Mid-infrared Absorption Spectra Research Articles

Methane cross-validation between three Fourier transform spectrometers: SCISAT ACE-FTS, GOSAT TANSO-FTS, and ground-based FTS measurements in the Canadian high Arctic

Abstract. We present cross-validation of remote sensing measurements of methane profiles in the Canadian high Arctic. Accurate and precise measurements of methane are essential to understand quantitatively its role in the climate system and in global change. Here, we show a cross-validation between three data sets: two from spaceborne instruments and one from a ground-based instrument. All are Fourier transform spectrometers (FTSs). We consider the Canadian SCISAT Atmospheric Chemistry Experiment (ACE)-FTS, a solar occultation infrared spectrometer operating since 2004, and the thermal infrared band of the Japanese Greenhouse Gases Observing Satellite (GOSAT) Thermal And Near infrared Sensor for carbon Observation (TANSO)-FTS, a nadir/off-nadir scanning FTS instrument operating at solar and terrestrial infrared wavelengths, since 2009. The ground-based instrument is a Bruker 125HR Fourier transform infrared (FTIR) spectrometer, measuring mid-infrared solar absorption spectra at the Polar Environment Atmospheric Research Laboratory (PEARL) Ridge Laboratory at Eureka, Nunavut (80° N, 86° W) since 2006. For each pair of instruments, measurements are collocated within 500 km and 24 h. An additional collocation criterion based on potential vorticity values was found not to significantly affect differences between measurements. Profiles are regridded to a common vertical grid for each comparison set. To account for differing vertical resolutions, ACE-FTS measurements are smoothed to the resolution of either PEARL-FTS or TANSO-FTS, and PEARL-FTS measurements are smoothed to the TANSO-FTS resolution. Differences for each pair are examined in terms of profile and partial columns. During the period considered, the number of collocations for each pair is large enough to obtain a good sample size (from several hundred to tens of thousands depending on pair and configuration). Considering full profiles, the degrees of freedom for signal (DOFS) are between 0.2 and 0.7 for TANSO-FTS and between 1.5 and 3 for PEARL-FTS, while ACE-FTS has considerably more information (roughly 1 DOFS per altitude level). We take partial columns between roughly 5 and 30 km for the ACE-FTS–PEARL-FTS comparison, and between 5 and 10 km for the other pairs. The DOFS for the partial columns are between 1.2 and 2 for PEARL-FTS collocated with ACE-FTS, between 0.1 and 0.5 for PEARL-FTS collocated with TANSO-FTS or for TANSO-FTS collocated with either other instrument, while ACE-FTS has much higher information content. For all pairs, the partial column differences are within ±3 × 1022 molecules cm−2. Expressed as median ± median absolute deviation (expressed in partial column units or as a percentage), these differences are 0.11 ± 9.60 × 1020 molecules cm−2 (0.012 ± 1.018 %) for TANSO-FTS–PEARL-FTS, −2.6 ± 2.6 × 1021 cm−2 (−1.6 ± 1.6 %) for ACE-FTS–PEARL-FTS, and 7.4 ± 6.0 × 1020 molecules cm−2 (0.78 ± 0.64 %) for TANSO-FTS–ACE-FTS. The differences for ACE-FTS–PEARL-FTS and TANSO-FTS–PEARL-FTS partial columns decrease significantly as a function of PEARL partial columns, whereas the range of partial column values for TANSO-FTS–ACE-FTS collocations is too small to draw any conclusion on its dependence on ACE-FTS partial columns.

Retrieval of xCO<sub>2</sub> from ground-based mid-infrared (NDACC) solar absorption spectra and comparison to TCCON

Abstract. High-resolution solar absorption spectra, taken within the Network for the Detection of Atmospheric Composition Change Infrared Working Group (NDACC-IRWG) in the mid-infrared spectral region, are used to infer partial or total column abundances of many gases. In this paper we present the retrieval of a column-averaged mole fraction of carbon dioxide from NDACC-IRWG spectra taken with a Fourier transform infrared (FTIR) spectrometer at the site in Ny-Ålesund, Spitsbergen. The retrieved time series is compared to colocated standard TCCON (Total Carbon Column Observing Network) measurements of column-averaged dry-air mole fractions of CO2 (denoted by xCO2). Comparing the NDACC and TCCON retrievals, we find that the sensitivity of the NDACC retrieval is lower in the troposphere (by a factor of 2) and higher in the stratosphere, compared to TCCON. Thus, the NDACC retrieval is less sensitive to tropospheric changes (e.g., the seasonal cycle) in the column average.

