Infrared external reflection

FTIR-ERS Fourier Transform Infrared external reflection spectroscopy... 

Teresa Iwasita and F. C. Nart provide a valuable perspective on the foundations, capabilities, and limitations of in-situ infrared external reflection spectroscopy of electrode surfaces, with emphasis on Fourier Transform instruments. In addition to the description of underlying principles and instrumentation, selected examples are given of the monitoring and interpretation of spectra of various species adsorbed at electrochemical interfaces. 

A consistent description of the structure of alkanethiol monolayers on gold has emerged from an array of spectroscopic and diffraction studies. X-ray photoelectron spectroscopic (XPS) studies support the presence of anisotropically chemisorbed alkanethiolates on gold [24-29]. Ellipsometric measurements [24-27, 30], capacitance studies [30] and XPS measurements [31] confirm monolayer film thickness. Fourier transform infrared external reflective spectroscopy (FTIR-ERS) shows that the chains tilt at about 30° off the surface normal, and the plane containing the carbon backbone is twisted out of the plane of tilt by about 50° [25-27, 30, 32, 33]. 

Claybom M, Umemura J, Merklin GT, Kattner J, Hoffinann H, Mendelsohn R, Flach CR, Frey BL, Com RM, Weibel SC, Rdseler A, Korte E-H (2002) Mid-infrared external reflection spectroscopy. In Griffiths PR, Chahners J (eds) Handbook of vibrational spectroscopy, vol 2. Wiley, Chichester, pp 969-1090... 

Characterization of these structures can be accomplished using a number of techniques, with the most useful being XPS [45], polarized infrared external reflectance spectroscopy (PIERS) [46-48], measurement of contact angles, ellipsometry [49,50] and, increasingly, STM/AFM [51-54]. Computation has also been very useful in understanding the order in these structures [55, 56]... 

These computations were also used to screen the most favorable configuration of a molecule on a given surface. For example, the most stable conformation of a carboxylic acid (oleate) molecule on a fluorite 111 surface was found to be a bidentate conformation (Mielczarski et al. 2002). As shown in Figure 2.7, the theoretically simulated conformation, as indicated through adsorption angles, matched well with that measured experimentally, as obtained through a sophisticated in situ infrared external reflection spectroscopy technique (Mielczarski et al. 1998). 

FIGURE 2.7 Conformation of adsorbed oleate molecule on fluorite 111 surface. The numbers in bracket denote experimental angles obtained through insitu infrared external reflection spectroscopy technique reported by (From Mielczarski, E., Mielczarski, J. A., and Cases, J. M., Langmuir, 14 1739,1998.)... 

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... 

Internal reference electrode IR-ERS Infrared external reflection... 

Hasegawa, T., Umemura, J. and Takenaka, T. (1993) Infrared external reflection study of molecular orientation in thin Langmuir-Blodgett films. J. Phys. Chem., 97, 9009-9012. 

Polarized infrared external reflection spectroscopy (PIERS) has been very useful in determining the structural details of these SAMs (11,15,20,21). PIERS results show that SAMs of alkanethiolates on gold are nearly analogs of two-dimensional crystalline alkanes. The alkane chains are predominantiy in a trans zig-zag conformation the few gauche conformations are concentrated near the ends of the chains (15,18,23). The alkane chains are tilted 26° relative to the surface normal. This tilt, which allows the chains to be close-packed, is a direct result of the lattice spacing of the sulfur atoms. 

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. 

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