Ultra-sensitive instrumentation in the UV-Vis-NIR spectral region with application on overtone spectroscopy.
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Fourier transform infrared spectroscopy (FT-IR) is a technique widely used in molecular spectroscopy. Technology advancements are such that FT-IR spectrometers can measure vibrational transitions occurring well within the visible (Vis) region of the electromagnetic spectrum. High overtone transitions of C-H, N-H, and O-H can be measured in the near-infrared (NIR) and the Vis regions using long path cells. Despite the large sensitivity of the FT-IR spectrometers and the use of long path cells, the absorption intensities of these transitions are so low that commercial FT-IR spectrometers would not register the absorption bands. Phase shift cavity ring down (PS-CRD) and thermal lens spectroscopy (TLS) are ultra-sensitive instrumental absorption techniques very commonly used in our laboratory. These are very powerful techniques for the detection of weak absorptions in the NIR-Vis region. In the case of high overtone spectra, CRD is ideal for gas samples whereas TLS is great for liquid samples. In our laboratory we use FT-IR and UV-Vis spectrometers as complementary techniques to CRDS and TLS. In order to simulate conditions in the atmospheres of other planets, we couple our instruments to a low temperature cryostat. In this way, these ultra-sensitive techniques can be applied to the study of gases in the atmosphere and liquid cryo-solutions to simulate the lakes of Saturn’s largest moon, Titan. This work shows the use of signal-to-noise ratios to determine the spectral quality of spectra obtained with the PS-CRD technique, particularly in situations where human eye cannot perceive the small differences. We evaluate the impact of some instrumental parameters involved in data acquisition process of the PS-CRD technique, as well as evaluate the spectral quality as a function of the On-Axis/Off-Axis position of the incoming beam. This work also introduces the use of a second pumping laser for the analysis of multi-component samples in thermal lens. The design facilitates the detection of solvent and solute exciting two different vibrational overtones. Finally, the determination of band asymmetry using spectral moment calculations is shown for fundamental bands and vibrational overtones. The usefulness of this method as a complementary feature to TL spectroscopy is discussed.
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