K.D. Rendulic

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.

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.

Strong rotational effects in the adsorption dynamics of H2/Pd(111): evidence for dynamical steering

Abstract We have used seeded beam techniques to determine the influence of the rotational energy of hydrogen and deuterium molecules on the adsorption dynamics. A strong decrease of sticking with increasing rotational energy has been observed. In the low translational energy regime where the sticking coefficient decreases with beam energy the strongest effects are encountered. This result points to the importance of dynamical steering rather than to adsorption via a classical molecular precursor. Because rotational energy is quantized, clear isotope effects between hydrogen and deuterium can be observed.

An investigation of vibrationally assisted adsorption: the cases H2/Cu(110) and H2/Al(110)

Abstract In the system H 2 /Cu(110) vibrationally assisted adsorption has been investigated by the use of seeded beams. Through a measurement of the internal energy contribution to adsorption at a constant translational kinetic energy of the hydrogen beam, the vibrational state of the adsorbed hydrogen can be determined. The seeded beam with excess vibrational energy adsorbs in the low translational kinetic energy range for H 2 as v = 1 whereas deuterium prefers the state v = 2. To obtain similar adsorption rates for H 2 /Al as in the case of H 2 /Cu one needs about 0.2 eV additional translational kinetic energy. For the adsorption of hydrogen on aluminum no vibrational contribution can be detected within the margin of error.

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.

Adsorption dynamics for the system hydrogen/palladium and its relation to the surface electronic structure

Abstract We have determined differential sticking coefficients for a monoenergetic nozzle beam of hydrogen on Pd(111) and Pd(110). In particular the energy dependence and the angular variation of the initial sticking coefficient were measured. The results indicate that adsorption of hydrogen on palladium occurs in parallel processes through a direct path with an activation barrier of perhaps 50 meV or less and a precursor path. There is relatively little difference in the adsorption properties of the (111) and the (110) plane. The appearance of a molecular precursor on the (111) plane can be related to the electronic structure of palladium, in particular to the absence of occupied Shockley surface states, as compared to Ni (111) and Pt (111). Pre-adsorbed potassium on a (110) plane acts as an inhibitor to adsorption. Different inhibiting mechanisms are observed for the direct adsorption path and the precursor path. At high potassium coverage the precursor path is completely suppressed.

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.

Sticking and desorption: a review

Abstract A systematic description of the adsorption dynamics as seen in different adsorption experiments is presented. A hierarchy of adsorption experiments is described, starting with molecular beam techniques involving a large number of variables as particle energy, angle of incidence and quantum state of the impinging molecule. The classification continues through adsorption of Maxwellian beams experiments with isotropic gas supply to equilibrium investigations. The application of the principle of detailed balancing is used to relate a particular adsorption experiment to a corresponding desorption experiment. A specific adsorption/desoprtpion dynamics observed in the experiments is brought in relation to microscopic processes on the surface.