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300 600 900 1200 edges te rrac e s Q M S am u 4 signal 0 100 200 300 T = 150 K D /graphite(0001) H 2.9 2.0 1.4 0.9 0.6 0 .23 0 .12 0 .06 0 .03 H D (g ) H = 4 .8*1 0 13 cm -2 s -1 D preexposure [M L] Q M S sig n a la m u 3 tim e [s] 450 500 550 D tem p . = 1 8 0 0 ...2 2 0 0 K graphite(0001) /A rrhenius analysis TD S /D ln ads D tem perature,1/T [10 -6 K -1 ] 0 1000 2000 3000 T ad = 150 K +H +D 130 15 eV x 50 C -H 1210 C -D 640 2650 C -H clean C -D 1950 phonons su rface in ten sity en erg y lo ss [cm -1 ] Adsorption and recombination via abstraction of H(D) on graphite (0001) surfaces Thomas Zecho 2 and Jürgen Küppers 1,2 1 Experimentalphysik III, Universität Bayreuth, D-95440 Bayreuth, 2 Institut für Plasmaphysik, D-85748 Garching, EURATOM Association Experimental setup kshop: Solid State Astrochemistry of Star Forming Regions – 17 April 2003, Leiden University oxidiced in air ZYH quality ZYA quality natural graphite flake Surface characterisation (SEM) [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) sis of thermal desorption from terraces 200 400 600 200 400 600 oxidiced in air ZYA/H temperature [K] dimer monomer suggested reconstruct ion: 200 400 600 Q M S am u 4 signal mixed graphite flake increasing D exposure 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 shape of desorption peak does not vary with surface quality higly oriented pyrolytic graphites (HOPGs) crystalline quality graphite flake H/D source The study was carried out in a UHV system with a base pressure of < 1*10 -10 torr, equipped with: • QMS (direct product detection) • TDS (thermal desorption spectroscopy) • AES, EELS, HREELS (electron spectrosopies) • H/D source 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. The H(D) atoms were produced by dissociation of H 2 (D 2 ) in a hot tungsten capillary. Thermal desorption from edges only surface with detectable edge desorption 300 600 900 1200 CD CD 2 CD 3 Q M S am u 4 signal CD 3 am u 19 C 2 D x am u 32 Q M S signal 300 600 900 1200 C 3 D x am u 48 temperature [K] x 10 Adsorption on terraces sp 2 0 10 20 coverage E p = 65 eV T = 150 K + flash 710 K -1450 K + flash 550 K + flash 480 K 6.5 eV - p lasm on satu ratio n in ten sity en erg y lo ss [eV ] surface effect E p variation 50 100 150 10 15 20 in te n s ity d e c re a s e [% ] E p [eV ] calculations (1/8 ML) by Jackson et al.: C-H(D) C-H(D) 1100(760) 2600(1900) HREELS EELS Abstraction from terraces 0,0 0,2 0,4 0,6 0 4 8 12 16 abs [A 2 ] in itia lD c o v e ra g e [M L] monomer dimer HOPG-ZYH 200 400 600 ? 630 K T = 150 K 540 K 580 K 490 K D 5.8 2.3 1.2 0.6 0.3 0.1 exposure [M L] Q M S signalam u 4 tem p eratu re [K ] 200 400 600 = 1 K /s T = 150 K H 5.8 1.2 0.6 0.3 0.1 0.05 560 K 500 K 445 K exposure [M L] Q M S signalam u 2 tem p eratu re [K ] Thermal desorption from terraces 3 desorption states sat = 0.2...0.4 ML strong isotope effect 0 20 40 60 80 100 0,0 0,5 1,0 1,5 o xid ice d in a ir g ra p h ite fla ke , D ZYH +2000°C ,D ZYH,D ZYH,H ZYA,D E act [eV ] coverage [% ] Activation barrier monomer desorption isotope effect coverage dependence D H HD Eley-Rideal + steering effect D D abs HOPG-ZYH HOPG-ZYH 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 thermal H(D) hydrogen and traces of hydrocarbon desorption at elevated temperatures + kT thermal desorption adsorption abstraction edge terrace reconstruction 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]. analysed with first order kinetics: adsorption: d[CD](t)/dt = σ ads Φ [C](t) abstraction: d[D2](t)/dt = σ abs Φ [CD](t) solution: [CD](t) = [CD] (1 – exp[ – (σ ads + σ abs ) Φ t ] ) with saturation coverage: [CD] = [C] 0 σ ads /(σ ads + σ abs ) Arrhenius description: σ ads = σ 0 exp[ -E act / (kT D ) ] H(D) uptake kinetics Activation barrier adsorption Analysis of saturation coverages (TDS) after dosing with variable D temperatures E act ~ 0.18 eV ~ every D with E > E act sticks HOPG-ZYA oxidiced in air hydrogen and hydrocarbon desorption from terrace edges x 10 direct evidence of H(D) adsorption on the graphite basal plane (1800 – 2200) K 10 m
