IR specular reflectance

Figures 3. Typical IR specular reflectance (a), IR diffuse reflectance (b) and UV-Vis-NIR reflectance (c) electrochromic data, applied potentials as shown. The diffuse reflectance data also show the effect of y-radiation, at exposures indicated ( top group = light state, bottom group = IR dark state).

Figs. 3-15. 3-16 IR Specular Reflectance electrochromic data for a poly(aromatic amine) using two differing device configurations (labeled P and 2 ). Courtesy Ashwin -Ushas Corp. Inc. 

Reflection-Absorption IR spectroscopy (RAIRS) where the linearly polarized IR beam is specularly reflected from the front face of a highly reflective sample, such as a metal single crystal surface (Figure 3.1(a)). This is also sometimes referred to as IRAS (IR reflection absorption). The IR beam comes in at grazing angle (i.e. almost parallel to the surface), and although absorption bands in RAIRS have intensities that are some two orders of magnitude weaker than in transmission studies on... 

Specular reflection IR spectroscopy has been used by Cole and coworkers to study the orientation and structure in PET films [36,37]. It has allowed characterizing directly very highly absorbing bands in thick samples, in particular the carbonyl band that can show saturation in transmission spectra for thickness as low as 2 pm. The orientation of different conformers could be determined independently. Specular reflection is normally limited to uniaxial samples because the near-normal incident light does not allow measuring Ay. However, it was shown that the orientation parameter along the ND can be indirectly determined for PET by using the ratio of specifically selected bands [38]. This approach was applied to the study of biaxially oriented PET bottles [39]. 

For some applications, especially such involving solid samples or fluids containing suspended particles, reflection spectroscopic systems are better suitable than transmission sensors. Apart from specular reflection, which provides comparatively little information and is of hardly any practical importance for IR sensing, two reflectrometric methods can be used to gain spectroscopic information about a sample diffuse reflection and transflection, a combination of transmission and diffuse reflection. 

Fig. 4 Schematic representation of the diffuse reflectance sampling accessory. Key A, blocker device to eliminate specular reflectance B, path of IR beam.

A majority of traditional NIR measurements are made on solid materials and these involve reflectance measurements, notably via diffuse reflectance. Likewise, in the mid-IR not all spectral measurements involve the transmission of radiation. Such measurements include internal reflectance (also known as attenuated total reflectance, ATR), external reflectance (front surface, mirror -style or specular reflectance), bulk diffuse reflectance (less common in the mid-IR compared to NIR), and photoacoustic determinations. Photoacoustic detection has been applied to trace-level gas measurements and commercial instruments are available based on this mode of detection. It is important to note that the photoacoustic spectrum is a direct measurement of infrared absorption. While most infrared spectra are either directly or indirectly correlated... 

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) is used to obtain spectra of powders and rough polymeric surfaces such as textiles and paper. IR radiation is focused onto the surface of the sample in a cup resulting in both specular reflectance (which directly reflects off the surface having equal angles of incidence and reflectance) and diffuse reflectance (which penetrates into the sample subsequently scattering in all angles). Special mirrors allow the specular reflectance to be minimized. 

Within the IR spectroscopy arena, the most frequently used techniques are transmission-absorption, diffuse reflectance, ATR, specular reflectance, and photoacoustic spectroscopy. A typical in situ IR system is shown in Fig. 7. Choosing appropriate probe molecules is important because it will influence the obtained characteristics of the probed solid and the observed structure-activity relationship. Thus, the probe molecules cover a range from the very common to the very rare, in order to elucidate the effect of different surfaces to very specific compounds e.g. heavy water and deuter-ated acetonitrile, CDsCN). The design of the IR cell is extremely important and chosen to suit the purposes of each particular study. For catalytic reactions, the exposure of catalytic metals must be eliminated in cell construction, otherwise the observed effect of the catalyst may not be accurate. 

Specular reflection is encountered when the reflecting medium is a smooth polished surface. The angle of reflection is identical to the incident angle of the radiation beam. If the surface is IR absorbent, the relative intensity of reflection is less for wavelengths that are absorbed than for wavelengths that are not. Thus, the plot of reflectance R, defined as the fraction of reflected incident radiant energy versus the wavelength (or wavenumber) appears similar to a transmission spectrum for the sample. 

The techniques used in this stage of the work were all IR spectroscopy using a Perkin-Elmer spectrometer 599 and appropriate attachments. Conventional transmission through a sodium chloride cell of path length 0.1 mm, and multiple specular reflectance from aluminium films with a mirror finish were both used. 

