Interaction of atomic hydrogen with metal surfaces

Received: 23 March 1998/Accepted: 21 August 1998     

Theory of surface processes

To understand kinetic processes at surfaces, a macroscopic theory is advanced based on the Onsager approach to nonequilibrium thermodynamics. A microscopic basis is then sought by re-examining the kinetic lattice gas model to generalize the underlying dynamics for adsorption, desorption and diffusion.Invited lecture presented at the workshop Dynamics and Kinetics of Interface Reactions (Appl. Phys. A 51/2+3)     

Angle and speed distributions of hydrogen desorbing thermally from metal surfaces

On the basis of the theory developed in a previous paper (Part I), the angle and speed distributions of D₂ molecules desorbing thermally from the Ni(111) surface are numerically calculated in the present paper (Part II). The dynamical motion of the activated complex of the recombinative desorption process is calculated by using a model potential surface, which involves a few parameters introduced to describe the necessary features of the potential surface at the transition state. Numerical calculations are carried out, and the results show that the present dynamical theory reproduces very well a variety of the characteristic features of the experimental results, e.g., sharply focused angular distribution, mean translational energy lower than 2k _B T at grazing angle, and the non-Maxwellian profile of the TOF distribution.     

Angle and speed distributions of hydrogen desorbing thermally from metal surfaces

A quantum theoretical treatment of the angle and speed distributions of recombinatively desorbing hydrogen from metal surfaces is proposed. The desorption rate is discussed in the framework of the transition state theory. The recombinative reaction process of hydrogen due to thermal activation leads to the formation of an activated complex in the transition state. In the vicinity of a saddle point on a three-dimensional potential energy surface, the translational motion of the activated complex in the direction perpendicular to the metal surface is accompanied by its center-of-mass vibrational motion parallel to the metal surface. In order to carry out the quantum mechanical calculation, the potential surface is replaced by a simplified model potential, which provides a square potential barrier along the surface normal. It is shown that, on leaving the potential barrier, the activated complex is reflected by the boundary of the potential barrier with a certain probability and, at the same time, the center-of-mass modes of vibration with frequencies v ₁ and v ₂ are coupled with the translational motion along the surface normal. Vibrational wave functions in the momentum representation are used to calculate the transmission coefficient, which is incorporated into the conventional rate formula. The angle-dependent speed distributions of desorbing molecules are derived from the rate formula.     

Kinetics of domain wall pinning in an incommensurate Pb overlayer on a Cu(110) surface

We have used grazing incidence X-ray scattering to study the kinetics of domain wall pinning during the transition between an incommensurate phase and a domain wall glass phase. The growth of both domain wall density and domain size are described by power-laws. The results are discussed in the context of recent theoretical models of growth.     

Combination of temperature-programmed thermal desorption and laser-induced thermal desorption

The combination of temperature-programmed thermal desorption with heating rates of the order of 1 K s^–1 and laser-induced thermal desorption with of the order of 10¹⁰ K s^–1 has proved to be a suitable method to determine both the frequency factor v and the activation energy E _d of desorption. The systems Ni(110)/N₂ and Ni(110)/CO serve as examples.     

On the adsorption and desorption of H2 at metal surfaces

Recent technological developments have made possible measurements of the distribution of internal levels of molecules desorbing from a hot surface. Such measurements provide new information concerning the desorption process and the potential energy surface (PES) that governs it. Associative, or re-combinative desorption is of particular interest because the distributions of internal levels reflect the manner in which the molecular bond is formed as the desorbing species leaves the surface. As the simplest associative desorption systems, H₂ and D₂ adsorbing on and desorbing from metal surfaces deserve special attention and serve as prototypes for systems with a more complex chemistry. In this note I review briefly from the theoretical point of view some features of the interaction of H₂ with metals and their relevance to associative adsorption and dissociative sticking.     

Investigation of self-organizing thiol films by optical second harmonic generation and X-ray photoelectron spectroscopy

The adsorption of self-organizing thiol films on polycrystalline gold substrates was investigated in situ by optical second harmonic generation. Growth of the films could be detected at submonolayer coverage. For a comparison with classical surface analytical methods the thiol coverage was measured ex situ by X-ray photoelectron spectroscopy. We find that formation of ordered thiol films up to monolayer coverage can be described by Langmuir adsorption kinetics.     

High-temperature desorption of benzene from zirconium surfaces

We report on the interaction of benzene with zirconium (0001) surfaces. Following adsorption at 150 or 170 K benzene desorbs near 715 K at exposures above one Langmuir. The high desorption temperature of benzene is indicative of the complicated kinetics that zirconium surfaces exhibit. For lower exposures benzene dissociates during heating and an increase of the oxygen content at the surface is detected. We propose that hydrogen from the dissociated layer(s) attracts subsurface oxygen and that an exchange of adsorbed carbon with this oxygen takes place.     

