The Reflection Techniques

When incident radiation interacts (from the vacuum) with a flat surface, the reflectance of the parallel and perpendicular polarized light (with respect to the incidence plane), Ry and Rx, are given by the relations below  

Total reflection of IR incident light at any wavelength from a surface can occur in two cases (i) if the beam arises from a transmitting medium and is incident on the surface of a conducting material (ii) if the beam arises and is incident within a transmitting medium, but the medium from which the beam arises has 

Figu re 3.1 (a) FTIR reflectance spectrum of a MgO monocrystal (incidence angle 26.5°, face [001]). (b) FTIR and FTFIR absorption / transmission spectra of MgO powder (reprinted with permission from G. Busca 

In the first case, it has been recognized that if molecules are adsorbed on the surface of a metal, only part of the grazing incident radiation is reflected, part being absorbed by the adsorbed species. This absorption is greatly increased if the incident radiation is polarized perpendicular to the metal surface. This is the basis of so-called Infrared Reflection Absorption Spectroscopy (IRRAS) [27], which is applied widely to surface studies on metal surfaces. In these conditions, an additional selection rule applies  

The second case refers to the so-called Internal Reflection Spectroscopy, that is used in the so-called Attenuated Multiple Total Internal Reflection technique 

---

A wide variety of in situ techniques are available for the study of anodic hhns. These include reflectance, eUipsometry, X-ray reflectivity, and SXRD. X-ray reflectivity can be used to study thick surface layers up to 1000 A. The reflectance technique has been used to study oxide growth on metals, and it yields information on oxide thickness, roughness, and stoichiometry. It the only technique that can give information on buried metal-oxide interfaces. It is also possible to get information on duplex or multiple-layer oxide hhns or oxide hhns consisting of layers with different porosity. Films with thicknesses of anywhere from 10 to 1000 A can be studied. XAS can be used to study the chemistry of dilute components such as Cr in passive oxide hhns. 

Infrared spectroscopy is a relatively simple technique, nondestructive, and versatile enough to analyze solids, liquids, and gases with a minimum of sample preparation. Polymers can be mixed with potassium bromide and then pressed into pellets. Films can be prepared from melt or cast from solution and can be studied easily. In bulk samples or powders, or if a concentration profile is needed, the reflectance technique is probably more suitable than transmission. 

The reflection technique has not been used as extensively as transmission. Its slow development may be attributed to several factors. It is used primarily on highly polished metal surfaces including those involved in fundamental studies of single crystals. The theoretical framework for reflection IR spectra has been developed only recently. Ultrahigh-vacuum techniques are required and modifications are needed for standard IR spectrometers. Since the reflection technique can be used with single crystals of metals, it is a bridge between the more sophisticated surface techniques used in surface science and the IR studies of the more practical catalysts. 

The conformational considerations lead us to expect that the monolayer thickness differs among the three classes that is accessible experimentally by the reflectivity techniques [94], For instance, fatty acid monolayers have thicknesses on the order of 20 A [95,96], whereas the vinyl polymer mono-layers of the first kind can have thicknesses of less than 10 A [97]. On the other hand, the other class of vinyl polymers gives rise to thicker mono-... 

A problem that may be encountered when analyzing a solid sample by transmittance spectroscopy is radiation scattering. Employing reflectance spectroscopy can sometimes reduce this problem. With this technique, the infrared spectra of most solid materials are easily obtained with little or no sample preparation. Spectra of a wide range of solid samples can be characterized with this technique, such as coatings on beverage containers and silicon wafers, polymer films, or other intractable samples. The reflectance technique, however, is less sensitive than the transmittance technique since about 80 /o of the infrared radiation is lost after being reflected off the sample surface. 

The X-ray images of extended lattice imperfections within the crystal arise fi om variations in beam intensity caused by local distortions of the lattice. Dislocation densities of up to 10 mm can be resolved in transmission studies or ten times this by the reflection technique. This resolution is lower than that obtainable by transmission electron microscopy, but the sample used may be thicker and does not have to be examined under high vacuum. X-ray beams also produce less radiation damage in the sample. When decomposition proceeds beyond a > 0.01, distortion of the lattice is such that the X-ray image loses resolution and the exposures required become even longer. Reflection data obtained at several different diffraction angles may be required to characterize the imperfections present. 

The application of the reflection technique to adsorption on single crystals will possibly offer a new lease on life to the use of infrared in fundamental gas-metal adsorption studies. Infrared studies that have contributed so much to CO adsorption studies, particularly those closely allied to catalytic systems, are likely, by application of the reflection technique, to be unique in their designation of adsorption complexes on single crystal surfaces. 

