Adsorption LEED experiments

Understanding of adsorption and of catalysis has been hindered by an oversimplified viewpoint, lack of detailed theory, and ignorance of surface structure. Our present knowledge of surface chemistry is still primitive, and only recently have we realized that there exists a variety and complexity of surface structure heretofore unexplored and hidden. The richness of adsorption phenomena has now become obvious, particularly from recent experiments using low energy electron diffraction (LEED). Adsorption is a much more complex process than imagined in the past a substantial fraction of our modern knowledge has come directly from LEED experiments and is very recent. 

The concept of surface reconstruction is a radical departure from classically held ideas about adsorption. Traditionally the term adsorption has implied that the substrate lattice is undisturbed by the arrival of adatoms, though small displacements or perturbations of substrate atoms are not necessarily ruled out. What has been meant classically is that the substrate lattice is never disconnected by adsorption that is, substrate atoms are never forced to move to altered positions associated with different lattice sites of the substrate. In surface reconstruction just this kind of disconnection is proposed. Many observers have concluded from LEED experiments that adsorbed atoms can be incorporated into a mixed layer containing foreign atoms as well as atoms of the host, with structures not resembling any of the three-dimensional compounds of the same atoms. Formation of such a two-dimensional layer having... 

The conclusion that reconstructive adsorption can occur is a controversial one, and many well-known workers have not accepted it (178). A resolution of the pros and cons still remains in limbo because convincing direct proof has not been easy to obtain, mainly because of the difficulties with interpretation of LEED intensities. However, there is a cumulative weight of indirect evidence, from sources other than LEED as well as from LEED experiments, that favors the reconstructionist position when the foreign atoms are especially reactive with the substrate. 

The similarity of behavior of NH3 on the W(IOO) and W(112) faces is illustrated by comparing the two sets of LEED experiments. In both of them adsorption of NH3 on the clean surface was followed by thermal breakup resulting from heating. Desorption into a mass spectrometer and work function measurements were made concurrently. 

LEED experiments are usually performed in a UHV chamber that is maintained at pressures below 10 mbar ( 10 Pa). The maintenance of a sufficiently low pressure is important to avoid residual gas adsorption and thus to keep the surface free of impurity during the measurement. In fact, a surface can be contaminated by a monolayer of gas in 1 s with a sticking coefficient of 1 with an ambient pressure of 10 mbar. Thus, the timescale of the experiment may be estimated to be 1000 s before the surface is contaminated with one monolayer of the residual gas. 

Based on electrochemical experiments combined with ex situ analysis by AES, LEED, and RHEED, Wang et al. (2001) suggested the formation of a (2 x 2) (2CO + O) adlayer on Ru(OOOl) at = 0.2 V in CO-samrated HCIO4, similar to the phase formed in UHV after CO adsorption on a (2 x 2)0-covered surface [Schiffer et al., 1997]. Erom the total charge density transferred after a potential step to 1.05 V in a CO-free electrolyte, they concluded that only 60% of the CO content in such an adlayer can be oxidized under these conditions [Wang et al., 2001]. 

An electrode can be characterized by LEED before and after the electrochemical experiment. Differences give information on adsorption and eventual movements of atoms over the surface. The great practical difficulty is the necessity of locating the electrode in exactly the same position for the two diffraction experiments. 

Sharp LEED patterns consistent with a c(4 x 2)CO superstructure could be readily obtained by exposing clean Ni(l 11) to 30 L CO in UHV at room temperature as reported in the literature [45-51], Also in accordance with information published elsewhere [52] was the increase in the background intensity of the rectangular Ni(110) LEED pattern following dosing, attributed to the presence of an amorphous CO overlayer. No AES spectra or LEED patterns were recorded after CO adsorption prior to the electrochemical experiments, to avoid electron-induced decomposition [53],... 

Similar results have been obtained for Ni(100)c(2 x 2)-CO and Ni(100)c(2 X 2)-0. For CO best agreement between theory and experiment is found for the adsorbate bonded in the atop or terminal position, in agreement with LEED observations. It has been emphasized by Plummer and Gustafsson that at present the interpretation of experimental data depends heavily on comparison with theoretical calculations for each individual adsorbate-surface combination. General symmetry arguments (selection rules), however, are now being formulated which should allow identification of the symmetry of the initial-state orbital or adsorption site, without complicated calculations. 

As described in Secs. 4.1, 4.1, and 6.2, adsorption of 1/2 ML Na on Al(lOO) at low temperature and at room temperature leads to the formation of two different c(2 X 2) structures. DFT calculations [67, 68] played a decisive role in solving the structure of the room temperature phase by showing that previously proposed models were incorrect, and predicting that the structure contained Na atoms in four-fold substitutional sites. This was later con rmed by a LEED study [20]. Comparison of theory and experiment for the two c(2 x 2)—Na phases is given in Table 18, from which a quantitative agreement can be seen. 

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