Surface infrared spectroscopy

▪ Abstract Advances in the field of vibrational optical activity (VOA) are reviewed over the past decade. Topics are surveyed with an emphasis on the theoretical and instrumental progress in both vibrational circular dichroism (VCD) and Raman optical activity (ROA). Applications of VOA to stereochemical and biological problems are reviewed, with a bias toward new kinds of experiments made possible by theoretical and instrumental advances. In the field of VCD, the most notable advances have taken place in the quality and size of ab initio calculations of VOA intensities and in the quality of step-scan Fourier transform instrumentation. For ROA, the most dramatic progress has occurred in the areas of theoretical formulation and high-throughput instrumentation. Applications of VOA now include all major classes of biological and pharmaceutical molecules. VOA's importance as a diagnostic tool will likely grow as the control of molecular chirality increases in research and industrial areas.

INFRARED AND RAMAN VIBRATIONAL OPTICAL ACTIVITY: Theoretical and Experimental Aspects

The experimental and the evaluation methods of infrared ellipsometry are outlined. The results provide a highly reliable basis for further interpretation, for example, material properties or molecular orientation. Various applications demonstrate the scope of the method.



Infrared ellipsometry is a reflection technique for determining the optical properties of a sample. It is advantageously employed to characterize layers down to nanometer thickness. The two experimental results obtained per spectral data point can be evaluated for the sample's optical constants (refractive and absorption index or the complex dielectric function) by strict application of optical theory. In addition, the layer thickness is available since it is independent of the wavelength and the generally applied interferometers cover a wide spectral interval. The evaluation can be extended to anisotropic optical properties.


Keywords:

infrared ellipsometry;
monolayers;
optical constants;
refractive index;
dielectric function

Infrared Spectroscopic Ellipsometry

Polarized external infrared reflection-absorption spectrometry at the air/water interface: Comparison of experimental and theoretical results for different angles of incidence

A Bomem MB100-based vibrational circular dichroism (VCD) spectrometer suitable for chiroptical studies in the infrared region is described. The optical layout utilizes the spectrometer's external collimated beam. Standard VCD optics consisting of a KRS-5 polarizer and a ZnSe photoelastic modulator are mounted on an optical breadboard. A ZnSe meniscus lens focuses the beam on a small-area MCT detector. Several optical designs were examined to find the optimal positions of the optical elements. The experiments aimed to improve the energy throughput, minimize polarization artifacts, and increase signal-to-noise ratio to such an extent that single-enantiomer VCD spectra could be recorded routinely and dependably. Examples are presented illustrating the high-performance characteristics of the instrument.

Alternative Design for Improved FT-IR/VCD Capabilities

The properties of a Fourier transform infrared (FT-IR) spectrometer designed around a polarizing Michelson interferometer (PMI) are analyzed with respect to applications in both conventional absorption as well as polarization spectroscopies. The PMI design is that of Martin and Puplett and consists of a metal grid polarizing beam splitter combined with rooftop retromirrors set to rotate the polarization direction in each beam by 90°. If used in conjunction with additional grid polarizers, positioned according to application, the result is an FT-IR instrument that can function both as a dual beam absorption spectrometer and as a polarization spectrometer. Operating in the former mode, the instrument is predicted to achieve speeds comparable to those achievable using a nonpolarizing dual beam Michelson interferometer but to cover a much wider spectral range due to the achromatic nature of metal grid polarizing beam splitters. In the latter mode, it is predicted to give at least double the effective throughput achievable with conventional instruments equipped with external polarization modulating optics, and again to cover a much wider spectral range. It would extend the capability of determining circular dichroism spectra into the far IR (down to ∼3 cm−1) for the first time. With the development of a process for forming a metal grid polarizer of fine spacing (∼0.1 μm) on a KBr substrate, the instrument would be capable of covering the spectral range 25 000 to 3 cm−1 or lower using only two beam splitters, i.e., in two ranges: 25 000 to 450 cm−1 using a KBr-supported metal grid, and 600 to 3 cm−1 or less using an unsupported wire grid.

Analysis of a Polarizing Michelson Interferometer for Dual Beam Fourier Transform Infrared, Circular Dichroism Infrared, and Reflectance Ellipsometric Infrared Spectroscopies

M. J. Dignam

Papers

Analysis of a Polarizing Michelson Interferometer for Dual Beam Fourier Transform Infrared, Circular Dichroism Infrared, and Reflectance Ellipsometric Infrared Spectroscopies