D. Tsankov

Alternative Design for Improved FT-IR/VCD Capabilities

A Bomem MB100-based vibrational circular dichroism (VCD) spectrometer suitable for chiroptical studies in the infrared region is described. The optical layout utilizes the spectrometers 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.

Comparative Study of an Anisotropic Polymer Layer by Infrared Spectroscopic Techniques

Infrared spectroscopic ellipsometry (IRSE), reflection absorption IR spectroscopy (RAIRS), IR transmission spectroscopy, and best-fit calculations are applied in a cooperative study to determine the anisotropic optical properties of a thin polyimide layer in the spectral range 4000–500 cm−1. The employed anisotropic uniaxial optical layer model afforded very good agreement between the calculated and the experimental spectra obtained by the different complementary IR methods. The main advantage of IRSE is that it is possible to obtain data for the optical constants and the thickness (d = 1.81 μm) of the polyimide layer simultaneously within one experiment. From the ellipsometric spectra it was concluded that the layer structure can be regarded as possessing uniaxial symmetry where the layer is isotropic in directions (x, y) parallel to the sample surface. A qualitative determination of the anisotropic parameters of vibrational bands is possible by calculation of the ellipsometric spectra. The evaluation procedure can be improved by evaluation of polarized reflection spectra, provided the reference standard has been calibrated by ellipsometry. The oscillator parameters are then derived more accurately from the separate s- and p-polarized reflection spectra rather than from their ratio, which is measured in ellipsometry.

Molecular orientation in octanedithiol and hexadecanethiol monolayers on GaAs and Au measured by infrared spectroscopic ellipsometry.

Infrared spectroscopic ellipsometry was used for determination of molecular orientation and for lateral homogeneity studies of organic monolayers on GaAs and Au, the organic layer being either octanedithiol or hexadecanethiol (HDT). The laterally resolved measurements were performed with the infrared mapping ellipsometer at the synchrotron storage ring BESSY II. The molecular orientation within the monolayers was determined by optical model simulations of the measured ellipsometric spectra. Different tilt angles were obtained for the monolayers of HDT and octanedithiol on GaAs: 19 degrees and >30 degrees , respectively. The tilt angle of the methylene chains for HDT on Au substrate (22 degrees ) is similar to the 19 degrees tilt which was obtained for the HDT monolayers on GaAs, thus suggesting similar molecular ordering of the thiolates on both substrates.

Total Reflection Prisms as Achromatic IR Retarders

Retarders based on total internal reflections are discussed. Quarterwave retardation can be obtained by a single reflection when one is using material of high refractive index (≥2.4), as it is available for the IR range. Alternatively, the influence of the beam divergence can be efficiently compensated for. Several optical designs and first results are presented.

FTIR Ellipsometry as a Tool for Studying Organic Layers: From Langmuir-Blodgett Films to Can Coatings

InstituteofSpectrochemistryandAppliedSpectroscopy,DepartmentBerlin,Albert-Einstein-Str.9,D-12489Berlin-Adlershof,Germany(Received July 20, 2001; in revised form August 9, 2001; accepted August 20, 2001)Subject classification: 68.47.Pe; 78.30.Jw; 78.66.Qn; S12The capabilities of infrared ellipsometry to examine structurally various thin organic layers depos-ited on different substrates are demonstrated in this contribution. The presented examples involve:Langmuir-Blodgett film on gold substrate, anisotropic film of polystyrene on a silicon wafer, im-planted fluorinated surface layer into a bulk polyethylene fuel tank and can coating deposited onelectrolytically-plated steel (tinplate). Different calculational procedures were used to determinethe film optical constants and thickness.

Structure and optical properties of thin As2S3–In2S3 films

This paper deals with the optical properties of thin As2S3–In2S3 films. The thin layers were deposited by thermal co-evaporation of As2S3 and In2S3. The composition of the coatings was controlled by x-ray microanalysis; it was found to be close to the expected one. The refractive index n and optical band gap were calculated from the transmittance and reflectance spectra. The results showed that the refractive index of thin As–S films is not affected by the addition of 1 at% indium and it increases from 2.46 to 2.58 for thin film with 13 at% In. A decrease in the changes in the refractive index, Δn, after exposure to light or annealing with addition of indium in arsenic sulfide is observed. To explain the influence of the indium on the photoinduced changes in the optical properties of thin As–S–In films, the glass structure was investigated by infrared spectroscopy. The calculated values of the optical constants were compared with those obtained from ellipsometric measurements.

FT IR-VCD spectra of six octadeoxynucleotides

Abstract The VCD spectra in the region of 1750 to 850 cm −1 of six octadeoxynucleotides consisting of systematically varying sequences of d(CGNNNNGC) 2 , where N = A, T, C, G, are reported and tentatively interpreted based on previously reported VCD spectra of related natural and synthetic polymers and oligomers. The CO and skeletal stretching vibrations of the bases exhibit features that are characteristic for base sequences and reveal details that are not apparent in the absorption spectra. Other features, notably an unassigned band near 1500 cm −1 and the PO 2 − and furanose modes near 1080 cm −1 are independent of base sequences.

A differential method for linear dichroic IR spectra - its theory and applications

Abstract The suggested method consists in measuring the IR spectrum of any oriented sample placed in the common beam of an IR-spectrometer with a polarizer in the sample beam, the zero line being set in the middle of the transmittance scale. The spectrum contains positive and negative bands depending on the orientation of the vibracional transition moments. This differencial technique has the features of a modulation spectroscopy method. The corresponding differential infrared linear dichroic (DIRLD) spectra exhibit simple pattern of the IR-active bands (positive, negative or missing) directly linked with the symmetry properties of the molecular vibrations.

VCD STUDY OF SYNTHETIC CHIRAL POLYMERS: STEREOREGULARITY IN POLY-MENTHYL METHACRYLATE

Abstract Menthyl methacrylate (MnMA) was polymerized both by free radical and anionic initiators in order to obtain polymers with low (poly-MnMA-rad) and high (poly-MnMA-an) isotactic content, respectively, and their VCD spectra were measured as CCl 4 solutions between 1650-850 cm −1 . Common growth of all VCD intensities, observed in the spectrum of poly-MnMA-an, indicate more and/or longer helical segments. The VCD spectra also revealed that the vibrations of the chain are most sensitive to changes in stereoregularity.

A method for measuring circular dichroism of induced cholesteric solutions in the infrared region

Abstract The principle of the suggested method consists in using two cholesteric filters for left and right circularly polarized light, placed in the sample and in the reference beam of a double beam IR spectrometer. The sample itself is inserted in the common beam of the spectrometer between the rotating sector mirror and the entrance slit of the monochromator. The cholesteric filters are used in the region of their selective reflection comprising an interval of 150–200 cm−1 where the absorption band of the nematic solvent of the induced cholesteric solution studied must also be present.