Overtone Absorption Bands Research Articles

Near-Infrared Dual-Range Methane Sensor Using the OAIC-HC Mode Based on VMD-SG-Assisted Optical Noise Suppression.

Based on tunable diode laser absorption spectroscopy and off-axis integrated cavity output spectroscopy, a dual-range methane hybrid sensor was constructed utilizing the overtone absorption band of CH4 gas molecules at 1653.7 nm. By simultaneously utilizing an off-axis integrated cavity and Herriott cell with an effective absorption path of 11 and 405 m, respectively, the two received photoelectric signals are decomposed into different frequency components by VMD and then reconstructed after SG filtering. Applying the global optimization algorithm DA-ELM to CH4 concentration inversion, the correlation coefficient R2 is as high as 0.9995. Through long-term stability verification, the instrument's standard deviation at 1 ppm is 27 ppb. After Allan-Werle deviation analysis, the sensor's limit of detection is 2.298 ppb at an integration time of 138 s. Using the self-developed sensor, the detection of dual-range trace CH4 gas is achieved, enabling a dynamic detection range of trace CH4 gas ranging from 400 ppb to 1000 ppm. The sensor realizes dual-range methane trace detection and actively controls methane emissions to improve environmental quality while taking into account the safety benefits of reducing production accidents.

Monitoring the Fractional Distillation Purification of Inorganic Hydrides by Diode Laser Absorption Spectroscopy

We present our findings on the use of diode laser spectroscopy (DLS) for monitoring СО2, Н2О, СН4, C2H2, C2H4, NH3, and other gas impurities during low-temperature fractional distillation purification of gaseous hydrides. We demonstrate a lineup of instruments and techniques for continuously monitoring impurities during purification of hydrides: NH3, AsH3, РН3, and SiH4. The IR sources used are distributed feedback diode lasers (DLs) having a single mode output fiber pigtail and covering the near-IR spectral region, from 0.7 to 2.0 μm, where overtone and combination absorption bands of the impurities of interest are located. The systems offer high sensitivity and speed in impurity concentration measurements. Owing to their small size and low energy consumption, they can readily be integrated into a process equipment (components of fractionation columns) and enable prolonged continuous monitoring of the purity of hydrides. We describe gas concentration measurement techniques that use conventional high-resolution differential absorption spectroscopy and new, modulation correlation approaches for monitoring a weak molecular absorption by an impurity of interest against a strong selective absorption background of a matrix substance (hydride under study). We present results of technological experiments concerned with the low-temperature fractional distillation of hydrides and the degree of impurity removal from them as monitored by DLS.

Lattice Rayleigh Anomaly Associated Enhancement of NH and CH Stretching Modes on Gold Metasurfaces for Overtone Detection.

Molecular overtones stretching modes that occupy the near-infrared (NIR) are weak compared to the fundamental vibrations. Here we report on the enhancement of absorption by molecular vibrations overtones via electromagnetic field enhancement of plasmonic nanoparallelepipeds comprising a square lattice. We explore numerically, using finite element method (FEM), gold metasurfaces on a transparent dielectric substrate covered by weakly absorbing analyte supporting N-H and C-H overtone absorption bands around 1.5 m and around 1.67 m, respectively. We found that the absorption enhancement in N-H overtone transition can be increased up to the factor of 22.5 due to the combination of localized surface plasmon resonance in prolonged nanoparticles and lattice Rayleigh anomaly. Our approach may be extended for sensitive identification of other functional group overtone transitions in the near-infrared spectral range.

Swept-wavelength lasers based on GaSb gain-chip technology for non-invasive biomedical sensing applications in the 1.7-2.5 μm wavelength range.

The infrared spectral region beyond 1.7 μm is of utmost interest for biomedical applications due to strong overtone and combination absorption bands in a variety of important biomolecules such as lactates, urea, glucose, albumin, etc. In this article, we report on recent progress in widely tunable swept-wavelength lasers based on type-I GaSb gain-chip technology, setting a new state-of-the-art in the 1.7 - 2.5 μm range laser sources. We provide an application example for the spectroscopic sensing of several biomolecules in a cuvette as well as an experimental demonstration of a non-invasive in-vivo sensing of human serum albumin through the skin.

