Reflection-absorption measurements

Figure 11,9 Block diagram of a polarization-modulation FT-IR spectrometer set-up for measuring reflection-absorption spectra from thin films on metal substrates.

In situ quantitation The quantitative analysis is performed by measuring the absorption of the chromatogram zone in reflectance at 2 = 440 nm (Fig. 1). 

Thin film nanostructures of the III-VI compound In2Se3 were obtained inside the pores (200 nm) of commercial polycarbonate membrane by automated ECALE methodology at room temperature [157], Buffered solutions with millimolar concentrations of In2(S04)3 (pH 3.0) and Se02 (pH 5.5) were used. The atomic ratio of Se/In in the deposited films was found to be 3/2. Band gaps from FTIR reflection absorption measurements were found to be 1.73 eV. AFM imaging showed that the deposits consist of 100 nm crystallites. 

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. 

Several precautions were taken to ensure the immobilization chemistry. First, the sulfhydryl groups containing the macromolecular fraction was spectrophotometrically determined according to the literature [15]. We found that every set of 150 base pairs contained approximately one disulfide group. Since the DNA fragment used has hundreds of base pairs, each DNA strand seems to have one disulfide as its terminal group. Next, we made IR spectral measurements in a reflection-absorption (RA) mode [14b]. A freshly evaporated gold substrate was immersed into the DNA solution for 24 h at 5°C. The substrate was carefully rinsed with deionized water, dried under vacuum and was immediately used for the measurements. An Au substrate treated with unmodified, native sonicated CT DNA solution was also prepared as the control measurement. The / -polar-ized radiation was introduced on the sample at 85° off the surface normal and data were collected at a spectral resolution of 4 cm with 2025 scans. 

FIG. 1 FT-IR spectra in midfrequency region. DNA-treated gold substrate measured in reflection-absorption mode (a) and transmittance spectrum of DNA cast on Cap2 (b). 

Figure 7.IS Some typical calibration curves for several substances measured by absorption in the reflectance mode. Substance identification 1 = practolol 2 azobenzene 3 - diphenyl-acetylene 4 alprenolol 5 = estrone and 6 - pamatolol. (Reproduced with permission from ref. 20. Copyright Elsevier Scientific Publishing Co.)...

In solvent-elimination LC-FTIR, basically three types of substrates and corresponding IR modes can be discerned, namely, powder substrates for diffuse reflectance (DRIFT) detection, metallic mirrors for reflection-absorption (R-A) spectrometry, and IR-transparent windows for transmission measurements [500]. The most favourable solvent-elimination LC-FTIR results have been obtained with IR-transparent deposition substrates that allow straightforward transmission measurements. Analyte morphology and/or transformation should always be taken into consideration during the interpretation of spectra obtained by solvent-elimination LC-FTIR. Dependent on the type of substrate and/or size of the deposited spots, often special optics such as a (diffuse) reflectance unit, a beam condenser or an FITR microscope are used to scan the deposited substances (typical diameter of the FITR beam, 20 pm). 

The unit cell (Table 1) and orientation matrix were determined from the XYZ centroids of 8192 reflections with I > 20c(7). The intensities (SAINT [8]) were corrected for beam inhomogeneity and decay, and the esd s adjusted using SADABS [9]. An absorption correction was applied (Tmin 0.949, Tmsx 0.983) and symmetry and multiply measured reflections averaged with SORTAV [10]. 

Figure 1. Optical path in the reflection-absorption measurement of thin film.

For measuring infrared absorption spectra of gases adsorbed on the surfaces of metal single crystals or polycrystalline foils, one uses reflection absorption infrared... 

Summarizing, infrared spectroscopy measures, in principle, force constants of chemical bonds. It is a powerful tool in the identification of adsorbed species and their bonding mode. Infrared spectroscopy is an in situ technique, which is applicable in transmission or diffuse reflection mode on real catalysts, and in reflection-absorption mode on single crystal surfaces. Sum frequency generation is a speciality... 

Infrared drying, ceramics processing, 5 656 Infrared dyes, 9 500 Infrared emission spectroscopy, 23 142 Infrared inks, 14 315 Infrared lasers, 22 180 Infrared LEDs, 22 175, 176 Infrared measurements, in growing amorphous silicon, 22 130 Infrared microspectroscopy, 16 486 Infrared reflection-absorption spectroscopy (irras), 24 72, 114-116. See also IR spectra... 

Figure 8.5. Palm of the hand. Fluorescence spectrum (Fb-mode, full line) and diffuse reflectance absorption spectrum (dashed line, normalized to X = 700 nm, reference filter paper) measured with a sensitized diode array spectrometer.

Figure 8.11. Diffuse reflectance absorption spectra of a strongly fluorescent sample (1,6-diphenylhexatriene adsorbed on porous alumina) (a) conventional measurement w ith monochromatic irradiation and detection via an integrating sphere (b) measurement in a fluorimeter with two monochromators. Reaction spectra during Irons - cis photoisomerization are also given (adapted from Ref. 26).

In addition to the indirect experimental evidence coming from work function measurements, information about water orientation at metal surfaces is beginning to emerge from recent applications of a number of in situ vibrational spectroscopic techniques. Infrared reflection-absorption spectroscopy, surface-enhanced Raman scattering, and second harmonic generation have been used to investigate the structure of water at different metal surfaces, but the pictures emerging from all these studies are not always consistent, partially because of surface modification and chemical adsorption, which complicate the analysis. 

Hence, if the scattering coefficient remains constant during a measurement, the absorption may be deduced from the reflectance. 

Specular reflectance infrared involves a mirrorlike reflection producing reflection measurements of a reflective material or a reflection-absorption spectrum of a film on a reflective surface. This technique is used to look at thin (from nanometers to micrometers thick) films. 

The IR and Raman spectra of benzotriazole, benzotriazole anion and its Cu(I) complex have been measured. The characteristic peaks in the IR spectrum of the triazole moiety in benzotriazole anion occur at 1163 cm , 1134 cm , and 1115 cm . A broad band with a main peak at 1151 cm occurs in the spectrum of the Cu(I)-BTA complex <85JST(l00)57i>. The chemisorption of benzotriazole on clean copper and cuprous oxide surfaces is investigated by combining XPS, UV-PE and IR reflection absorption spectroscopy (IRAS). Coordination geometry including the triazole-... 

Adsorption and phase formation of uracil on massive Au[ (lll)-(110)] singlecrystal and Au (111 - 20 nm) film electrodes in 0.1 M IT2SO4 has been studied in electrochemical measurements and applying ATR surface-enhanced infrared reflection absorption spectroscopy [299]. At E < 0.15 V (versus trapped hydrogen electrode), uracil molecules are disordered and planar oriented. Close to the pzc, a 2D condensed physisorbed film of planar-oriented molecules interconnected by directional hydrogen bonds, is formed. 

There are some potential problems that should be taken into account when interpreting such spectra. A spectrophotometer measures transmission (and maybe also reflection) but not absorption. What is measured as absorption is a transmission measurement that is mathematically manipulated to convert it to absorption. Absorption is usually measured as absorbance. A, which by definition is given by... 

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