An absolute calibration method of an ethyl alcohol biosensor based on wavelength-modulated differential photothermal radiometry.

In this work, laser-based wavelength-modulated differential photothermal radiometry (WM-DPTR) is applied to develop a non-invasive in-vehicle alcohol biosensor. WM-DPTR features unprecedented ethanol-specificity and sensitivity by suppressing baseline variations through a differential measurement near the peak and baseline of the mid-infrared ethanol absorption spectrum. Biosensor signal calibration curves are obtained from WM-DPTR theory and from measurements in human blood serum and ethanol solutions diffused from skin. The results demonstrate that the WM-DPTR-based calibrated alcohol biosensor can achieve high precision and accuracy for the ethanol concentration range of 0-100 mg/dl. The high-performance alcohol biosensor can be incorporated into ignition interlocks that could be fitted as a universal accessory in vehicles in an effort to reduce incidents of drinking and driving.

Dynamics of mid-infrared light absorption related to photoexcited charge carriers in Ge/Si quantum dots

Mid-infrared optical absorption spectra of Ge/Si quantum dot structures in polarized light were obtained under conditions of interband optical excitation. Mid-infrared absorption changes in comparison to equilibrium conditions for certain light polarization in spectral range of 0.25 – 0.6 eV. The sign of the effect is found to be different in various spectral ranges. Transients of photoinduced absorption contain fast and slow components. The fast decay component of the absorption is related to the direct recombination of localized holes and electrons in quantum dots while a slow component is determined by significantly less probable processes of indirect in real space recombination.

The determination of some crystallographic parameters of quartz, in the sand dunes of Ouargla, Algeria

In this work, the crystallographic phase, degree of crystallinity, crystal system, space group and unit cell parameters of quartz in the Ouargla region (Algeria) sand dunes have been determined using Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The mid-infrared absorption spectrum showed the characteristic doublet for α-quartz at 796 and 779cm−1. The observed principle diffractions at the d-spacings of 3.7937, 3.3539 and 1.8204Å confirm the presence of α-quartz in Ouargla sand. The calculated absorption ratio (degree of crystallinity) of A796/A779≅1.756 indicated that our quartz exhibits a highly crystalline nature. The crystallographic parameters of quartz in Ouargla sand have been determined through analysis of X-ray diffraction data, carried out using the simulation code DICVOL06. These parameters were found as: hexagonal crystal system, space group P3221 (154), unit cell parameters: a=b=4.9294Å, c=5.4093Å and V=113.832Å3.

POLYCYCLIC AROMATIC HYDROCARBON CLUSTERS AS SOURCES OF INTERSTELLAR INFRARED EMISSION

Polycyclic aromatic hydrocarbons (or PAHs) have been the subject of astrochemical research for several decades as principal sources of the interstellar aromatic infrared emission bands. PAH clusters could possibly contribute to these emission bands, but a lack of data on their infrared properties has made this hypothesis difficult to evaluate. Here we investigate homogeneous neutral PAH clusters by measuring the mid-infrared absorption spectra of the five nonlinear PAH molecules phenanthrene, chrysene, pyrene, perylene, and benzo[ghi]perylene within solid argon ice at a fixed temperature of 5 K. We attribute observed spectral shifts in their principal absorption bands as a function of argon/PAH ratio to clustering of the PAH molecules within the argon matrix. These shifts are related to the cluster structures forming in the matrix and the topology of the monomer PAH molecule. We predict that interstellar PAH molecules that are relatively large (no fewer than 50 carbon atoms per molecule) and compact will have clusters that contribute to the asymmetrically red-shaded profile of the interstellar 11.2 μm emission band.

Mid-infrared light absorption by photo-excited charge carriers in Ge/Si quantum dots

Mid-infrared optical absorption spectra of Ge/Si quantum dot structures in polarized light were obtained under conditions of additional interband optical excitation. Photo-excited electron-hole pairs captured to quantum dots cause mid-infrared absorption changes for certain light polarization in spectral range of 0.25 – 0.6 eV. The sign of the effect is found to be different in different spectral ranges. There is an increase of absorption at the long-wavelength edge of the spectrum and a decrease of absorption at the short-wavelength edge. Absorption increase is considered to be related to contribution of optical transitions of non-equilibrium holes from the quantum dot ground states, while absorption decrease is associated with suppression of interband-like processes with generation of hole inside the dot and bound electron outside of the dot under conditions of full occupation of appropriate discrete states.

