A. Winkler

Wide range nozzle beam adsorption data for the systems H2/nickel and H2/Pd(100)

We have performed angle resolved measurements of the sticking coefficient for H2 on Ni(111), Ni(110), Ni(100) and Pd(100). Beam energies of 100 K ⩽ E/2k ⩽, 2800 K were used. For H2/Ni(111) normal energy scaling is obeyed; all available data on adsorption/desorption kinetics can be quantitatively explained assuming a distribution of one-dimensional activation barriers. On the other surfaces there exists a mixture of activated and non-activated adsorption paths. Adsorption of sulfur on Ni(110) and Pd(100) preferentially blocks the nonactivated path leading to predominantly activated adsorption. In all cases quantitative agreement with time-of-flight measurements obtained in permeation/desorption was observed. Previous discrepancies between low and high temperature data could be removed to obtain a consistent description of adsorption/desorption kinetics.

Kinetics and energetics of oxygen adsorption on Pt(111) and Pt(112)- A comparison of flat and stepped surfaces

A comparative study of oxygen adsorption on Pt(111) and Pt(112) has been performed using temperature programmed desorption, isothermal desorption, Auger spectroscopy, LEED and isotopic measurements. On Pt(112) three molecular adsorption states (α1, α2, α3) and two atomic adsorption states (β1, β2) have been found. The β2-state exhibits repulsive lateral interaction whereas the β1-state shows attractive interaction. The adsorption kinetics at Tad = 87 K involves a precursor state. For Pt(112) at 87 K, the sticking coefficient is 0.97 at zero coverage and remains constant in the low coverage regime. On Pt(111) at 87 K, the sticking coeffient increases with increasing oxygen coverage at low coverage, with s0 = 0.29. This suggests that empty Pt sites near an O2-covered Pt site experience an enhanced reactivity with O2. Tad = 300 K the adsorption kinetics are governed by direct dissociative adsorption with an activation barrier of ≈2 kal/mol on Pt(111), yielding an initial sticking probability of 0.05, whereas a complicated adsorption behavior is obtained for Pt(112) with s0 = 0.53. The conversion of molecular oxygen into atomic oxygen is discussed as well as the influence of subsurface oxygen and “clean-off” effects on the adsorption kinetics.

Adsorption kinetics for hydrogen adsorption on nickel and coadsorption of hydrogen and oxygen

Abstract The influence of surface structure and surface impurities on adsorption kinetics has been investigated by measurements of the adsorption of hydrogen and of the coadsorption of hydrogen and oxygen on various nickel surfaces. The initial sticking coefficients for hydrogen on Ni(111), Ni(S)-[8(111) × (100)] and Ni(110) are found to have the values 0.05, 0.24 and 0.96, respectively. The saturation coverage of hydrogen amounts to 1.0 monolayer on Ni(111), 1.1 monolayers on Ni(S)-[8(111) × (100)] and 2.3 monolayers on Ni(110). Coadsorption supplies information about the influence of surface contamination on the sticking coefficient. In general, contamination on the surface reduces the sticking coefficient. In the special case of hydrogen adsorption on Ni(111) the initial sticking coefficient is increased by preadsorption of oxygen. This behavior can be described by involving a precursor state and two different types of adsorption sites on the surface. It is found that there exists a finite probability for dissociation even on occupied adsorption sites.

A search for vibrational contributions to the activated adsorption of H2 on copper

Abstract We have performed seeded-beam experiments to separate the influence of kinetic energy and vibrational energy in the sticking of H 2 on copper. There is a contribution of vibration (about 20%) to the sticking coefficient. A widening of the angular variation of the sticking coefficient for the vibrationally hot beam indicates coupling of the translational and vibrational energy. Finally, the isolated effect of vibration shows a slight isotope effect favoring H 2 over D 2 ; this points towards tunneling during adsorption.

The sticking coefficient of H2 on Ni(111) as a function of particle energy and angle of incidence: A test of detailed balancing

Abstract The sticking coefficient of H 2 /Ni(111) changes proportionally to the beam energy. The angular distribution of the probability of adsorption varies with cos 3.5 θ; the angular distribution of desorption is found to change as cos 4.5 θ at 300 K. Assuming validity of detailed balancing, the adsorption data suggest an energy distribution for desorption which agrees with existing time-of-flight measurements.

