /(σads + σabs ) x 10 Activation barrieradsorption Analysis of saturation coverages (TDS) after dosing with variable D temperatures EELS HOPG-ZYA Ep variation Eact~ 0.18 eV surface effect Arrhenius description: σads = σ0 exp[ -Eact / (kTD) ] temperature [K] direct evidence of H(D) adsorption on the graphite basal plane • 3 desorption states ~ every D with E > Eact sticks hydrogen and hydrocarbon desorption from terrace edges • Qsat = 0.2...0.4 ML • strong isotope effect Analysis of thermal desorption from terraces Activation barrier monomer desorption shape of desorption peak does not vary with surface quality graphite flake Summary • first experimental evidence of H(D) adsorption and • abstraction reactions on the graphite basal plane • measurements are in excellent agreement with theory • additional dimer reconstruction structure is suggested increasing D exposure [1] L. Jeloaica, V. Sidis, Chem. Phys. Lett. 300, 157 (1999) [2] X. Sha, B. Jackson, Surf. Sci. 496, 318 (2002) [3] T. Zecho, A. Güttler, X. Sha, B. Jackson, J. Küppers, J. Chem. Phys. 117, 18 (2002) [4] X. Sha, B. Jackson, J. Chem. Phys. 116 (16), 7158 (2002) [5] T. Zecho, A. Güttler, X. Sha, D. Lemoine, B. Jackson, J. Küppers, Chem. Phys. Lett. 366, 188-195 (2002) monomer dimer • coverage dependence • isotope effect