The polarization dependence of the photon absorbance in metal surface systems also brings about the so-called surface selection rule, which states that only vibrational modes with d5mamic moments having components perpendicular to the surface plane can be detected by RAIRS [22,21 and 24]. This rule may in some instances limit the usefulness of the reflection technique for adsorbate identification because of the reduction in the number of modes visible in the IR spectra, but more often becomes an advantage thanks to the simplification of the data. Furthermore, the relative intensities of different vibrational modes can be used to estimate the orientation of the surface moieties. This has been particularly useful in the study of self-... 

Infrared analysis of adhesive samples representing various percent conversion was achieved using reflectance spectroscopy. The transmission mode was Ineffective due to sample thickness and opacity. Limitation of the reflectance technique for measurement of bulk adhesive polymerization was apparent on examination of the sample surface nearest the UV source which Indicates complete cure (Irrespective of Irradiation time for t >0 sec). Consequently, the sample surface furthest from the UV source was used to Indicate conversion. Area ratio of absorption bands at 830 and 810 cm was calculated for this determination. The 810 cm band Intensity, attributable to CH2 deformation of carbon double bond functionality. Is directly proportional to conversion. Spectra of samples representing O-IOOZ conversion are shown In Figure 4. 

Ellipsometry involves measurement of the change of polarization that occurs when polarized radiation is reflected from a specular surface. Ellipsometry is an extension of the reflection technique in which the polarization of the reflected radiation rather than just its intensity is measured. Ranges of operation from ultra-violet to infrared, UV/Visible is typical. 

Infrared spectroscopy is the fastest and cheapest of the spectroscopic techniques used by organie and polymer chemists. As indicated, it is the measurement of the absorption of IR frequencies by organic compounds placed in the path of the beam of light. The samples can be solids, liquids, or gases and can be measured in solution or as neat liquid mulled with potassium bromide (KBr) or mineral oil. Recent developments in attenuated total refleetion (ATR) and diffuse reflectance techniques have made the analysis of solid adhesives possible. In fact, for bulk samples or powders, the reflectance technique is probably more suitable than transmission. 

Basically, there are two categories of FTIR spectroscopies reflection and nonreflection techniques [38], The latter class comprises either acoustic detection or emission from the sample itself. The techniques recognized here are photoacoustic spectroscopy (PAS), emission spectroscopy (EMS), and photothermal beam deflection spectroscopy (PBDS). These techniques will not be considered further in this chapter. The reader is referred to the literature [39-42], For adhesion studies the reflection techniques (SRIRS) are more important. The major classes of sampling techniques in SRIRS are ... 

Thin unsupported solid sheets often show interference fringes that could be mistaken for absorption bands. So do organic coatings On metals when examined by the reflection technique. Interference fringes can be a nuisance, but there are ways of eliminating them. 

By contrast, the refraction acoustical technique involves the recording of refracted sound waves from the seabed and subbottom. Compared with the reflection technique, the refraction technique requires stronger energy sources and takes more time. In addition, the source and detectors must be spaced further apart. However, the refraction method provides deeper subbottom penetration. It is not commonly used in offshore engineering work. A typical arrangement for a seismic refraction survey that shows the required energy source and receiver close to or in contact with the seafloor is shown in Figure 3.7. 

The reflection technique is more limited in resolution and magnification than is transmission. Reflection optical microscopy of adhered surfaces is often revealing, but the very limited depth of field is a severe handicap. Nevertheless, examination of surfaces after testing or failure is a simple, but important, first step in determining locus of failure (see Stress distribution mode of failure). Surface reflectivity of specimens is often improved by deposition of a layer or metal. The reflection microscope is especially useful when operated in dark-field mode, where topographical differences are accentuated. 

In the reflectivity technique, the intensity of specularly reflected x-rays is measured as a function of the incidence angle. Since the reflection coefficient for x-rays is less than unity, it is totally reflected from the sample surface, up to the critical angle of incidence. This critical angle is proportional to the square root of electron density and, hence, of the mass density. The density measured in this way can be used to derive the sp lsp ratio or, said otherwise, the extent to which a film is graphite like (p = 2.25 g/cm ) or diamond like (p = 3.51 g/cm ). The results of a study of the density variation with the time of deposition of diamond-like films (converted to film thickness) in the initial stages of their growth can be found in Ref. 71. 

Many of the transmission techniques discussed earlier in this chapter were developed over 50-plus years ago and involve a certain amount of skill and sometimes significant amounts of manual sample preparation. Back then labor was cheaper, and it was okay for it to take many minutes or hours for one sample to be analyzed. However, in the 21st century, labor costs are significantly higher, and all labs are under pressure to do more with less to increase profitability. Therefore, the main criterion the author uses to evaluate sample preparation techniques is speed and ease of use. For many samples, reflection techniques are faster and easier than transmission techniques. My approach, then, is that for a given sample I will try reflection techniques first. If the reflection technique works well, it will do so quickly and I can move on to the next sample. If the reflection technique does not work well, I find out quickly so not much time is wasted. Then, I will try one of the transmission techniques appropriate for the particular sample. 

---