Infrared reflection-absorption measurement

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. 

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. 

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. 

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... 

There are two alternatives for acquiring infrared spectra from the deposited samples. Samples deposited on an optically flat germanium disk with an aluminum backing are transferred to a separate module for reflectance-absorption measurements. The disk is rotated continuously during sample collection, with the result that the chromatogram is deposited as a track near the perimeter of the disk. Rotation of the disk in the spectrometer module allows either a continuous scan of the deposited chromatogram... 

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 analysis of thin films is often performed by infrared spectroscopy. Compared to reflection absorption measurements on metal surfaces using p-polarized fight at grazing incidence, investigation of self-supported thin films or of thin films on transparent substrates shows weaker infrared absorption bands. Weaker absorption bands are caused by the absence of the surface enhancement mechanism and the poorer reflectivity. On the other hand, due to the absence of the metal selection rule, spectra of p- and s-polarization can be recorded in the case of freestanding films or of transparent substrates. Complex spectral features may arise... 

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. 

Modeling of electrochemical interfaces under UHV conditions supplements the aforementioned approaches. For example, it exploits the controlled modification of the metal/vacuum interface by the various components typical of electrochemical interfaces (ions, solvent molecules) and registration of infrared reflection-absorption spectra along with work function measurements [92]. A gradual increase in the surface coverage by solvent molecules, which changes from a submonolayer up to multilayers at various doses of adsorbed... 

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]. 

Spin coating is a useful method for laying down onto an infrared transparent window a uniform thin layer of a few micrometers thickness of a polymer from a solution. A film can conveniently be cast from a polymer that is soluble in a convenient solvent at a concentration of 1-2%. An example of a spin-coater is shown in Figure 4 a clean 13 mm diameter window is attached to the sample stub, onto which several drops of the solution are laid down from a pipette, after which the rotor is spim for a few seconds. Thin films on gold-coated disks suitable for reflection-absorption measurements (see the section Thin films on metallic substrates ), may also be prepared by the spin-coating procedure. 

Figure 9 Optical setup for external reflection infrared measurements of Langmuir-Blodgett monolayers on liquid water surfaces. The arrow indicates the shuttling direction of the Langmuir trough in order to switch between the sample (monolayer on water) and the reference (pure water) position. (Reprinted with permission from Flach CR, Gericke A, and Mendelsohn R (1997) Quantitative determination of molecular chain tilt angles in mono-layer films at the air/water interface infrared reflection/absorption spectroscopy of behenic acid methyl ester. Journal of Physical Chemistry S 101 58-65 American Chemical Society.)...

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