J. Küppers

Adsorption of hydrogen and deuterium atoms on the (0001) graphite surface

Adsorption of H and D on HOPG surfaces was studied with thermal desorption (TDS), electronic (ELS), and high-resolution electron-energy-loss (HREELS) spectroscopies. After admission of H (D) from thermal (2000 K) atom sources to clean graphite surfaces TD spectra revealed recombinative molecular H2 (D2) desorption in a main peak around 445 K (490 K) and a minor peak at 560 K (580 K). After admission of higher fluences the main peak shifts to 460 K (500 K) and develops a shoulder at 500 K (540 K). The saturation coverages were calculated as 0.4±0.2 for H and D and initial sticking coefficients of 0.4±0.2 were obtained. Through leading edge analysis of the TD spectra desorption activation energies for H and D were determined as 0.6 and 0.95 eV, respectively. EL spectra suggest a 16% loss of the sp2 character of the surface carbon 2sp electrons upon D adsorption. HREEL spectra of H (D) graphite covered surfaces reveal in addition to two graphite-intrinsic optical phonon losses vibrational features at 1210 an...

The hydrogen surface chemistry of carbon as a plasma facing material

Recent progress in plasma performance in experiments on controlled thermonuclear fusion will lead to next-generation fusion experiments with the ultimate goal to make a power-generating fusion reactor reality. A major issue in the design of fusion experiments is the selection of the first wall material. Due to its outstanding thermal properties and its low Z carbon is and will be a first choice material, either elemental or in compounds and composites. Given the fact that in magnetically confined hydrogen plasmas of high density and temperature substantial wall fluxes of H species occur, the hydrogencarbon surface chemistry, in particular chemical erosion and H retention, becomes a concern. Although for more than a decade materials science studies on the Hcarbon interaction have been performed, the elementary reaction steps of this interaction have become clear only recently. This report reviews work performed on the Hcarbon surface chemistry under the aspects of chemical erosion. It presents in detail model studies directed towards identifying elementary reaction steps. Related fields, e.g. the HC surface chemistry as relevant for the production of hard a-C:H coatings and low pressure diamond synthesis are covered in the review.

Interaction of H atoms with Cu(111) surfaces: Adsorption, absorption, and abstraction

The interaction of H (D) atoms with clean and D (H) covered Cu(111) surfaces was studied with TDS and direct product detection methods. H (D) atoms exhibit an initial sticking coefficient of 0.22. Due to abstraction, the surface saturation coverage is achieved at Θ=0.34, significantly less than the half monolayer coverage obtained through exposure of energetic H2 molecules to Cu(111) surfaces. Adsorbed H (D) desorbs recombinatively between 250 and 400 K. Desorption of absorbed H (D) via gaseous H2(D2) around 200 (210) K was observed according to a zero-order rate law with an activation energy of 0.40 (0.35) eV. Abstraction of D (H) by H (D) at 80 K lead to gaseous HD and D2(H2) formation. About 1% of the adsorbed species occurred in homonuclear products. Throughout the abstraction reaction the HD rate was found strictly proportional to coverage and flux, in line with a purely quasifirst-order, exponentially decreasing Eley–Rideal-type product rate. However, this phenomenology as well as the occurrence of ...

H atom impact induced chemical erosion reaction at C:H film surfaces

Abstract C:H film surfaces which are subjected to a flux of thermal H atoms erode chemically via hydrocarbon, probably methyl, production. At the present H flux the erosion reaction is effective above 400 K and below 700 K, with a maximum around 600 K. The erosion efficiency at this temperature is ≈ 0.01 C atom per incoming H. A kinetic analysis of the erosion reaction and competing hydrogenation and dehydrogenation surface reactions under impact ofH reveals an activation energy of ≈37 kcal mol for the H atom impact induced erosion. As the efficiency of the erosion reaction depends on the incoming H flux, it may contribute as an important reaction in low-pressure diamond synthesis.

Abstraction of D adsorbed on Pt(111) surfaces with gaseous H atoms

The impact of gaseous H atoms at D covered Pt(111) surfaces at 85 K leads to the formation of gaseous HD and D2 products. The kinetics of formation of these products was measured simultaneously with H exposure for different initial D coverages. The HD and D2 rates as a function of H fluence from the reaction start exhibit common characteristics; a rate step, a rate maximum, and a subsequent exponential rate decay. The HD and D2 rate steps were observed not to scale linear with the D coverage and to increase if on D covered surfaces H was coadsorbed prior to reaction. Of the observed phenomena, only the exponential decay of the HD rate is in line with expectations if an Eley–Rideal mechanism acts in the present reaction. D2 formation, the HD rate step as a function of D coverage, and the presence of a H coadsorbate contradict the Eley–Rideal picture. The results suggest that the reactions towards HD and D2 proceed via hot atom type mechanisms.