Correlation between crystallinity and mid-infrared optical absorption spectra of silicon supersaturated with sulfur

ABSTRACTWe prepared silicon hyperdoped with sulfur by ion-implantation followed by pulsed laser melting. Effects of laser fluence during pulsed laser melting and of post-annealing on the silicon hyperdoped with sulfur are investigated. The structure of hyperdoped layer changes from poly-to mono-crystal with increasing laser fluence. Interface between sulfur-implanted-layer and single-crystal substrate disappear above 1.1 J/cm2. The spectral intensity of mid-infrared (MIR) optical absorption increases with crystallinity and spectral shape depends on whether the melt depth during pulsed laser melting reaches interface between implanted layer and single-crystal silicon substrate or not. The MIR absorption intensity rapidly decreases with thermal annealing temperature and almost disappears at 750 °C. The activation energy of conductivity decreases with increasing laser fluence and further decreases with increasing post thermal-annealing temperature. The insulator-metal transition is observed for the sample annealed at 750 °C. These results indicate that there is no direct correlation between MIR optical absorption band and insulator-metal transition.

Support vector machine classification of volatile organic compounds based on narrow‐band spectroscopic data

In this work, a list of volatile organic compounds (VOCs) that are associated with targets susceptible to versatile security issues – such as drug trafficking, explosives carrying, or human presence in forbidden areas – are monitored and discriminated through algorithmic processing of their midinfrared (MIR) spectroscopic properties. Usually, such tasks are relatively straightforward by identifying the absorption peaks of the investigated compounds in extended spectral recordings, from a few hundred up to many thousands of wavenumbers (cm−1). Nevertheless, the physical mechanisms and instrumentation for obtaining so broad spectral profiles may prove to be complex, especially in field measurements, while data acquisition and processing may also prove to be time‐consuming. In the proposed approach, support vector machine algorithmic training is applied in order to evaluate the potential of exploiting very narrow spectral MIR absorption bands that are optimal for reliable prediction analysis and training. The probabilistic classification performance of these bands is evaluated and compared with the prediction performance when using wider MIR absorption spectra. Depending on the data set and the list of the associated VOCs, spectral data recording within a span up to several tens of wavenumbers – at regions where absorption is detectable – prove to be enough for efficient VOC classification. Copyright © 2014 John Wiley & Sons, Ltd.

FTIR measurements of mid-IR absorption spectra of gaseous fatty acid methyl esters at T=25–500 °C

Gas-phase mid-infrared (IR) absorption spectra (2500–3400cm−1) for eleven fatty acid methyl esters (FAMEs) have been quantitatively measured at temperatures between 25 and 500°C using an FTIR spectrometer with a resolution of 1cm−1. Using these spectra, the absorption cross section at 3.39μm, corresponding to the monochromatic output of a helium–neon laser, is reported for each of these fuels as a function of temperature. The data indicate that the 3.39μm cross section values of saturated FAMEs vary linearly with the logarithm of the number of CH bonds in the molecule.

Correlation between Raman sum and optical conductivity sum in La2−xSrxCuO4

In a strongly correlated electron system, the single-particle spectral function changes into a coherent peak and incoherent humps which extend over 1 eV. The incoherent parts lose the symmetry and k dependence, so that the Raman spectra with different symmetries become identical and they are expressed by the optical conductivity. We found that the B1g and B2g spectra in La2−xSrxCuO4 become identical above 2000 cm−1 in the underdoped phase, if Fleury–Loudon type B1g two-magnon scattering is removed. The first Raman susceptibility moment correlates with the generalized optical conductivity moment. The good correlation arises from the incoherent states of a hump from 1000 to 4000 cm−1. The hump is the only structure of the incoherent electronic states in the mid-infrared absorption spectra below 1.4 eV at low carrier densities. The energy is twice the separated dispersion segments of the spin wave in the k⊥ stripe direction. The incoherent state is formed by the magnetic excitations created by the hole hopping in the antiferromagnetic spin stripes in the real space picture.

