Infrared external reflection-absorption

Infrared spectroscopy experiments are performed at the University of the Saarland, Saarbriicken, using the external reflection absorption spectroscopy (ERAS) technique. Absorption peaks are attributed to molecular eigenvibrations which in some cases are localized at specific atom groups within the molecules. 

Infrared external reflection spectroscopy (IR-ERS) is generally based on single external reflection of IR radiation at the surface of a metal or metal film. This method is also called reflection-absorption infrared spectroscopy (RAIRS) or grazing angle... 

Infrared external reflection spectroscopy is used extensively in the study of thin films on metallic substrates, the incident beam is directly reflected from the metallic surface, and the transmitted beam is attenuated in the regions of substrate absorption for other materials. The reflected beam interacts with molecules situated at the surface of partially transmitting materials. The resulting reflectance spectra are functions of polarization and incidence angle of the incident beam and provide a quantitative measure of the surface concentrations and an indirect measure of the structure and orientation of molecules in the surface layer. 

The three most commonly applied external reflectance techniques can be considered in terms of the means employed to overcome the sensitivity problem. Both electrically modulated infrared spectroscopy (EMIRS) and in situ FTIR use potential modulation while polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) takes advantage of the surface selection rule to enhance surface sensitivity. 

A majority of traditional NIR measurements are made on solid materials and these involve reflectance measurements, notably via diffuse reflectance. Likewise, in the mid-IR not all spectral measurements involve the transmission of radiation. Such measurements include internal reflectance (also known as attenuated total reflectance, ATR), external reflectance (front surface, mirror -style or specular reflectance), bulk diffuse reflectance (less common in the mid-IR compared to NIR), and photoacoustic determinations. Photoacoustic detection has been applied to trace-level gas measurements and commercial instruments are available based on this mode of detection. It is important to note that the photoacoustic spectrum is a direct measurement of infrared absorption. While most infrared spectra are either directly or indirectly correlated... 

Fourier transform infrared microscopes are equipped with a reflection capability that can be used under these circumstances. External reflection spectroscopy (ERS) requires a flat, reflective surface, and the results are sensitive to the polarization of the incident beam as well as the angle of incidence. Additionally, the orientations of the electric dipoles in the films are important to the selection rules and the intensities of the reflected beam. In reflectance measurements, the spectra are a function of the dispersion in the refractive index and the spectra obtained are completely different from that obtained through a transmission measurement that is strongly influenced by the absorption index, k. However, a complex refractive index, n + ik can be determined through a well-known mathematical route, namely, the Kramers-Kronig analysis. 

Using the surface of liquid water as the reflective substrate, infrared reflection-absorption spectra of phospholipid layers have been obtained (123-125). The infrared beam is taken external to the spectrometer with two CaF2 lenses and a flat mirror, with another flat mirror directing the reflected beam to the detector. A Langmuir film balance was designed (124) which allowed compression of the phospholipid layers to over 50 dyne/cm. 

Geriche A., Huhnerfiiss H., In situ investigation of saturated long-chain fatty acids at the air/water interface by external infrared reflection-absorption spectrometry, J. Pf s. Chem. 97 (1993) pp. 12899-12908. 

Information on the conformational state of the hydrocarbon chains and their orientation has been obtained from external infrared reflection absorption spectroscopy (IRRAS). The first systematic IRRAS studies on phospholipid Langmuir monolayers were reported by Dluhy et al ) (see, for instance fig. 3.62). For DPPC monolayers in the LE phase the positions of the conformation-sensitive symmetric and anti-symmetric C-H stretching bands in the IRRAS spectra were found to be at the same positions as for bilayer systems of DPPC above the Kralft temperature. In the LC phase the frequencies of these bands indicate that the hydrocarbon chains of the lipid molecules are in the all-trans ) conformation (i.e. zig-zag) and analysis of polarized IRRAS spectra show that their average tilt is ca 35° relative to the monolayer normal. This is in reasonable agreement with the tilt angle of 30° obtciined from X-ray diffraction on DPPC monolayers (30°). 

R. Mendelsohn, J.W. Brauner, and A. Gericke, External Infrared Reflection Absorption Spectrometry of Monolayer Films at the Air-Water Interface, Ann. Rev. Phys. Chem. 46 (1995) 305. (Review theory and practice of IRRAS as applied to Langmuir monolayers determination of conformational states of hydrocarbon tails and H-bonding, ionization states of head groups, and molecular orientation illustrated with experimental results on monolayers of single-chain amphiphiles, phospholipids and proteins.)... 

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. 

Alpers W and Hiihnerfuss H (1989) The damping of ocean waves by surface films a new look at an old problem. J Geophys Res 94 6251-6265 Benvegnu DJ and McConnell HM (1992) Line tension between liquid domains in lipid monolayers. J Phys Chem 96 6820-6824 Gericke A, Michailov AV, and Hiihnerfuss H (1993) Polarized external infrared reflection-absorption spectroscopy at the air/water interface comparison of experimental and theoretical results for different angles of incidence. Vib Spectroscop 4 335-348... 

Mendelsohn R, Brauner J W and Gericke A 1995 External infrared reflection absorption spectrometry of monolayer films at the air-water interface Ann. Rev. Rhys. Chem. 46 305-34... 

Surface enhanced infrared absorption (SEIRA) has been observed in external reflection spectroscopy [185], for further details, see Sect. 5.2.5. 

One of the most commonly applied IR techniques developed to overcome these problems is the external reflectance technique. In this method, the shong solvent absorption is minimized by simply pressing a reflective working electrode against the IR transparent window of the electrochemical cell. The sensitivity problem, that is, the enhancement of the signal/noise ratio in the case of external reflectance techniques is solved by various approaches. These are, for instance, electrochemically modulated infrared spectroscopy (EMIRS), in situ FTIR (which use potential modulation), and polarization modulation infrared reflection absorption spectroscopy (PM-IRAS, FTIR) [86,117-123]. 

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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