Reflection-absorption spectroscopy measurement method

A variant of IRRAS is polarization modulation IR reflection absorption spectroscopy (PM-IRRAS). In this method, the polarization of the IR beam incident on the sample is modulated between parallel and perpendicular polarization. When the sample is metallic, only the parallel-polarized light yields signals from adsorbed molecules, because the electric field amplitude of perpendicular-polarized light vanishes at the metal surface. This statement is the basis for the metal surface selection rule 100,109). When the medium above the sample (gas or liquid phase) is isotropic, both polarizations are equivalent. The PM-IRRAS method thus enables the measurement of signals from adsorbates on a metal surface in the presence of an absorbing gas or liquid phase. 

The infrared reflection-absorption spectroscopy was performed on a Bruker IFS 66 spectrometer (Karlsruhe, Germany) equipped with a MCT detector and a modified external reflection attachment P/N 19650 of SPECAC (Orpington, UK). This included a miniaturized Langmuir-trough, permitting thermostatic measurements. An extensive description of the method can be found in Gericke et al. (1993). The IRRAS set-up as well as the experimental approach can be inferred from the schematic sketch shown in Fig. 2. 

During a number of years we have applied surface orientated analytical methods to the study of protein adsorption on solid surfaces. These Investigations Include in situ studies with elllpsometry, surface potential and capacitance measurements (1.2) We have applied spectroscopic techniques like infra- ed reflection absorption spectroscopy (IRAS, 3-5) and ESCA (5-7) to investigate details in the Interaction between organic molecules and surfaces. Spectroscopic techniques have also been used to... 

Due to its very nature, the electrode/electrolyte interface may conveniently be studied by reflection-absorption spectroscopy. The first attempts in the infrared wavelength range were made with internal reflection spectroscopy. This allows multiple reflections at the electrode surface to increase the signal, which was otherwise too weak for direct measurement.However, due to inherent difficulties of this method (e.g., the need for a transparent substrate, the necessity for a thin metal layer as electrode), specular external reflection spectroscopy now is preferred for the in situ investigation of electrode processes. 

In the case of infrared spectroscopy, various types of infrared measurement methods (transmission method, attenuated total reflection (ATR) method, reflection-absorption (RA) method, surface-enhanced method, etc.) may be applied. 

In the field of not only traditional metallurgy but also recently developed nano-technology, it is very interesting and important what change is introduced when it is surrounded by other atoms. Such a change in electronic states has been investigated as chemical shift detected by X-ray (XPS) and UV (UPS) photoemission spectroscopy [1] as well as X-ray emission and absorption spectroscopy [2,3]. Also, such a chemical shift has been simulated by theoretical calculation [4]. However, many problems have been unsolved. In the case of XPS and UPS, since the most outer layers of substances are analyzed, the spectra are easily affected by absorbed gaseous molecules. Also, with the X-ray emission and absorption spectroscopy it is difficult to analyze the complicated X-ray transition states for substances composed of heavy metal elements. Therefore, a complementary method has been demanded for the spectroscopy such as XPS, UPS and X-ray emission and absorption spectroscopy. The coefficient y of the electronic contribution to heat capacity, Cp, near absolute zero Kelvin reflects the density of states (DOS) in the vicinity of Fermi level (EF) [5]. Therefore, the measurement of y is expected to be one of the useful methods to clarify the electronic states of substances composed of heavy metal elements. 

IR, Raman, NMR, ESR, UPS, XPS, AES, EELS, SIMS) [1]. However, some industrial carbon materials such as amorphous carbon films and carbon black cannot be easily characterized from the local-structure point of view by these methods, because these materials usually take amorphous and complex structures. Recently, soft X-ray emission and absorption spectroscopy using highly brilliant synchrotron radiation [2] has been utilized to characterize various carbon materials, because information on both the occupied and unoccupied orbitals, which directly reflect the local structure and chemical states, can be provided from the high-resolution soft X-ray measurements. We have applied the soft X-ray spectroscopy to elucidate the local structure and chemical states of various carbon materials [3]. Additionally, we have successfully used the discrete variational (DV)-Xa method [4] for the soft X-ray spectroscopic analysis of the carbon materials, because the DV-Xa method can easily treat complex carbon cluster models, which should be considered for the structural analysis of amorphous carbon materials. 

Extending the equipment, the authors (Beale et al., 2005) recently added energy dispersive X-ray absorption spectroscopy (XAS). Raman and UV-vis spectra are recorded by illuminating opposite sides of a catalyst bed in a vertical tubular reactor and detecting the scattered and reflected light as described above. XAS is performed in the same horizontal plane but in transmission and with the beam orthogonal to the incident radiation of the other two methods. Example spectra were recorded for samples at 823 K. A combination of UV-vis (fiber optics) and XAFS spectroscopy for investigation of solids has also been described by Jentoft et al. (2004), who reported UV-vis measurements of samples at 773 K. 

Surface analytical methods — X-ray absorption spectroscopy, XAS — Figure. Electrochemical cell for in situ XAS measurements in reflection, set up with a gracing incident X-ray beam, beam shaping slit, ionization chambers for the intensity measurement of incoming (ii) and reflected beam (I2) and beam stop for the direct nonreflected beam [vii]... 

Higo, N. et al. Validation of reflectance infrared spectroscopy as a quantitative method to measure percutaneous absorption in vivo. Pharmaceutical Research 70(10) 1500-1506, 1993. 

Reflection spectroscopy is used for more accurate quantification of the radiation reflected by a sample the intensity, spectral composition, angular distribution and polarisation can be analysed. This method is particularly apt for measuring samples that are impervious to light, that is to say, wherever absorption spectroscopy cannot be used. 

In absorption photometry the pathlength of the cuvette is usually fixed. In conventional clinical chemical methods a dilution of the sample is necessary both to run the assay under optimized conditions and to make sure that the developed color of the reaction product is within the measurable absorbance range of a spectrophotometer. The thickness of the reagent carrier in reflec-tometry which is calculated by means of the Kubelka-Munk theory, is assumed to be infinite and hence of negligible significance. Hence, the linear range in reflection spectroscopy may be expected to exceed that of absorption spectroscopy with a consequential reduction in the frequency of sample dilution prior to measurement. 

Infrared spectroscopy (IR) is a fairly simple in situ method. Since the absorption coefficients of molecular vibrations are rather low, it is impossible to detect the IR absorption of a molecule adsorbed or bonded to the semiconductor surface, merely by an ordinary vertical transmission measurement. This problem was solved by using attenuated total reflection (ATR) spectroscopy, as introduced by Harrick [17], and first applied to semiconductor-liquid junctions by Beckmann [18,19]. In this technique, the incident IR light beam is introduced via a prism into a semiconductor, at such an angle that total internal reflection occurs at the semiconductor-liquid interface, as illustrated... 

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