Vibrational Spectroscopy IRRAS

Within catalysis and surface science, vibrational spectroscopy techniques are important tools to identify adsorbates, study their binding behavior, can provide information on adsorption sites. In the case of infrared spectroscopy, the most common form of vibrational spectroscopy, the vibrations in molecules are excited by the absorption of photons in the infrared range. Since the IR has only been used for few measurements within this work the experimental setup is described below but no theory is presented here, but can be found elsewhere [37, 101-104]. In order to measure infrared adsorption spectra on single crystal surfaces, a variation of IR spectroscopy is used, called infrared reflection adsorption spectroscopy (IRRAS). A schematic sketch of the IRRAS setup of the nanocat is depicted in Fig. 3.11. 

The IR beam is focused onto the single crystal at a grazing angle and, during the reflection, of the / -component of the IR light on the single crystal excites those vibrations of the adsorbed molecule for which the component of the dipole moment perpendicular to the surface changes (this is an additional surface selection rule for IRRAS).29 

Experimental details—At best a peak-to-peak noise level of 0.05 % (transmission at 4cm resolution, a mirror velocity of 1.89 and averaging over 512 scans) is achieved. Data acquisition is performed by the software (IR software, Thermo Scientific Nicolet, U.S.A.), all spectra have been background corrected. 

---

Some characteristics of, and comparisons between, surface-enhanced Raman spectroscopy (SERS) and infrared reflection-absorption spectroscopy (IRRAS) for examining reactive as well as stable electrochemical adsorbates are illustrated by means of selected recent results from our laboratory. The differences in vibrational selection rules for surface Raman and infrared spectroscopy are discussed for the case of azide adsorbed on silver, and used to distinguish between "flat" and "end-on" surface orientations. Vibrational band intensity-coverage relationships are briefly considered for some other systems that are unlikely to involve coverage-induced reorientation. 

With infrared reflection absorption spectroscopy (IRRAS), it is possible to obtain information about the orientation of enzyme molecules adsorbed on flat metal surfaces (3,4). Electric dipole-transition moments oriented perpendicular to a flat metal surface show enhanced IR absorbance. IR bands due to vibrations of groups with transition moments oriented parallel to the surface are not observed. The IR-beam component which is polarized perpendicular to the plane of incidence (parallel to the surface) contains no information and can be eliminated by using a polarizer. 

Another class of techniques monitors surface vibration frequencies. High-resolution electron energy loss spectroscopy (HREELS) measures the inelastic scattering of low energy ( 5eV) electrons from surfaces. It is sensitive to the vibrational excitation of adsorbed atoms and molecules as well as surface phonons. This is particularly useful for chemisorption systems, allowing the identification of surface species. Application of normal mode analysis and selection rules can determine the point symmetry of the adsorption sites./24/ Infrarred reflectance-adsorption spectroscopy (IRRAS) is also used to study surface systems, although it is not intrinsically surface sensitive. IRRAS is less sensitive than HREELS but has much higher resolution. 

Vibrational spectroscopy (ATR-FTIR, IRRAS, Raman) Identification of interfacial molecules orientational order (second-rank order parameter S )), and conformational order. ATR-FTIR restricted to the ATR-crystal/fluid interface. 

In conclusion, strained surfaces can show very original structures and new catalytic properties. In order to associate the modified catalytic properties to the peculiar structures generated, one has to asume that these original structures are still present under the reactive mixture, at high pressure. Measurements under pressure of reactants are then necessary to measure both the surface structure and the surface species as reaction intermediates. Up to now, only very few data are available in that field. Recent developments around techniques such as STM [79-80], grazing X-ray Diffraction [81]. .. and optical vibrational spectroscopies such as IRRAS[82-83] using a polarized light and SFG [79] have demonstrated the possibility to realise such observations. 

Leverette, C.L. and Dluhy, R.A. (2004) Vibrational characterization of a planar-supported model bilayer system utilizing surface-enhanced Raman scattering (SERS) and infrared reflection-absorption spectroscopy (IRRAS). Colloids and Surfaces A, 243, 157-167. 

Although optical vibrational techniques are less sensitive than electron-based spectro-metric methods, these techniques are employed extensively for thin-film characterization because of the specific and characteristic vibrational spectrum shown by various functional groups and molecules present in the film. The most commonly used vibrational spectroscopic techniques are infrared (IR) and Raman spectroscopy. Because of the interference caused by absorption of IR by the underlying substrate, IR reflection-adsorption spectroscopy (IRRAS) and its polarization modulation (PM) analog, PM-IRRAS, which uses the polarization selectivity of surface adsorption, are typically employed to characterize thin films (Gregoriou and Rodman, 2006). 

Recent work in our laboratory has shown that Fourier Transform Infrared Reflection Absorption Spectroscopy (FT-IRRAS) can be used routinely to measure vibrational spectra of a monolayer on a low area metal surface. To achieve sensitivity and resolution, a pseudo-double beam, polarization modulation technique was integrated into the FT-IR experiment. We have shown applicability of FT-IRRAS to spectral measurements of surface adsorbates in the presence of a surrounding infrared absorbing gas or liquid as well as measurements in the UHV. We now show progress toward situ measurement of thermal and hydration induced conformational changes of adsorbate structure. The design of the cell and some preliminary measurements will be discussed. 

Successful IR spectroscopy of ultrathin films is very sensitive to the choice of the method and the optical geometry of the experimental set-up, maximizing spectral contrast and the amount of information obtained about the film. These choices should be made on the basis of a comparison of band intensities in film spectra calculated for different experimental conditions. In this section, this approach will be demonstrated using a 1-nm weakly absorbing hypothetical layer that models an isotropic organic monolayer with optical constants 2 = 1-3 and 2 = 0.1 in the region of the vCH vibrations (v = 2800 cm ). The layer is assumed to be located on a Ge or A1 substrate. The spectra were calculated for /7-polarized reflection IRRAS and ATR and single transmission. 

Figure 3.70. Calculated absorbances forp-polarized radiation In IRRAS spectra of hypothetical absorbate vibration at 3000 cnr on silicon (ns = 3.42) as function of angle of incidence for different cone angles k of incident radiation. Film parameters dz = 1 nm, nz = 1.5, /cz = 0.1. Reprinted, by permission, from FI. Brunner, U. Mayer, and FI. Floffmann, Appl. Spectrosc. 51, 209 (1997), p. 215, Fig. 7. Copyright 1997 Society for Applied Spectroscopy.

The complexation of PABA with nucleotides was further studied using polarization modulated infrared reflection absorption spectroscopy (PM-IRRAS). PM-IRRAS spectra of PABA films exhibit all the characteristic vibrations of polyaniline and boronic acid (Figure 3.27) [93, 94]. After complexation with NAD+ (Figure 3.27, b) and NADH (Figure 3.27, c), the disappearance of the free B-OH group vibration at 986 cm and increase in the intensity of the asymmetric B-O bond vibration at 1330 cm indicate the formation of boronate ester. The new vibrations at 1080 and 1470 cm have been attributed to ribose and adenine moieties, respectively [153]. The vibrations at 1218 (NAD+), 1245... 

---