Emergence of very broad infrared absorption band by hyperdoping of silicon with chalcogens

We report the near through mid-infrared (MIR) optical absorption spectra, over the range 0.05–1.3 eV, of monocrystalline silicon layers hyperdoped with chalcogen atoms synthesized by ion implantation followed by pulsed laser melting. A broad mid-infrared optical absorption band emerges, peaking near 0.5 eV for sulfur and selenium and 0.3 eV for tellurium hyperdoped samples. Its strength and width increase with impurity concentration. Its strength decreases markedly with subsequent thermal annealing. The emergence of a broad MIR absorption band is consistent with the formation of an impurity band from isolated deep donor levels as the concentration of chalcogen atoms in metastable local configurations increases.

Long-term validation of tropospheric column-averaged CH<sub>4</sub> mole fractions obtained by mid-infrared ground-based FTIR spectrometry

Abstract. At the Izaña Atmospheric Research Center, high-resolution mid-infrared solar absorption spectra have been recorded for more than 12 yr using Fourier Transform InfraRed (FTIR) spectrometers. We use the spectral fitting algorithm PROFFIT to retrieve long-term time series of methane (CH4) from the measured spectra. We investigate the total column-averaged dry air mole fractions of methane (totXCH4) obtained from a profile scaling and a profile retrieval, and apply two approaches for deriving the tropospheric column-averaged dry air mole fractions: firstly, we use the FTIR hydrogen fluoride (HF) total column amounts as an estimator for the stratospheric CH4 contribution and a posteriori correct the totXCH4 data of a profile scaling retrieval accordingly (troXCH4post); secondly, we directly determine the tropospheric column-averaged dry air mole fractions of methane (troXCH4retr) from retrieved CH4 profiles. Our theoretical estimation indicates that the scaling retrieval leads to totXCH4 amounts that are subject to a large smoothing error, which can be widely avoided by applying a profile retrieval (for the latter we estimate an overall precision of 0.41%). We compare the different FTIR CH4 data to Izaña's Global Atmospheric Watch (GAW) surface in-situ CH4 data (CH4GAW), which in the case of the Izaña Atmospheric Research Center high mountain observatory are very representative for the free tropospheric CH4 amounts. Concerning totXCH4, the agreement between the FTIR data product and the in-situ measurement is rather poor documenting that totXCH4 is not a valid free tropospheric CH4 proxy, as it is significantly affected by the varying stratospheric CH4 contribution and it rather follows the variation in the tropopause altitude. The a posteriori correction method as applied here only removes a part of this stratospheric CH4 contribution. In contrast the profile retrieval allows for a direct estimation of the tropospheric column-averaged CH4 amounts. Results of the profile retrieval analysis correlate well with the CH4GAW data (correlation coefficient of 0.60, FTIR-GAW scatter of 0.97%), and both data sets show very similar annual cycles and trend behaviour for the 2001–2010 time period. Furthermore, we find a very good absolute agreement between the troXCH4retr and CH4GAW (mid-infrared FTIR/GAW scaling factor of 0.9987) suggesting that mid-infrared FTIR data can be well combined with the surface in-situ GAW data. Our study strongly supports the value of mid-infrared ground-based FTIR CH4 profile retrievals as well as the robustness of the approach for achieving total and tropospheric column-averaged XCH4 data of high quality.

Dual beam photoacoustic infrared spectroscopy of solids using an external cavity quantum cascade laser

Quantum cascade laser-based instrumentation for dual beam photoacoustic (PA) spectroscopy is described in this article. Experimental equipment includes a 4.55 μm (2141-2265 cm(-1)) continuous wave external cavity quantum cascade laser (EC-QCL), two gas-microphone PA cells, and two lock-in amplifiers. Correction for the time and wavenumber dependence of the laser output is effected through real-time division of the PA signals derived from the sample and reference channels. Source-compensated mid-infrared absorption spectra of carbon black powder and aromatic hydrocarbon solids were obtained to confirm the reliability of the method. Absorption maxima in the EC-QCL PA spectra of hydrocarbons are better defined than those in Fourier transform infrared spectra acquired under similar conditions, enabling the detection of several previously unknown bands.

Infrared absorption nano-spectroscopy using sample photoexpansion induced by tunable quantum cascade lasers

We report a simple technique that allows obtaining mid-infrared absorption spectra with nanoscale spatial resolution under low-power illumination from tunable quantum cascade lasers. Light absorption is detected by measuring associated sample thermal expansion with an atomic force microscope. To detect minute thermal expansion we tune the repetition frequency of laser pulses in resonance with the mechanical frequency of the atomic force microscope cantilever. Spatial resolution of better than 50 nm is experimentally demonstrated.

