Ultrahigh vacuum surface spectroscopy

Ultrahigh vacuum surface spectroscopies can provide far greater breadth and depth of information about surface properties than can yet be achieved using in situ spectroscopies at the aqueous/metaI interface. Application of the vacuum techniques to electrochemical interfaces is thus desirable, but has been plagued by questions of the relevance of the emersed, evacuated surfaces examined to the real electrochemical interfaces. This concern is accentuated by surface scientists observations that in UHV no molecular water remains on well-defined surfaces at room temperature and above (1). Emersion and evacuation at room temperature may or may not produce significant changes in electrochemical interfaces, depending.on whether or not water plays a major role in the surface chemistry. 

In order to verify the existence of this barrier to dissociation, we have used molecular beam techniques coupled with ultrahigh vacuum surface electron spectroscopies to monitor the extent of dissociation as a function of the transla-... 

The data obtained from these radiotracer measurements are very useful in the discussion of the result of IR-spectroscopic studies. On the other hand, radiochemical measurements could be supported by ex situ electron spectroscopies of ultrahigh vacuum surface science. 

The present paper reviews the physical and chemical evidence for the above rules obtained over the last several years from ultrahigh vacuum surface science studies of molybdenum single crystals chemically modified by 0, C, S, and B. Additionally, the results of recent studies of methylcyclopropane hydrogenolysis will be presented which illustrate the influence of surface acid/base sites on catalytic hydrocarbon conversions. The surface coverage of each modifier was determined by quantitative Auger electron spectroscopy or x-ray photoelectron spectroscopy (XPS). The atomic structure of oxygen, carbon, and sulfur adlayers below one monolayer (ML)... 

A great advantage of using polymers is easy preparation of thin films from their solutions. However, when they are not prepared in ultrahigh vacuum, surfaces of solids are in general covered by contaminants and therefore not clean from surface science. This hindered use of electron spectroscopies for valence electronic... 

Because of the generality of the symmetry principle that underlies the nonlinear optical spectroscopy of surfaces and interfaces, the approach has found application to a remarkably wide range of material systems. These include not only the conventional case of solid surfaces in ultrahigh vacuum, but also gas/solid, liquid/solid, gas/liquid and liquid/liquid interfaces. The infonnation attainable from the measurements ranges from adsorbate coverage and orientation to interface vibrational and electronic spectroscopy to surface dynamics on the femtosecond time scale. 

A relatively new arrangement for the study of the interfacial region is achieved by so-called emersed electrodes. This experimental technique developed by Hansen et al. consists of fully or partially removing the electrode from the solution at a constant electrical potential. This ex situ experiment (Fig. 9), usually called an emersion process, makes possible an analysis of an electrode in an ambient atmosphere or an ultrahigh vacuum (UHV). Research using modem surface analysis such as electron spectroscopy for chemical analysis (ESCA), electroreflectance, as well as surface resistance, electrical current, and in particular Volta potential measurements, have shown that the essential features (e.g., the charge on... 

GDS instruments are viable alternatives to the traditional arc and spark-source spectroscopies for bulk metals analysis. Advantages of GDS over surface analysis methods such as AES, XPS and SIMS are that an ultrahigh vacuum is not needed and the sputtering rate is relatively high. In surface analysis, GD-OES, AES, XPS and SIMS will remain complementary techniques. GD-OES analysis is faster than AES (typically 10 s vs. 15 min). GD-OES is also 100 times more sensitive than... 

As mentioned previously, this can be attributed in part to the lack of structure-sensitive techniques that can operate in the presence of a condensed phase. Ultrahigh-vacuum (UHV) surface spectroscopic techniques such as low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and others have been applied to the study of electrochemical interfaces, and a wealth of information has emerged from these ex situ studies on well-defined electrode surfaces.15"17 However, the fact that these techniques require the use of UHV precludes their use for in situ studies of the electrode/solution interface. In addition, transfer of the electrode from the electrolytic medium into UHV introduces the very serious question of whether the nature of the surface examined ex situ has the same structure as the surface in contact with the electrolyte and under potential control. Furthermore, any information on the solution side of the interface is, of necessity, lost. 

X-ray photoelectron spectroscopy is indeed quite informative, but requires the use of expensive instrumentation. Also, the detection of photoelectrons requires the use of ultrahigh vacuum, and therefore can mostly be used for ex situ characterization of catalytic samples (although new designs are now available for in situ studies [146,147]). Finally, XPS probes the upper 10 to 100 A of the solid sample, and is only sensitive to the outer surfaces of the catalysts. This may yield misleading results when analyzing porous materials. 

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