RAIR spectroscopy

As with any system, there are complications in the details. The CO sticking probability is high and constant until a 0 of about 0.5, but then drops rapidly [306a]. Practical catalysts often consist of nanometer size particles supported on an oxide such as alumina or silica. Different crystal facets behave differently and RAIRS spectroscopy reveals that CO may adsorb with various kinds of bonding and on various kinds of sites (three-fold hollow, bridging, linear) [307]. See Ref 309 for a discussion of some debates on the matter. In the case of Pd crystallites on a-Al203, it is proposed that CO impinging on the support... 

Fig. 4. Schematic drawing of a reflection-absorption infrared (RAIR) spectroscopy experiment.

In order to understand RAIR spectroscopy, it is convenient to model the experiment (see Fig. 4). Consider a thin film with refractive index n =n ik and thickness d supported by a reflecting substrate with refractive index ni = ri2 — iki- The refractive index of the ambient atmosphere is o- Infrared radiation impinges on the film at an angle of incidence of 6 . The incident radiation can be polarized parallel to or perpendicular to the plane of incidence. 

A powerful characteristic of RAIR spectroscopy is that the technique can be used to determine the orientation of surface species. The reason for this is as follows. When parallel polarized infrared radiation is specularly reflected off of a substrate at a large angle of incidence, the incident and reflected waves combine to form a standing wave that has its electric field vector (E) perpendicular to the substrate surface. Since the intensity of an infrared absorption band is proportional to / ( M), where M is the transition moment , it can be seen that the intensity of a band is maximum when E and M are parallel (i.e., both perpendicular to the surface). / is a minimum when M is parallel to the surface (as stated above, E is always perpendicular to the surface in RAIR spectroscopy). 

Two important features of RAIR spectroscopy can be observed by comparing the RAIR spectrum in Fig. 7 with the k spectrum in Fig. 6, remembering that the... 

As discussed above, it is desirable to use large, almost grazing angles of incidence in RAIR spectroscopy in order to maximize the sensitivity of the technique to ultra-thin films. Using such large angles of incidence usually requires the substrates to be at least a few centimeters in length. This severely limits the spatial... 

The SAM was obtained by immersing the clean substrate for 48 h in an ethanol solution of ferrocenylhexyl isocyanide. The strong v(N=C) peak at 2147cm" observed in FTIR spectra of free ferrocenylalkyl isocyanide (on a KBr plate) is not present in the RAIR spectra of this isocyanide on a nickel surface. Considering the surface selection rules for RAIR spectroscopy, the absence of a v(N=C) peak in the RAIR spectrum indicates that the chemisorbed isocyanides are bonded through both their carbon and nitrogen atoms, and they adopt an orientation in which the N=C bond is parallel to the surface. 

VEEL or RAIR spectroscopy to provide insight into temperature-dependent surface reactions. The TPD technique also provides information about the overall C/H composition of the hydrocarbon layers at different temperatures. Whereas we have made reference to, and taken into account, experimental results obtained from the use of such nonvibrational techniques in many cases, particularly when considering spectral interpretaions, it has not been feasible for us to systematically cover such papers that do not also include vibrational spectroscopic work. 

The success of the isotope dilution experiment for CO on Pt(lll) was accompanied by a serious difficulty in reconciling the magnitude of the shift, which determines Oy/3Q), with the intensity of the band, which also determines Oy/3Q). When due allowance is made for the resultant surface field and geometric factors (36) in RAIR spectroscopy the intensity is almost consistent with the vibrational polarizability av = 0.057 X3 (39), corresponding to the gas phase intensity, as has been concluded for CO adsorbed on copper films (40) from infrared studies and for CO on Pt(lll) (41) and Cu(100) (42) from high resolution electron energy loss spectroscopy. This value of av is an order of magnitude smaller than that deduced from the frequency shift. 

Some of the techniques described in this chapter used most widely today are Auger electron spectroscopy, X-ray photoelectron spectroscopy, electron-probe micro-analysis, low energy electron diffraction, scanning electron microscope, ion scattering spectroscopy, and secondary ion mass spectroscopy. The solid surface, after liberation of electrons, can be analyzed directly by AES, XPS, ISS, and EPMA (nondestructive techniques), or by liberation of ions from surfaces using SIMS (involving the destruction of the surface). Apart from the surface techniques, reflectance-absorbance infrared (RAIR) spectroscopy has also been employed for film characterization (Lindsay et al., 1993 Yin et al., 1993). Some... 

One approach to obtaining structural information on surface films is reflectance-absorbance infra-red (RAIR) spectroscopy and Fourier Transform IR (Foster, 1999). Model materials were prepared by thermal and thermooxidative decomposition of zinc diisopropyl-dithiophosphate. Lubricant-derived... 

RAIR spectroscopy of the oxidized monolayers produced complex spectra due to dimers, degradation products, and polymer within the oxidized film. Infrared bands providing spectral information of changes occurring in the polymer films include the 1513, 820, and 740 cm" bands for the 0.0 V spectra, and 1590 and 860 cm" bands in the +0.65 V spectra. The 1513 cm" band observed is assigned to the benzoid vibration for the fully reduced (0.0 V) polymer structure. In the 0.0 V spectra, the 1513 cm" (benzoid band) band is accompanied by two active out-of-plane deformation vibrations at 820 and 740 cm" which are due to 1,2,4-ring substitution." ... 

---