Reflection---Absorption Sampling Technique

This technique is used to study thin (down to submonolayer) films adsorbed on reflective substrates such as metals. Experimentally it involves measuring the change in the reflectance spectrum of the substrate that accompanies thin film formation. Various acronyms for the technique are used infrared reflection—absorption spectroscopy (IRRAS, IRAS) and reflection—absorption infrared spectroscopy (RAIRS). The Basics of IRRAS spectra are described in Chapter 5.2. 

The depth of penetration of the electrical field from the surface of the metal substrate into the adsorbed sample is between 5 and 500 nm. This enables investigation of submonolayers. IRRA spectra differ from conventional transmission spectra of bulk compounds, because only vibrations with transition dipole moments perpendicular to the surface wiU be excited. Since the evanescent field decays rapidly, vibrating groups closer to the surface yield larger absorption bands. Moreover, the polarization status of incident radiation is crucial, only p-polarized light will interact. 

Another practical consideration is film thickness. In the case of IRRAS of thick films, we observe a superposition of two spectra, one spectrum due to molecules close to the surface (with some enhanced spectral bands) and one due to the bulk sample (conventional transmission spectrum). In the case of very thick samples, the bulk spectrum dominates and the angle of incidence is not so important. 

In the case of thicknesses between 0.1 and 1 pm, both types of spectra have to be taken into consideration. 

Compared to other sampling techniques for surface investigations, a great advantage of IRRAS results from propagation of the probe photon in a non-vacuum environment. This enables the spectrometer to be set up outside an ultra-high vacuum chamber, which considerably simplifies operation. 

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In contrast to infrared spectrometry there is no decrease in relative sensitivity in the lower energy region of the spectrum, and since no solvent is required, no part of the spectrum contains solvent absorptions. Oil samples contaminated with sand, sediment, and other solid substances have been analysed directly, after being placed between 0.5 mm 23-reflection crystals. Crude oils, which were relatively uncontaminated and needed less sensitivity, were smeared on a 2 mm 5-reflection crystal. The technique has been used to differentiate between crude oils from natural marine seepage, and accidental leaks from a drilling platform. The technique overcomes some of the faults of infrared spectroscopy, but is still affected by weathering and contamination of samples by other organic matter. The absorption bands shown in Table 9.1 are important in petroleum product identification. 

In the diffuse reflectance mode, samples can be measured as loose powders, with the advantages that not only is the tedious preparation of wafers unnecessary but also diffusion limitations associated with tightly pressed samples are avoided. Diffuse reflectance is also the indicated technique for strongly scattering or absorbing particles. The often-used acronyms DRIFT or DRIFTS stand for diffuse reflectance infrared Fourier transform spectroscopy. The diffusely scattered radiation is collected by an ellipsoidal mirror and focussed on the detector. The infrared absorption spectrum is described the Kubelka-Munk function ... 

Electrochemists have used IR spectroscopy for many years to probe electrodeelectrolyte interfaces 107). The most popular technique is IR reflection absorption spectroscopy (IRRAS) 108). A schematic comparison of the principle of ATR and IRRAS experiments is shown in Fig. 37. One advantage of the ATR over the IRRAS technique for catalytic applications concerns diffusion. In IRRAS experiments, the IR beam passes through a thin liquid film between a window and the sample. This... 

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... 

This technique involves a reflectivity measurement, therefore, the Kramers-Kroenig transform is required to convert the resulting reflectance spectrum to a pseud-absorbance spectrum. This is a simple procedure for normal incidence measurements. Optically thick samples are desirable. Additional interfaces, such as air pockets, in the sample can induce reflection absorption bands - resulting in a spectrum of convolved reflection and reflection absorption features. In some cases, optically smooth protein and starch films can be prepared (without cryogenic slicing),... 

Reflection-absorption (RA) or grazing-angle spectroscopy, is a very useful technique that gives information about the direction of transition dipoles in a sample. Figure 3 present an optical setup for a grazing-angle experiment (22). 

Besides spectroscopic techniques such as infrared-reflection-absorption spectroscopy (IRRAS) and X-ray photoelectron spectroscopy (XPS), SFM-based stiffness imaging was applied in order to detect radiation-induced variations of surface stiffness [180]. For that purpose, when exposing the PE-film to the VUV-radiation, the film was covered with a Ni mesh. Thus, the PE-film was partially masked and exposed to the VUV radiation only within the square-shaped holes of the mesh. After having finished that treatment and having removed the mesh, the sample surface was scanned in force modula-... 

There are three types of reflectance techniques specular, diffuse and reflection-absorption as illustrated in Figure 9.22. Specular reflectance is applied to samples with smooth and polished surfaces, diffuse reflectance is applied to samples with rough surfaces, and reflection-absorption is applied to IR-transparent thin films on IR opaque substrates. The specular and diffuse techniques are more widely used and are introduced in more detail in the following text. 

IR spectroscopy is one of the few analytical techniques that can be used for the characterization of solid, liquid, and gas samples. The choice of sampling technique depends upon the goal of the analysis, qualitative identification or quantitative measurement of specific analytes, upon the sample size available, and upon sample composition. Water content of the sample is a major concern, since the most common IR-transparent materials are soluble in water. Samples in different phases must be treated differently. Sampling techniques are available for transmission (absorption) measurements and, since the advent of FTIR, for several types of reflectance (reflection) measurements. The common reflectance measurements are attenuated total reflectance (ATR), diffuse reflectance or diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and specular reflectance. The term reflection may be used in place of reflectance and may be more accurate specular reflection is actually what occurs in that measurement, for example. However, the term reflectance is widely used in the literature and will be used here. 

Specular reflectance techniques basically involve a mirror-like reflection from the sample surface that occurs when the reflection angle equals the angle of incident radiation. It is used for samples that are reflective (smooth surface) or attached to a reflective backing. Thus, specular techniques provide a reflectance measurement for reflective materials, and a reflection-absorption (transflectance) measurement for the surface films deposited on, or pressed against reflective surfaces (Figure 9). 

If the reflectivity is very high, diffraction losses may become dominant, in particular for cavities with a large separation d of the mirrors. Since the TEMqo mode has the lowest diffraction losses, the incoming laser beam has to be mode-matched by a lens system to excite the fundamental mode of the resonator but not the higher transverse modes. Similar to intracavity absorption, this technique takes advantage of the increased effective absorption length Leff = LUX — R), because the laser pulse traverses the absorbing sample 1/(1 - R) times. 

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