Interferometric time delay correction for Fourier transform spectroscopy in the extreme ultraviolet

We demonstrate a Fourier transform spectrometer in the extreme ultraviolet (XUV) spectrum using a high-harmonic source, with wavelengths as short as 32 nm. The femtosecond infrared laser source is divided into two separate foci in the same gas jet to create two synchronized XUV sources. An interferometric method to determine the relative delay between the two sources is shown to improve the accuracy of the delay time, with corrections of up to 200 asec required. By correcting the time base before the Fourier transform, the frequency resolution is improved by up to an order of magnitude.     

Uniform time-sampling Fourier transform spectroscopy

We show that uniform time sampling of both the reference and the target channels in a continuous scanning Fourier transform spectrometer is a simple and versatile way of extending the Nyquist limit shorter than the wavelength of the reference channel. We also discuss the benefits of recording the reference channel when intensity calibrating the target data.     

Apodizing functions for Fourier transform spectroscopy

Apodizing functions are used in Fourier transform spectroscopy (FTS) to reduce the magnitude of the sidelobes in the instrumental line shape (ILS), which are a direct result of the finite maximum optical path difference in the measured interferogram. Three apodizing functions, which are considered optimal in the sense of producing the smallest loss in spectral resolution for a given reduction in the magnitude of the largest sidelobe, find frequent use in FTS [J. Opt. Soc. Am.66, 259 (1976)]. We extend this series to include optimal apodizing functions corresponding to increases in the width of the ILS ranging from factors of 1.1 to 2.0 compared with its unapodized value, and we compare the results with other commonly used apodizing functions.     

Fourier transform spectroscopy of LaS in the infrared

We recorded the emission spectrum of diatomic lanthanum sulfide on the Los Alamos Fourier transform spectrometer. In the region 7500-16,000 cm(-1), we identified over 120 bands and assigned them to the A(2)∏(r)-X(2)Σ(+) and B(2)Σ(+)-X(2)Σ(+) transitions. Each of these bands is four headed.     

High-information time-resolved Fourier transform spectroscopy at work

The first instrumental setup, to our knowledge, that is capable of recording in a few hours the time-resolved Fourier transform (TRFT) interferograms of gas-phase spectra that cover several thousands of inverse centimeters with spectral- and time-resolution limits that are equal, at best, to 2.5 x 10(-3) cm(-1) and 2 ns, respectively, is reported. It was developed on the stepping-mode Connes-type interferometer of the Laboratoire de Photophysique Moléculaire Université de Paris Sud. Also, for the first time, to our knowledge, these high-resolution TRFT spectra, illustrated with the Doppler-limited emission spectra of the N(2) transitions (B-A) and (B'-B) between 5500 and 11 000 cm(-1) and of the atomic Ar lines between 1800 and 4000 cm(-1), are recorded in the infrared spectral range. To obtain identical results that have the same signal-to-noise ratio, we should have increased the recording time of our unique previous high-information TRFT spectra by approximately 50,000. In other words, one hour is now long enough to obtain what would previously have required six years to record.     

Fourier transform Raman spectroscopy of thin films

Conventional Raman measurements of dyes in the visible region exhibit resonant effects which enhance the Raman scattering cross-section of the chromophores by several orders of magnitude but make scattering from other parts of the molecules, such as the hydrocarbon chains, unobservable. Taking advantage of the benefits inherent to Fourier transform (FT) spectroscopy, Raman spectra can now be measured routinely with an FT IR spectrometer and a continuous wave Nd:YAG laser. By coupling the laser excitation into a thin film waveguide, we have recently observed FT Raman spectra of a thin film of polystyrene. The advantages of using integrated optics with FT Raman spectroscopy for Langmuir-Blodgett films of dye molecules are also discussed.     

Correction of instrument line-shape distortions in Fourier transform spectroscopy

A recovery procedure has been developed to correct instrument line-shape distortions observed in Fourier transform spectroscopy. The procedure can be described as a phase-error correction performed in the spectral domain to correct for path-difference-dependent phase errors observed in sharp spectral features. The technique has been applied successfully to high-resolution atmospheric emission spectra. The inherent broadening of the real features has been separated accurately from instrumental distortions. Using models for the path-difference-dependent error sources and data from two narrow window regions at 50 and 118 cm(-1), we show that the distortion has a simple dependence on the spectral frequency.     

