Reflection-absorption spectroscopy measurements

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. 

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... 

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. 

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. 

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. 

FTIR spectroscopy has become a standard technique for investigating Idle structure and level of orientational order of ultrathin films, that is, films in the submicron range (2.5 to 500 nm) (9). By combining transmission spectroscopy with infrared reflection-absorption spectroscopy (this technique is also referred to as grazing incidence reflection), the orientation of the functional groups of the molecules in the film can be investigated. These measurements are nondestructive to the film and can be conducted at a range of temperatures and pressures. 

Generally, the assumption is made that scattering does not depend on the wavenumber so that the conversion of the measured reflectance spectrum R by means of the Kubelka-Munk function F R), results in an absorption-proportional representation. As for ATR and reflection-absorption spectroscopy, also the diffuse-reflectance spectmm does not consist of dispersion features but band-like structures. For changes in low absorption, the sensitivity of diffuse reflectance is greater than the one of transmittance, while strong absorption bands are less pronounced in the diffuse-reflection (see Fig. 6.4-18). Therefore, diffuse-reflection spectra resemble poorly resolved transmittance spectra. For diffuse reflectance spectra where R is in the order of 0.01 or below, the function -log R or just I / R is equally well suited for conversion (Olinger and Griffiths, 1988). Such level are found with compact samples such as polymer foams or varnishes with filler (Otto, 1987 Korte and Otto, 1988). 

Different types of reflectance spectroscopy depend upon the reflecting behavior of the radiation on the solid. Fig. 8 illustrates various categories used to distinguish techniques for reflecting radiation off solids. Specular reflection spectroscopy is used to measure the reflectance spectrum of a smooth, glossy surface. In reflection-absorption spectroscopy, the radiation passes through a thin surface film on a reflective... 

The properties of the dual-film electrode were characterized by in situ Fourier transform infrared (FTIR) reflection absorption spectroscopy [3]. The FTIR spectrometer used was a Shimadzu FTIR-8100M equipped with a wide-band mercury cadmium teluride (MCT) detector cooled with liquid nitrogen. In situ FTIR measurements were carried out in a spectroelectro-chemical cell in which the dual-film electrode was pushed against an IR transparent silicon window to form a thin layer of solution. A total of 100 interferometric scans was accumulated with the electrode polarized at a given potential. The potential was then shifted to the cathodic side, and a new spectrum with the same number of scans was assembled. The reference electrode used in this experiment was an Ag I AgCl I saturated KCl electrode. The IR spectra are represented as AR/R in the normalized form, where AR=R-R(E ), and R and R(E ) are the reflected intensity measured at a desired potential and a base potential, respectively. 

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. 

The FTIR spectrum of the PTFE film deposited by laser ablation was identical to that of the target [54], but that of the film produced by SR etching showed some visible differences (see Fig. 29). Obviously, the C-F2 deformation bands at 640 and 513 cm-1 appear much smaller in the bottom trace. To understand why these 640 and 513 cm-1 bands were so small in the SR case, we measured both normal and oblique transmission of FTIR with an incident angle of 0 and 80° [58]. Two FTIR spectrometers (PERKIN-ELMER and JASCO) were used to measure spectra in the range 400-3000 cm-1. For a cross-check, the film was also deposited on a metallic surface and infrared reflection absorption spectroscopy [62] was carried out to confirm our oblique transmission measurements. Typical changes in the FTIR transmission... 

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... 

Polarization-modulation infrared reflection-absorption spectroscopy (PM-IR-RAS) spectra were recorded with a Bruker ITS 66/S Fourier transform infrared spectrometer equipped with a PMA 37 polarization modulation module and a ititrogen-cooled MCT detector. The infrared beam was first p-polarized with a ZnSe wire grid polarizer (Specac) before passing through a photoelastic modulator (Hinds Instruments, PEM-90), which modulated at a frequency of 74 kHz. A lock-in ampHfier (Stanford model SR-830) was used to obtain the PM-IRRAS spectra. The half-wave retardation frequency was set at 4000 cm . The PM-IR-RAS spectra were recorded as S= R -Rs)/(R +Rg). A total of 250 scans at a resolution of 4 cm were collected for each measurement at an angle of incidence of 82.5° with respect to the normal to the sample surface. 

The third example is the reflection measurement at a rotating disk electrode (RDE). Scherson and his coworkers have developed near-normal incidence UV-visible reflection-absorption spectroscopy at RDEs [50-52]. Both (AR/R)dc and (AR/R)er have been measured under hydrodynamic conditions. The use of an RDE enables them to quantitatively control the diffusion layer concentration profile of the solution phase species, especially the species generated electro-... 

This technique is used to study thin (down to submonolayer) films adsorbed on reflective substrates such as metals. Experimentally it involves measuring the change in the reflectance spectrum of the substrate that accompanies thin film formation. Various acronyms for the technique are used infrared reflection—absorption spectroscopy (IRRAS, IRAS) and reflection—absorption infrared spectroscopy (RAIRS). The Basics of IRRAS spectra are described in Chapter 5.2. 

The controlled deposition of ruthenium on well-defined surfaces, such as Pt(hkl) [95-103] and Au hkl) [38-40], has been characterized by electrochemical measurements, Fourier transform infrared reflection-absorption spectroscopy (FT-IRRAS), XPS and STM measurements. The interest in these studies is mainly concentrated on the ruthenium modification of a platinum surface because of its extreme importance in electrocatalysis. It has been demonstrated that a ruthenium-deposited Pt( 111) substrate showed an extremely high activity in methanol oxidation compared to ruthenium-deposited Pt(hkl) electrodes with other crystallographic orientations [98, 99]. 

The experimental problem of reflectance spectroscopy is the usually small effect of a thin surface film on the reflected light. The change of the intensity during reflection is measured in UV/vis reflectance absorption spectroscopy and IR reflectance absorption spectroscopy. In the case of IR reflectance absorption spectroscopy, another problem is the absorption of the electrolyte. This has led to the development of thin-layer cells. 

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