Chemical, energetic, and geometric heterogeneity of device-quality (1 0 0) surfaces of single crystalline silicon after HFaq etching

An analysis, based on angle-resolved X-ray photoelectron spectroscopy, multiple-internal-reflection infrared spectroscopy, and atomic force microscopy, of device-quality (1 0 0)silicon surfaces after etching in dilute aqueous solution of HF is presented. The analysis shows that the surface is mainly formed by a heterogeneous distribution of SiH, SiH₂and SiH₃ terminations, but contains sub-stoichiometric oxidized silicon. The analysis shows moreover the existence of a form of reduced silicon, not consistent with the currently accepted picture of the native HF_aq-etched surface.     

In situ investigation of the catalytic reaction H2+< ![IGNORE[O2→H2 O with second-harmonic generation

2 +O₂→H₂ O in the pressure range 0.2 Torr≤p_tot≤10 Torr on Pt(111) surface. At a catalyst temperature of T=700 K the equilibrium oxygen coverage θ_o is determined as a function of hydrogen partial pressure α. The experimentally obtained θ_o is modelled in a two step process considering the mass transport in the gas phase as well as the catalytic reaction on the surface. In this pressure range the mass transport in the gas phase changes from molecular flow conditions to laminar flow, inducing a strong modification of the gas phase present at the catalyst through different diffusivities of the reactants as well as through desorbing reaction products from the catalyst. It is shown that these gas phase alterations have to be taken into account for a proper modelling of the surface mechanism. Simulation calculations allow one to identify the sequential hydrogen addition reaction as the main reaction path for water production in this parameter range. Excellent agreement with previous investigations is obtained for the determined activation energies of the water-producing reaction steps equal to E^f _H2O≥0.7 eV. Received: 20 September 1998 / Revised version: 15 December 1998     

Steps,facets and nanostructures: investigations of Cu (11n) surfaces

Step structure and dynamics and adsorbate-induced reconstruction of stepped surfaces are useful features for investigating fundamental surface phenomena and their practical consequences. In this respect, vicinal Cu (11n) surfaces with n=3, 5 and 9 were studied by scanning tunneling microscopy (STM) at 300 K. While the regular monoatomic steps fluctuate for Cu (119), they are apparently stabilized for n≤5 by their strong repulsive interaction and this leads to a very low kink activity. In addition to the regular steps, the formation of double steps was observed as a particular phenomenon on those surfaces. These double steps determine the dynamic behavior at room temperature. By applying existing models for the fluctuations of the step positions in space and time, the formation energy for kinks at these double steps was determined to be 0.16 eV for Cu (115). According to this evaluation, diffusion along step edges is the dominating mass-transport mechanism. A model for the structure of the double steps and for the atomic displacement processes necessary for kink migration at these steps is presented. Complete faceting is observed for these surfaces upon oxygen adsorption at elevated temperatures (around 500 K) and was studied in detail for n=5 and 9. Both surfaces reconstruct into two types of {104} facets and a third facet, the orientation of which is determined by the macroscopic crystal orientation. The facet size is governed by the formation kinetics and can be controlled by varying the crystal temperature or the oxygen partial pressure. The formation kinetics is discussed as a nucleation and growth process and the relevant parameters are given. Facets within a range of size between 5 nm and 100 nm could thus be produced. They remained stable at ambient atmosphere, up to about 620 K, and also if covered by additional metal layers such as Ni. Received: 1 June 2001 / Accepted: 23 July 2001 / Published online: 3 April 2002     

Adsorption active sites -Key factor on the adsorption ability of pyridine molecules on Ag surfaces

Experimental results on the study of pyridine molecules adsorbed on Ag films and Ag foils under atmospheric and ultra-high vacuum environments by the secondharmonic generation technique suggest that micro-scaled active sites existing in the evaporated Ag film play the key role for this kind of adsorption.From Su-Zhou University     

The coefficient of surface stiffness for a pinned interface

We derive nonperturbatively — within the meanfield version of the Landau-Ginzburg-Wilson theory — the expressions for the constrained order parameter, for the coefficient of surface stiffness, and for the effective interface potential of a fluctuating interface in the presence of a flat substrate. The derived expressions display the typical dependence on the distancel between the interface and the substrate; the exponentially decaying contributions are multiplied by polynomials inl. We show that the expressions for the coefficient of surface stiffness and for the effective interface potential have identical form as those proposed recently by Jin and Fisher (Phys. Rev.B 47, 7365 (1993)).Dedicated to Herbert Wagner on the occasion of his 60th birthday     

Influence of steps and defects on the dissociative adsorption of molecular hydrogen on silicon surfaces

-4 . Neither the absolute values nor the temperature dependence of adsorption on the terraces are affected by the misorientation. Received: 3 November 1998     

Electronically stimulated desorption

In the valence region, desorption of neutrals predominates; nevertheless most research has been on ions up to recently. New methods have made the investigation of neutrals easier, and angle as well as energy distributions have been reported; on this basis the salient mechanisms can be deduced both for chemisorbed and physisorbed layers. Desorption of ions and, in particular, of fragment ions, is strongly enhanced following higher excitations. These can most easily be reached via Auger decay of adsorbate core holes. Considerable detail of understanding is obtainable through polarization-dependent measurements with synchrotron radiation. The importance of many-body interactions at surfaces (increased localization by hole-hole interaction; increased delocalization by screening) becomes obvious. The field of stimulated desorption has profitted strongly from the improved understanding derived from the mature state of surface spectroscopies.