Application of Fourier transform infrared spectroscopy in cement Alkali quantification

Alkali in cement is responsible for the Alkali–silica-reaction phenomenon that manifests itself in the form of premature cracking in concrete structures such as bridge decks and concrete pavements. X-ray fluorescence spectroscopy (XRF) is commonly used for cement Alkali quantification but a simpler and faster analytical procedure based on Fourier transform infrared spectroscopy (FTIR) has been expanded for this purpose. An analytical absorption band at 750 cm^?1 in the FTIR spectra of cement samples belonging to Alkali solid solution of tricalcium aluminate [C₃A(ss)] is used for Alkali quantification. Regression analysis of a plot correlating FTIR absorption band area ratio (750/923 cm^?1) to equivalent Alkali Na₂O _e (Na₂O _e  = % Na₂O + 0.658 × % K₂O) measured by XRF shows a linear correlation coefficient, R ², of 0.97. High Alkali cement samples show a higher microstructural disorder coefficient, C _d, which is a reactivity criterion introduced by Bachiorrini and co-authors (Proceedings of the seventh international conference on concrete alkali-aggregate reactions? 1986) for ASR-susceptible aggregates. Results of this research indicate applicability of FTIR technique to quantitatively predict cement vulnerability to ASR through the \( A_{{750\,{\text{cm}}^{ - 1} }} \) to \( A_{{923\,{\text{cm}}^{ - 1} }} \) band area ratio and the magnitude of the disorder coefficient (C _d).     

Characterization of magnetic paper using Fourier transform infrared spectroscopy

Magnetic papers were prepared by using the co-precipitation method. The spectral data of the magnetic fibres were obtained by using the photoacoustic Fourier transform infrared spectroscopy (FTIR-PAS) and attenuated total reflection (ATR). It was found that the elevated loading degree increased the IR absorption and reduced the tensile strength of the paper. The partial-least-squares analyses showed that the FTIR-ATR data were strongly correlated with the degree of loading and the correlation obtained was better than that of the FTIR-PAS spectral data.     

Quantitative determination of moisture in lubricants by Fourier transform infrared spectroscopy

This paper describes the development of a practical Fourier transform infrared (FT-IR) method for the determination of moisture in lubricants through the combined use of signal transduction and differential spectroscopy to circumvent matrix effects. The acid-catalyzed stoichiometric reaction of 2,2-dimethoxypropane (DMP) with moisture to produce acetone was used to provide IR signals proportional to the amount of moisture present in oils. Calibration standards were prepared by spiking polyalphaolefin (PAO) gravimetrically with water using dioxane as a carrier. For FT-IR analysis, standards and samples were diluted with acidified isooctane and then split, with one aliquot treated with DMP and the other with a blank reagent. The spectra of the two aliquots were collected, and a differential spectrum was obtained so as to ratio out the invariant spectral contributions from the sample. Quantitation for moisture was based on measurement of the peak height of the nu(C=O) absorption of acetone at 1717 cm(-1), yielding a standard error of calibration of approximately 40 ppm H2O. The method was validated by standard addition of water in dioxane to PAO containing added base as well as to new and used oils. In all cases the method responded quantitatively to standard addition, the average standard error of prediction being approximately 80 ppm, with the results showing only a minor dependence on the oil formulation. From an analytical perspective, the FT-IR method is both more reproducible and more accurate than Karl Fischer methods and has advantages in terms of environmental considerations, sample size, and speed of analysis as well as the variety of oil types that can be handled. Signal transduction/differential spectroscopy may have broader utility as an alternative means for the determination of low levels of moisture in complex matrices.     

Fluorescence lifetime imaging and Fourier transform infrared spectroscopy of Michelangelo's David

We developed a combined procedure for the analysis of works of art based on a portable system for fluorescence imaging integrated with analytical measurements on microsamples. The method allows us to localize and identify organic and inorganic compounds present on the surface of artworks. The fluorescence apparatus measures the temporal and spectral features of the fluorescence emission, excited by ultraviolet (UV) laser pulses. The kinetic of the emission is studied through a fluorescence lifetime imaging system, while an optical multichannel analyzer measures the fluorescence spectra of selected points. The chemical characterization of the compounds present on the artistic surfaces is then performed by means of analytical measurements on microsamples collected with the assistance of the fluorescence maps. The previous concepts have been successfully applied to study the contaminants on the surface of Michelangelo's David. The fluorescence analysis combined with Fourier transform infrared (FT-IR) measurements revealed the presence of beeswax, which permeates most of the statue surface, and calcium oxalate deposits mainly arranged in vertical patterns and related to rain washing.