Vibrational spectroscopy surface studies

Dumas P, Weldon M K, Chabal Y J and Williams G P 1999 Molecules at surfaces and interfaces studied using vibrational spectroscopies and related techniques Surf. Rev. Lett. 6 225-55... 

In addition to the surface/interface selectivity, IR-Visible SFG spectroscopy provides a number of attractive features since it is a coherent process (i) Detection efficiency is very high because the angle of emission of SFG light is strictly determined by the momentum conservation of the two incident beams, together with the fact that SFG can be detected by a photomultiplier (PMT) or CCD, which are the most efficient light detectors, because the SFG beam is in the visible region, (ii) The polarization feature that NLO intrinsically provides enables us to obtain information about a conformational and lateral order of adsorbed molecules on a flat surface, which cannot be obtained by traditional vibrational spectroscopy [29-32]. (iii) A pump and SFG probe measurement can be used for an ultra-fast dynamics study with a time-resolution determined by the incident laser pulses [33-37]. (iv) As a photon-in/photon-out method, SFG is applicable to essentially any system as long as one side of the interface is optically transparent. 

J. Phys. Chem. B, 106, 5143-5154. Somorjai, G. A. and Rupprechter, G. (1999) Molecular studies of catalytic reactions on crystal surfaces at high pressures and high temperatures by infrared-visible sum frequency generation (SFG) surface vibrational spectroscopy. J. Phys. Chem., 103, 1623-1638. 

Chen Z, Shen YR, Somoijai GA. 2002. Studies of polymer surfaces by sum frequency generation vibrational spectroscopy. Annu Rev Phys Chem 53 437-465. 

Schlapka A, Kasberger U, Menzel D, Jakob P. 2002. Vibrational spectroscopy of CO used as a local probe to study the surface morphology of Pt on Ru(0001) in the submonolayer regime. Surf Sci 502/503 129. 

Vibrational spectroscopy provides the most definitive means of identifying the surface species arising from molecular adsorption and the species generated by surface reaction, and the two techniques that are routinely used for vibrational studies of molecules on surfaces are Infrared (IR) Spectroscopy and Electron Energy Loss Spectroscopy (HREELS) (q.v.). 

The book has been written as an introductory text, not as an exhaustive review. It is meant for students at the start of their Ph.D. projects and for anyone else who needs a concise introduction to catalyst characterization. Each chapter describes the physical background and principles of a technique, a few recent applications to illustrate the type of information that can be obtained, and an evaluation of possibilities and limitations. A chapter on case studies highlights a few important catalyst systems and illustrates how powerful combinations of techniques are. The appendix on the surface theory of metals and on chemical bonding at surfaces is included to provide better insight in the results of photoemission, vibrational spectroscopy and thermal desorption. 

Carbon monoxide on metals forms the best-studied adsorption system in vibrational spectroscopy. The strong dipole associated with the C-O bond makes this molecule a particularly easy one to study. Moreover, the C-0 stretch frequency is very informative about the direct environment of the molecule. The metal-carbon bond, however, falling at frequencies between 300 and 500 cm1, is more difficult to measure with infrared spectroscopy. First, its detection requires special optical parts made of Csl, but even with suitable equipment the peak may be invisible because of absorption by the catalyst support. In reflection experiments on single crystal surfaces the metal-carbon peak is difficult to obtain because of the low sensitivity of RAIRS at low frequencies [12,13], EELS, on the other hand, has no difficulty in detecting the metal-carbon bond, as we shall see later on. 

Cuf1101-HC00 The decomposition of formic acid on metal and oxide surfaces is a model heterogeneous reaction. Many studies have since shown that it proceeds via a surface formate species. Thus on Cu 110) adsorbed formic acid is found at low temperature. On heating to 270 K deprotonation occurs, giving rise to the surface formate, which in turn decomposes at 450 K with evolution of H2 and C02- In previous studies, particularly with vibrational spectroscopy, it had been demonstrated that the two C-0 bonds are equivalent and that the symmetry is probably C2v [19]. A NEXAFS study by Puschmann et al. [20] has subsequently shown that the molecular plane is oriented perpendicular to the surface and aligned in the <110> azimuth. 

Vibrational spectroscopy is the experimentalist s most powerful tool for studying the effects of changes in local environment on individual chemical bonds. Studies of simple adsorbates like CO which have strong characteristic absorption bands have contributed greatly to our understanding of adsorption processes at surfaces (1). As shown here and in other papers in this symposium, recent experimental developments have led to a renewed effort to use the vibrational spectroscopy of adsorbates as a probe for understanding the physical chemistry of metal/electrolyte interfaces. 

The use of neutron reflectivity at liquid interfaces, which is a method sensitive to both surface roughness and surfactant layer thickness, was reviewed with the examples of polydimethylsiloxane-surfactant layers.633 Sum-frequency generation (SFG) vibrational spectroscopy was applied to study surface restructuring behavior of PDMS in water in an attempt to understand antifouling properties of silicones.6 ... 

In addition to the indirect experimental evidence coming from work function measurements, information about water orientation at metal surfaces is beginning to emerge from recent applications of a number of in situ vibrational spectroscopic techniques. Infrared reflection-absorption spectroscopy, surface-enhanced Raman scattering, and second harmonic generation have been used to investigate the structure of water at different metal surfaces, but the pictures emerging from all these studies are not always consistent, partially because of surface modification and chemical adsorption, which complicate the analysis. 

Since 1905, when Coblentz obtained the first IR spectrum, vibrational spectroscopy has become an important analytical research tool. This technique was then applied to the analysis of adsorbates on well-defined surfaces, subsequently moving towards heterogeneous reaction studies. Terenin and Kasparov (1940) made the first attempt to employ IR in adsorption studies using ammonia adsorbed on a silica aerogel containing dispersed iron. This led to a prediction by Eischens et al. from Beacon Laboratories in 1956 that the IR technique would prove to be extremely important in the study of adsorption and catalysis. For an excellent review article in IR spectroscopy, see Ryczkowski and references therein and for a more recent review with applications, see Topsoe. ... 

The interaction of N2 with transition metals is quite complex. The dissociation is generally very exothermic, with many molecular adsorption wells, both oriented normal and parallel to the surface and at different sites on the surface existing prior to dissociation. Most of these, however, are only metastable. Both vertically adsorbed (y+) and parallel adsorption states (y) have been observed in vibrational spectroscopy for N2 adsorbed on W(100), and the parallel states are the ones known to ultimately dissociate [335]. The dissociation of N2 on W(100) has been well studied by molecular beam techniques [336-339] and these studies exemplify the complexity of the interaction. S(Et. 0n Ts) for this system [339] in Figure 3.36 (a) is interpreted as evidence for two distinct dissociation mechanisms a precursor-mediated one at low E and Ts and a direct activated process at higher These results are similar to those of Figure 3.35 for 02/ Pt(lll), except that there is no Ts... 

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