Adsorption and desorption dynamics as seen through molecular beam techniques

Abstract A description of the history of knowledge about adsorption and desorption dynamics is given. The individual stations include the encounter with non-cosine, non-Maxwellian distributions of adsorbing and desorbing particles; detailed balancing in its development as a tool to relate adsorption and desorption data is described. A further section treats the concept of precursor mediated adsorption and its verification by molecular beam methods. The problem of surface defects is briefly touched. Refinements in the molecular beam techniques finally lead to the possibility to gain state resolved dynamics data for adsorption and desorption processes.

The role of surface defects in the adsorption and sesorption of hydrogen on Ni(111)

On a smooth Ni(111) surface both the β 1 and the β 2 state show activated adsorption. The desorption flux is highly peaked towards the surface normal. Defects introduce non-activated adsorption sites which lower the desorption temperatures in flash desorption. At the same time a cosine dependence of the desorption flux is obtained. Because of its small sticking coefficient the β 1 state is already influenced by small defect concentrations; the β 2 state is only affected on highly defective surfaces.

Quantitative characterization of a highly effective atomic hydrogen doser

An atomic hydrogen doser of the Bertel type was characterized in terms of the degree of dissociation and angular distribution of the effusing particles. In this doser hydrogen is dissociated in a tungsten tube which is heated by electron bombardment. Various experimental techniques were used to determine the degree of dissociation as function of temperature and gas flux. It is shown that simple equilibrium considerations cannot be applied to obtain the degree of dissociation accurately. Nevertheless, for sufficiently small gas flux and temperatures above 1850 K, the degree of dissociation approaches 100%. The angular distribution was determined by a gold foil on a goniometer as detector, which is sensitive to atomic hydrogen only. The experimental results were compared with Monte Carlo simulations. A strongly forward focused distribution is observed which allows efficient atomic hydrogen dosing. This doser was used to measure absolute initial sticking coefficients for atomic hydrogen on various single cry...

Reaction kinetics of atomic hydrogen with deuterium on Ni(110)

Abstract The interaction of atomic hydrogen with clean and deuterium precovered Ni(110) surfaces at 150 K has been investigated using thermal desorption mass spectrometry. The initial sticking coefficient of atomic hydrogen is 0.9 ± 0.1. Above saturation at 1.5 monolayers for dissociative H2 adsorption, atomic hydrogen absorption leads to the occupation of subsurface sites with a penetration probability of 1 × 10−2. Impingement of atomic hydrogen on a deuterium covered Ni(110) surface leads to deuterium abstraction via HD production with a removal coefficient of 0.26 HD molecules per H-atom (Eley-Rideal mechanism). In addition, removal of deuterium by associative D2 desorption has been observed at Ts = 150 K, with a removal coefficient of 0.04 D2 molecules per H-atom (collision induced recombinative Langmuir-Hinshelwood desorption). Interaction of atomic deuterium with a hydrogen covered surface yields an HD removal coefficient of 0.38 HD molecules and an H2 removal coefficient of 0.06 H2 molecules per D-atom.

Aluminum hydride desorption from hydrogen covered aluminum single crystal surfaces

Formation and desorption of aluminum hydride from hydrogen covered Al(110), Al(100), and Al(111) has been investigated, using thermal desorption spectroscopy. Both desorption of aluminum hydride and molecular hydrogen has been detected with branching ratios depending on surface structure. Production of aluminum hydride is negligible on the rough (110) surface, whereas on the flat (111) plane up to 50% of the adsorbed hydrogen is desorbed in form of aluminum hydride. Furthermore, aluminum hydride formation is strongly enhanced with an increase of the heating rate. Desorption of molecular hydrogen follows a close to zero order reaction, with a desorption energy around 17±1 kcal/mol. Aluminum hydride desorption can be described best by a fractional order (≊0.5), with a desorption energy of 27±1 kcal/mol. Angular distribution measurements reveal desorption distribution functions of D(Θ)≊cos Θ−cos3 Θ for aluminum hydride and D(Θ)≊cos2 Θ−cos15 Θ for molecular hydrogen, strongly dependent on surface structure.