A kinetic study of the interaction of gaseous H(D) atoms with D(H) adsorbed on Ni(100) surfaces

The kinetics of reactions which occur upon subjecting D(H) covered Ni(100) surfaces with H(D) atom fluxes were investigated. At 120 K surface temperature in the H→Dad reaction HD and D2 were observed as reaction products, in the D→Had reaction HD and H2 were reaction products. As the reaction temperature was well below the hydrogen desorption temperature, a direct reaction step, like in the Eley–Rideal (ER) mechanism, is suggested to operate for HD production. However, the characteristics of the HD formation kinetics observed in the present study contradict an essential element of the ER: mechanism the rate of HD formation is not proportional to the surface coverage of the adsorbed reaction species D or H under impact of a flux of H or D atoms. Therefore, a modification of the mechanistic description of atom/surface reactions seems necessary. This modification should allow for reaction products which are completely unaccounted for in the ER picture: D2 from H→Dad and H2 from D→Had reactions. The observed ...

A hot-atom reaction kinetic model for H abstraction from solid surfaces

Measurements of the abstraction reaction kinetics in the interaction of gaseous H atoms with D adsorbed on metal and semiconductor surfaces, H(g)+D(ad)/S→ products, have shown that the kinetics of the HD products are at variance with the expectations drawn from the operation of Eley–Rideal mechanisms. Furthermore, in addition to HD product molecules, D2 products were observed which are not expected in an Eley–Rideal scenario. Products and kinetics of abstraction reactions on Ni(100), Pt(111), and Cu(111) surfaces were recently explained by a random-walk model based solely on the operation of hot-atom mechanistic steps. Based on the same reaction scenario, the present work provides numerical solutions of the appropriate kinetic equations in the limit of the steady-state approximation for hot-atom species. It is shown that the HD and D2 product kinetics derived from global kinetic rate constants are the same as those obtained from local probabilities in the random walk model. The rate constants of the hot-atom kinetics provide a background for the interpretation of measured data, which was missing up to now. Assuming that reconstruction affects the competition between hot-atom sticking and hot-atom reaction, the application of the present model at D abstraction from Cu(100) surfaces reproduces the essential characteristics of the experimentally determined kinetics.

The role of sticking and reaction probabilities in hot-atom mediated abstraction reactions of D on metal surfaces by gaseous H atoms

Recent experiments on the abstraction of D adsorbed on metal surfaces with gaseous hydrogen atoms revealed a kinetics of HD formation which is not compatible with the operation of Eley–Rideal (ER) mechanisms. Furthermore, homonuclear products were observed during abstraction, which are not expected through an ER reaction scheme. It was therefore suggested that hot-atom (HA) mechanisms are more appropriate to explain the measured kinetics and products. Random walk calculations of the abstraction kinetics are presented based on a model which exclusively relies on elementary reaction steps which are HA mediated processes. Within this model, the ratio of two variables, the probabilities for hot-atom sticking at empty sites ps and hot-atom reaction with adsorbed species pr, was found to control the kinetics of HD and D2 formation. The essential features of measured kinetic data at Ni(100), Pt(111), and Cu(111) surfaces were reproduced by simple and reasonable assumptions on ps/pr.

Abstraction of D chemisorbed on graphite(0001) with gaseous H atoms

Abstract The kinetics of HD formation during admission of H atoms to D covered HOPG surfaces was measured at 150 K as a function of the initial D coverage. The phenomenology of the HD kinetics is as expected for an Eley-Rideal reaction or hot-atom reaction scenario with preference for reaction over sticking for hot atoms. It can be described by an abstraction cross-section, which varies between 17 (low coverage) and 4 A 2 (high coverage). The large abstraction cross-section at low coverages is in accordance with a steering effect of the adsorbed D on the incoming H as predicted by theory.

Interaction of thermal H atoms with Ni(100) H surfaces: through surface penetration and adsorbed hydrogen abstraction

Abstract Considerable quantities, > 3 monolayers, of hydrogen (deuterium) were absorbed in subsurface sites at Ni(100) surfaces via impact of thermal H (D) atoms. Competitive to a direct transition (penetration) into the bulk with an initial probability of 4.5 × 10 −2 impinging H (D) atoms abstract adsorbed D (H) with an initial abstraction probability R = 0.25 and cross section of 1.6 A 2 . The filling of subsurface sites obeys a Langmuir-type rate law. An adsorbed H (D) monolayer does not affect the probability for penetration into the bulk. Above 150 K subsurface H (D) move to the surface and either recombine with adsorbed H (D) followed by recombinative desorption or get adsorbed at the surface if they encounter an empty site. The desorption kinetics from subsurface sites is in quantitative agreement with thermodynamic bulk Ni H data. Isotope effects were neither observed for bulk penetration nor for abstraction.