Reflection/transmission spectra

Fig. 30. IR reflection transmission spectra for InAs films formed on a smooth, annealed Au surface and on a roughened Au surface. The substrate was formed by taking a well annealed, smooth Au on glass substrate and vapor depositing more Au on one half, at room temperature, so that a series of 40 bumps were formed over one half of the surface. The two deposits were thus formed in the same electrodeposition run. Adapted from ref. [282],...

This characteristic of RAIR can be observed experimentally. Fig. 8 shows the transmission spectrum of polydimethylsiloxane (PDMS) while Fig. 9 shows the RAIR spectrum of a thin film of PDMS spin-coated onto a chromium substrate. It can be observed that the bands near 1024 and 1095 cm have similar intensities in the transmission spectra but the band at higher frequencies is clearly much more intense in the RAIR spectrum. This change in relative intensity when PDMS is deposited onto a reflecting substrate is related to optical effects and is not related to orientation effects. 

The ATR technique is a commonly used infrared internal reflection sampling technique. It samples only the surface layer in contact with the ATR element the sampling depth probed is typically of the order of 0.3-3 pm [1]. Unless software corrected, compared with a transmission spectrum, the relative intensity of bands within an ATR spectrum increase in intensity with decreasing wavenumber. Several FTIR instrument companies now supply "ATR-correction" software developed to correct for the different relative intensities of bands observed between ATR and transmission spectra, so that ATR spectra can be more easily compared to and searched against transmission spectra. 

We conclude that when log f(R3.) is plotted against the detection wavelength, the resulting diffuse reflectance spectrum will be identical to the transmission spectrum of the compound. The only difference will be a displacement in the ordinate by the magnitude of -log s. 

Infrared spectra of zeolitic samples can be measured in several different modes. These include transmission, diffuse reflectance, attenuated total internal reflection (ATR) and emission. Transmission and diffuse reflectance are by far the most widely used of these techniques. In the transmission mode, the sample is placed directly in the infrared beam of the instrument and the light passing through or transmitted is measured by the detector. This transmitted signal (T) is ratioed to the open beam (no sample) signal (To) to get the transmission spectrum of the sample. The transmission spectrum is converted to an absorbance spectrum ... 

Many polymers are too tough to be ground even at liquid nitrogen temperatures. Consequently, surface techniques are often used. Internal reflectance or attenuated total reflectance (ATR) is the second most commonly used infrared technique [38-40]. For soft or pliable polymers or solutions, ATR is an extremely versatile technique and the spectrum is similar to a transmission spectrum. Unlike transmission, the spectrum obtained is independent of sample thickness. 

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. 

Another application is the reporting of the PAS spectrum of heavily n-dopped acetylene which is the prototype organic metal and for which the transmission spectrum or reflectance spectrum is very difficult to obtain199). The fibrous nature of the material and its reactivity with air presented no problems for the gastight PAS sample cell. 

A = solution absorption spectrum B = diffuse reflectance spectrum C = crystal transmission spectrum D = transmission spectrum as KC1 pellet E = absorption spectrum of melt at 400 °C maximum extinction coefficient in parentheses following the band maxima. 

Diffuse Reflection. Using a set of flat and elliptical mirrors, this device can measure a sufficient amount of light diffused by a sample dispersed in KBr powder (Fig. 10.20). By comparing the diffused reflection obtained with neat KBr, a result resembling the transmission spectrum is obtained. Kubelka-Munk s correction can be used to improve the spectrum. 

To validate the whole technique, a transmission spectrum of the neat liquid silane was compared with one obtained by specular reflectance from an aluminium mirror surface on which a film of silane had been cast from 1% solution in anhydrous methanol. These two spectra were virtually indistinguishable, as regards both position and intensity of all peaks. They contained a considerable number of peaks, most of which could be assigned with complete satisfaction in terms of the known structure of y-glycidoxypropyltrimethoxy-silane. 

Diffuse reflection from powder sample is a complex combination of transmission, internal and external reflections, and scattering. It is dependent on the particle size, absorption and refractive indices of the studied material. The case of proper prepared powder diffuse reflection R carries the information primarily about the transmission spectrum of the sample (Willey 1976 Fuller and Griffiths 1978). The traditional method of the absorption spectra (K) calculation on the base of the diffuse reflection R is the Kubelka-Munk equation K = (1 - R)2S/2Rc, where S is the scattering coefficient, concentration of the studied material is c = 1 in our case. 

An empirical relationship has been shown between the contact angles for wettability of a polymer film and the degree to which photooxidation products have accumulated in the surface layers of the film. Changes in wettability of polymer films during photooxidation are markedly dependent on the nature of the polymer. In the detection and identification of the earliest processes and products of surface photooxidation, the wettability method is far more sensitive than the infrared transmission or attenuated reflectance spectra and is about as sensitive but more specific than the ultraviolet transmission spectrum. Contact angle measurements themselves can be used as leads in the selection of solvents for the separation and identification of photooxidation products formed in the surface layers of a polymer film and are potentially useful in establishment of rates of specific processes. 

The sample absorbs the evanescent field so that the totally reflected wave is attenuated accordingly the reflectance spectrum closely resembles a transmission spectrum. Usually such spectra can actually be inteipreted and evaluated as transmittance, and commercially available accessories are designed to support this. With angles of reflection well above the critical one, some sensitivity is lost which however is regained by multiple internal reflections. How pronounced the actual spectrum is, can be adjusted by the number of reflections used, i.e. the sample-coated area of the slab whether both surfaces, one surface, or just a fraction. In any case the area of a given reflection should fully be covered to avoid spectral dilution. 

The term reflection absorption classes a group of experiments where the sample to be analyzed is placed on a mirror so that the absorption of the sample reduces the reflectance, and a sort of transmission spectrum of the sample results. The sample thickness ranges from many times the wavelength down to fractions of a monolayer. At either end of this scale special experimental modifications provide improved sensitivity with respect to the actual thickness. 

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