Figure 4. Variation in the specular reflectance IR spectra of films of different thicknesses of silane A187 on aluminium mirror surfaces. The sensitivity used for the monolayer is ten times that used for the other spectra.

Figure 8. (a) FTIR spectrum of 1 in CC14, (b) C-H stretching region of the grazing angle external specular reflection IR spectrum of a monolayer of 1 on an aluminized silicon wafer, and (c) as in (b) for a monolayer of OTS on an aluminized silicon wafer. Spectra are recorded at 76° incidence, 1000 scans, 2 cm resolution, and the baselines have been adjusted to zero absorbance. 

Exposure of a Id and a 4a treated electrode to CO. An electrode treated with Id and 4a in the aforementioned manner was placed in a 50 mL 3-neck flask containing 30 mL of 0.1 M CH2C12 solution of Bu4NPF6. CO was then bubbled into the solution for 10 min and a cyclic voltammogram was subsequently recorded. The wave assigned to Id had shifted +120 mV relative to the 4a wave. Similarly, when a lxl cm2 Au electrode treated with Id and 4a was placed in a vial and purged with CO for 10 min, specular reflectance IR indicated surface conversion of Id to 2d. 

Many types of spectroscopy have been coupled with electrochemistry including electronic absorption spectroscopy, X ray (see X-Ray Absorption Spectroscopy), infrared (IR) (see Vibrational Spectroscopy), or Raman spectroscopy, eUipsometry, specular reflectance, and electron spin resonance (ESR). Electronic absorption spectroscopy, one of the most prominent electrochemistry coupled with spectro-electrochemical techniques, will be discussed here as will... 

Light (or near-ir and uv radiation) that is incident on opaque minerals is partly absorbed and partly reflected by them. There are two kinds of reflection processes that occurring when light is reflected from a flat polished surface of the mineral (specular reflectance) and that occurring when the light is reflected from the mineral after it has been finely powdered (diffuse reflectance). The latter arises from radiation that has penetrated the crystals (as in an electronic absorption spectrum) and reappeared at the surface after multiple scatterings in this case there will also be a specular component to the reflectance from light that is reflected from the surfaces of the particles. The specular reflectance of a flat polished surface of an opaque mineral measured at normal incidence can be related to the n and k terms of the complex refractive index (N) in which ... 

The commercial spectrometers designed primarily for undertaking transmittance measurements in the near-ir-visible-near-uv range commonly have attachments that enable specular reflectance measurements to be made on polished blocks (see Wendlandt and Hecht, 1966 Hedel-man and Mitchell, 1968). Specular reflectance measurements of small... 

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. 

Fourier transform infrared spectroscopy (FT-IR) is useful for identifying organic and inorganic compounds by comparison with library references. Perkin Elmer System 2000 offers near IR, mid IR, far IR 15,000-15,030 cm, transmittance (T), specular reflectance (SR Ref. 6) and diffuse reflectance (DR), horizontal and vertical attenuated total reflectance (ATR) microscope (>10-gm spot, 10,000-10,580 cmy ... 

Figure 10.19 FTIR imaging system. Left, an optical pathway in an IR microscope associated with a spectrometer. The sample can he examined in transmission (left), or in specular reflection modes (right). This accessory is installed in a spectrometer whose beam is deflected. The Cassegrain optic has the advantage that the light is reflected at the surface of mirrors rather than having to pass through optical lenses. Right, model 400 UMA (reproduced courtesy of Varian Inc. USA).

The elegant experiment, which apparently avoids the problem of solvent absorption, is attenuated total reflectance and this was the first in-situ IR technique to be developed. More recently, transmission and specular reflectance modes using IR radiation have also been carefully investigated and it has become evident that each of these three methods has both advantages and disadvantages. Before considering each technique in detail, it will be useful to develop a consistent theoretical framework. 

The technique developed by Bewick for use with a dispersive IR spectrometer is termed electrically modulated infrared spectroscopy (EMIRS) and is essentially a direct development of the UV-VIS specular reflectance technique [50], modulated specular reflectance spectroscopy (MSRS). As in MSRS, radiation is specularly reflected form a polished electrode surface while the electrode potential is modulated with a square wave between a base potential and the working potential at which the process of interest occurs the wavelength range of interest is then slowly scanned. Only that... 

In situ electro-optical reflection is a very promising means for meeting this need. There are two such methods, internal and external reflection methods the latter includes specular reflection spectroscopy, ellip-sometry, IR reflection spectroscopy, and surface enhanced Raman scattering (SERS). 

The specular reflection spectroscopy is being applied extensively in our research, but it provides only limited data on the molecular level. Thus SERS, IR, or internal reflection spectroscopy used in conjunction with the present method should help to obviate this problem. 

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