Vibrational spectroscopies RAIRS

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. 

The primary techniques used in this study include X-ray photoelectron spectroscopy (XPS), reflection-absorption infrared spectroscopy (RAIR), and attenuated total reflectance infrared spectroscopy (ATR). XPS is the most surface-sensitive technique of the three. It provides quantitative information about the elemental composition of near-surface regions (< ca. 50 A sampling depth), but gives the least specific information about chemical structure. RAIR is restricted to the study of thin films on reflective substrates and is ideal for film thicknesses of the order of a few tens of angstroms. As a vibrational spectroscopy, it provides the type of structure-specific information that is difficult to obtain from XPS. The... 

There is a number of vibrational spectroscopic techniques not directly applicable to the study of real catalysts but which are used with model surfaces, such as single crystals. These include reflection-absorption infrared spectroscopy (RAIRS or IRAS) high-resolution electron energy loss spectroscopy (HREELS, EELS) infrared ellipsometric spectroscopy. 

At the time of a recent review [9], there remained very few examples of vibrational studies of adsorbate, or localised substrate modes, at metal oxide surfaces. By far the majority of studies concerned the characterisation by HREELS of phonon modes (such as Fuchs-Kliewer modes) pertaining to the properties of the bulk structure, rather than the surface, or to electronic transitions. Such studies have been excluded from this review in order to concentrate on the vibrational spectroscopy of surface vibrations on well-characterised metal oxide surfaces such as single crystals or epitaxially grown oxide films, for which there is now a substantial literature. Nevertheless, it is important to briefly describe the electronic and phonon properties of oxides in order to understand the constraints and difficulties in carrying out RAIRS and HREELS with sufficient sensitivity to observe adsorbate vibrations, and more localised substrate vibrational modes. 

FT-RAIRS measurements of CO have also been used to identity facets of oxide supported Cu particles [78, 82]. The low sensitivity of RAIRS on single crystal ZnO(OOOl) prevented the observation of adsorbed CO or CO2, despite their observation in NEXAFS [78], although the local metallic dielectric allowed CO to be observed on the Cu particles. There appear to be no examples of HREELS being used to carry out vibrational spectroscopy of adsorbates on oxide supported metal particles. A HREELS study of Ag on MgO(lOO) films [95] was used only to characterise the Ag induced attenuation in the substrate Fuchs-Kliewer phonons, and the appearance of the metal/oxide interfacial plasmon at higher energies. HREELS has also been used to characterise the oxide/oxide interface between NiO and thin film MgO(lOO) [96]. Similar measurements of substrate phonon attenuation were made in HREELS studies on Pt films grown on ZnO(OOOl) [97]. 

Figure 5 shows the SFG vibrational spectra of carbon monoxide obtained at 10 -700 Torr of CO and at 295 K. When the clean Pt(lll) surface was exposed to 10 L (1 L=10 Torr sec) of CO in UHV, two peaks at 1845 cm and 2095 cm were observed which are characteristic of CO adsorbed at bridge and atop sites. LEED revealed that a c(4 X 2) structure was formed in which an equal number of carbon monoxide molecules occupied atop and bridge sites [15]. Such results are in agreement with previous HREELS [16] and reflection-absoiption infrared spectroscopy (RAIRS) [17] studies. ITie much higher relative intensity of atop bonded CO to bridge bonded CO in the SFG spectra is due to the specific selection rule for the SFG process [18]. As mentioned earlier, SFG is a second order, nonlinear optical technique and requires the vibrational mode under investigation to be both IR and Raman active, so that the SFG intensity includes contributions from the Raman polarizability as well as the IR selection mle for the normal mode. 

NMR, EXAFS/XANES, and STM (see Section 2.4.2). Procedures specific to chemisorbed states 17.29,30,68-70 include measurement of changes in electrical or magnetic character, vibrational spectroscopies, (HREELS, DRIFTS/RAIRS etc), calorimetry and thermal desorption. This short list is far from being comprehensive, and the reader is reminded that the purpose of this work is not to instmct in the use of these techniques, but rather to present and evaluate the results they generate. The principles concerned are only mentioned where understanding of the results necessitates it. This is somewhat in the spirit of Jerome K. Jerome, who in his Preface to Three Men in a Boat (to say nothing of the dog) advised his readers not to use it as a manual for a River Thames holiday they would, he said, be wiser to stay ay home. 

RAIRS results in a spectrum that is similar to that of a transmission measurement For grazing incidence geometry, only molecular vibrations giving rise to a dynamic dipole perpendicular to the surface are infrared active this is termed the surface selection rule for surface vibrational spectroscopy. 

Characterization of the Unmodified and PLL-g-PEG-Modified Surfaces. 3.2.1. RAIRS Measurements of the PLL(20)-g[3.5]-PEG(2) Monolayer. Reflection-absorption infrared spectroscopy (RAIRS) is well suited to study adsorbates on metallic surfaces, which are highly reflective. It relies on reflecting an infrared beam at near-grazing incidence from the metallic surface on which the thin film of interest has been deposited. Only the component of the vibrational transition dipole moments perpendicular to the surface plane contributes to the absorption spectra. The intensity of an absorption band is proportional to the squared cosine of the angle between the transition dipole moment and the surface normal. Therefore, RAIRS provides information not only on fimctional groups but also on orientation and conformation of adsorbed molecules or molecular entities. Metal oxides... 

Since both HREELS and RAIRS are vibrational spectroscopies, and the same selection rules apply, their information contents must overlap. This is demonstrated in Fig. 8 [2], in which the HREELS and RAIRS spectra from a Cu(l 11) surface covered with about 10 molecular layers of cyclohexane at low temperature are shown. The vibrational spectra appear at the same energetic positions in both techniques, but it should be noted that whereas RAIRS has the advantage of better energy resolution HREELS is able to record spectra down to losses close to 0 cm. For reasons of IR transmission of window materials, the cutoff in RAIRS is in the region 400-800 cm". ... 

Several available FTIR techniques such as attenuated total reflection [5-7] have been utilized for coatings hardened on a metal surface, diffuse reflectance [8] for ultra-thin films, and transmittance [9, 10] for clear or transparent coatings. The FTIR is useful for determining parameters such as residual porosity and molecular bonding. If used in concert with a microscope, excellent spatial resolution can be obtained. Hasik et al. [11] studied a polysiloxane-silsesquioxane system with FTIR spectroscopy while developing SiCO ceramic. The authors noticed the transformation in vibration bands, in their material, as a function of chemical compositions and temperature. A quartz crystal microbalance and reflection-absorption infrared spectroscopy (RAIR) was used to study silane reaction over oxidized aluminum surface [12]. 

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