Raman and mid-infrared spectroscopic study of geometrically frustrated hydroxyl cobalt halides at room temperature

Mid-infrared absorption and Raman spectra of the geometrically frustrated material series, hydroxyl cobalt halides ß-Co2(OH)3Cl and ß-Co2(OH)3Br, are first, to the best of our knowledge, measured at room temperature, to study the corresponding relationship between their vibrational spectral properties and crystal microstructures. Through the comparative analysis of the four spectra we have categorically assigned the OH-related vibration modes of hydroxyl groups in the trimeric hydrogen bond environment (Co3 ≡OH)3 … Cl/Br, and tentatively suggested vibration modes of O-Co-O, Co-O and Cl/Br-Co-Cl/Br units. These results can also become the basis for analysing their low-temperature spectral properties, which can help to understand the underlying physics of their exotic geometric frustration phenomena around phase transition temperatures.

Vibrational spectroscopic properties of botallackite-structure basic copper halides

Mid-infrared (4000–400 cm −1) absorption and Raman (4000–95 cm −1) spectra of the series of geometrically frustrated materials, botallackite-structure basic copper halides, α-Cu 2(OH) 3Cl and α-Cu 2(OH) 3Br polycrystalline samples were first, to the best of our knowledge, measured, respectively, to study the corresponding relationship between their vibrational spectral properties and crystal microstructures. Through the comparative analysis to the four spectra, the authors have definitely assigned or tentatively suggested the vibrational modes of hydroxyl groups in the trimeric hydrogen bond environment (OH) 3 Cl/Br, and atomic units O–Cu–O, Cu–O and Cl/Br–Cu–Cl/Br, etc. These results can be propitious to the low temperature spectral property of α-Cu 2(OH) 3Cl and α-Cu 2(OH) 3Br which must help to understand the underlying physics of their exotic geometric frustration phenomena at low temperatures.

Motional modes in bulk powder and few-molecule clusters of tris(8-hydroxyquinoline aluminum) and their relation to spin dephasing

The ensemble averaged spin dephasing rate of localized electrons in the organic molecule tris(8-hydroxyquinoline aluminum) or Alq3 has been found to be significantly larger in bulk powder than in single- or few-molecule clusters confined within 1–2 nm sized nanocavities [B. Kanchibotla et al., Phys. Rev. B 78, 193306 (2008)]. To understand this observation, we have compared the midinfrared absorption spectra of bulk powder and single- or few-molecule clusters. It appears that molecules have additional vibrational modes in bulk powder possibly due to multimerization. Their coupling with spin may be responsible for the increased spin dephasing rate in bulk powder.

Mid-infrared spectroscopic properties of geometrically frustrated basic cobalt chlorides

Three types of FTIR spectrometers were employed to measure the mid-infrared (4000—400 cm-1) absorption spectra of geometrically frustrated hydroxyl cobalt chloride Co2(OH)3Cl, and the intrinsic absorption peaks in the functional group region and fingerprint region were selected and assigned to corresponding vibrational modes according to its known crystal structure. In the assigning process, great emphasis was laid on analyzing the exact experimental data of hydroxyl stretching vibration mode vOH, that is, estimating the free vOH of the Co3—OH group without any hydrogen bond (H-bond), to obtain the red-shift which reflects the formation of an H-band in Co2(OH)3Cl. A 156 cm-1 red-shift is obtained theoretically which demonstrates the presence of non-negligible weak H-bonds, and eventually result in the discovery of the rarely reported trimeric H-bond in the field of crystalline materials, which consists of three independent hydroxyl donors and only one Cl- acceptor. We explained the relative weakness of this kind of hydrogen bond which may have a critical effect on the lattice symmetry and magnetic structure.

Trimeric Hydrogen Bond in Geometrically Frustrated Hydroxyl Cobalt Halogenides

The mid-infrared absorption spectra of geometrically frustrated hydroxyl cobalt halogenides Co2(OH)3Cl and Co2(OH)3Br are measured by FTIR spectrometers, and the stretching vibrational modes of hydroxyl groups are found to be 3549cm−1 and 3524cm−1 respectively. Through finding their true terminal O-H group stretching vibration frequencies, we obtain 107cm−1 and 99cm−1 red shift caused by the corresponding O-H···Cl and O-H···Br hydrogen bonds. Rarely reported trimeric hydrogen bonds (Co3≡O-H)3···Cl/Br are pointed out to demonstrate the relative weakness of this kind of hydrogen bond which may have a critical effect on the lattice symmetry and magnetic structures.