The kinetics of adsorption and abstraction for the system H(D) on Al(111) and Ni(111) has been investigated using an efficient atomic beam source, thermal desorption spectroscopy, and multiplexed mass spectrometry. The initial sticking coefficient of atomic hydrogen on Ni(111) is 1.0 and independent of the angle of incidence. For H on Al(111) the initial sticking coefficient is 0.6 and increases with increasing angle of incidence (S(φ)∼cos−0.4 φ). On Al(111) preadsorbed oxygen leads to a decrease of the initial sticking coefficient for H down to 0.1; potassium on the other hand has no significant influence on the initial sticking probability. On both surfaces, Ni(111) and Al(111), abstraction of deuterium proceeds at a surface temperature of 150 K due to impinging H atoms (H+D→HD), with an initial abstraction coefficient of 0.20 on Al(111) and 0.12 on Ni(111). In the case of Al(111) this coefficient is nearly independent of the initial D coverage and therefore the abstraction reaction cannot be described by a simple Eley–Rideal process, but rather by a hot-precursor mechanism. In addition to abstraction there is also a small probability for the removal of an adsorbed species by a collision induced desorption process. Abstraction of deuterium by impinging H-atoms is strongly affected by modification of the Al(111) surface, either by oxygen or by potassium. Oxygen decreases the HD abstraction coefficient, probably by inhibiting the hot-precursor state of H on the surface. Potassium increases the HD abstraction rate, most likely by increasing the lifetime of atomic hydrogen in the precursor state.

Adsorption and desorption of water as well as the interaction of water with atomic deuterium on a clean and oxidized Al(111) surface has been studied using temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). Quantitative determination of the sticking coefficient for water reveals a value of unity on the clean and the oxidized surface. In the desorption spectra three different peaks of water and hydrogen can be observed. The alpha-peak for water at 160 K can be attributed to evaporation of water multilayers. Already at this low temperature parallel dissociation of water leads to the formation of aluminum hydroxides. The simultaneous desorption of molecular hydrogen can be explained by a reaction assisted process, due to the formation of hot H-atoms during the dissociation process of water. The beta-peak at about 320 K and the gamma-peak at 650 K, both for water and hydrogen, are due to decomposition/recombination of different aluminum hydroxides. Dosing a thick ice layer with atomic deuterium reveals only a weak interaction. The reaction coefficient for dissociation of the water molecule or the replacement of an H-atom of the water by a D-atom is about 4%. (C) 1999 Elsevier Science B.V. All rights reserved.

We have investigated the desorption kinetics of subsurface hydrogen (deuterium) from Ni(111) with angle resolved thermal desorption spectroscopy. For the first time we directly compare the desorption from subsurface states populated either by dosing with hydrogen atoms or by implantation of molecular hydrogen ions. For the various dosing modes subsurface desorption proceeds at clearly different temperatures. The angular distribution of the desorption probability for all subsurface states can be described by a cosnΘ function with n<1, whereas desorption from the surface states is strongly forward focused (n=4.5). The initial sticking coefficient of H (D) on the clean Ni(111) surface is 1.0 and independent of the angle of incidence. The “initial absorption” coefficient for atomic hydrogen at the H-saturated surface is about 0.1. Interaction of atomic hydrogen with the H-saturated surface takes place in form of abstraction, adsorption and absorption simultaneously.