Comparison of XCO abundances from the Total Carbon Column Observing Network and the Network for the Detection of Atmospheric Composition Change measured in Karlsruhe

Abstract. We present a comparison of Karlsruhe XCO records (April 2010–December 2014) from the Total Carbon Column Observing Network (TCCON) and from the spectral region covered by the Network for the Detection of Atmospheric Composition Change (NDACC). The Karlsruhe TCCON Fourier transform infrared (FTIR) spectrometer allows us to record spectra in the mid-infrared (MIR) and near-infrared (NIR) spectral region simultaneously, which makes Karlsruhe a favourable FTIR site to directly compare measurements from both spectral regions. We compare XCO retrieved from the fundamental absorption band at 4.7 µm (as used by NDACC) and first overtone absorption band at 2.3 µm (TCCON-style measurements). We observe a bias of (4.47 ± 0.17) ppb between both data sets with a standard deviation of 2.39 ppb in seasonal variation. This corresponds to a relative bias of (4.76 ± 0.18) % and a standard deviation of 2.28 %. We identify different sources which contribute to the observed bias (air-mass-independent correction factor, air-mass-dependent correction factor, isotopic identities, differing a priori volume mixing ratio profiles) and quantify their contributions. We show that the seasonality in the residual of NDACC and TCCON XCO can be largely explained by the smoothing effect caused by differing averaging kernel sensitivities between the MIR and NIR spectral region. This study aims to improve the comparability of NDACC and TCCON XCO validation data sets as desired for potential future satellite missions and model studies.

Reflectance spectroscopy of low atomic weight and Na-rich minerals: Borates, hydroxides, nitrates, nitrites, and peroxides

We have measured reflectance spectra (0.35–20μm) of a suite of minerals and synthetic compounds that contain low-Z (⩽Na) elements as the major cation and/or the major anion in oxides/oxyhydroxides, and are relevant to planetary geology and astrobiology. The suite comprises Na-borates, Na-, K-, Ca-hydroxides, nitrates, nitrites, and peroxides. Na-borate spectra exhibit B–O fundamental vibrations between 7 and 14μm, and overtones/combinations of these bands in the 1.55, 1.75, 2.15, and 2.25μm regions. Na-, K-, and Ca-hydroxide reflectance spectra are characterized by OH and metal–OH fundamental vibrations near 3, 8, and 18μm, and a number of overtone and combination absorption bands at shorter wavelengths, and a characteristic metal–OH band near 2.35μm. The nitrate and nitrite spectra exhibit fundamental N–O vibrations in the 7–14μm region and numerous combinations and overtones that are still detectable to as low as ∼1.8μm. Na-peroxide is largely spectrally featureless below 24μm, making its detection problematic, while H-peroxide has many OH-related absorption features below 2.5μm that differ in position from those of H2O ice and liquid. The results of this study indicate that the borates, hydroxides, nitrates, nitrite, and hydrogen peroxide can all be uniquely identified using characteristic absorption features that are present below 2.5μm. However, some of these features are weak, and their detectability will depend on the types and abundances of any accessory phases that may be present.

Temperature dependence of the second oh overtone absorption band of vibrations of the water newly melted immediately after freezing

The second overtone band shape of water and its temperature dependence are identical to those of melted and distilled water.

Broadening and shifting coefficients of rotation–vibrational lines in the fundamental and first overtone bands of HCl and HBr induced by oxygen and air

We report FTIR measurements of the line broadening and shifting coefficients in the fundamental and first overtone absorption bands of HCl and HBr in mixtures with oxygen and synthetic air. Most of the data for HBr and the overtone of HCl are believed to be published for the first time.

Temperature and multi-species measurements by supercontinuum absorption spectroscopy for IC engine applications

The first supercontinuum (SC) absorption spectroscopy measurements showing the feasibility of quantitative temperature evaluation are presented to the best of the authors' knowledge. Temperature and multi-species measurements were carried out at a detection rate of ~2 MHz in a high-temperature flow cell within a temperature range from 450 K to 750 K at 0.22 MPa, representing conditions during the suction and compression stroke in an internal combustion (IC) engine. The broadband SC pulses were temporally dispersed into fast wavelength sweeps, covering the overtone absorption bands 2ν(1), 2ν(3), ν(1) + ν(3) of H2O and 3ν(3) of CO2 in the near-infrared region from 1330 nm to 1500 nm. The temperature information is inferred from the peak ratio of a temperature sensitive (1362.42 nm) and insensitive (1418.91 nm) absorption feature in the ν(1) + ν(3) overtone bands of water. The experimental results are in very good agreement with theoretical intensity ratios calculated from absorption spectra based on HiTran data.

Laser-induced grating spectroscopy of highly excited overtone and combination vibrational states of methane

Laser-induced grating spectroscopy is applied to investigate weak overtone and combination absorption bands of methane. The rotationally-resolved spectra of transitions to the polyads Pn, n=8–10, 12 recorded at pressures ranging from 0.2 to 4bar and temperatures 298K and 124–130K cover a broad interval between 891 and 618nm (11220–16180cm−1) and contain a significant amount of prominent narrow lines which correspond to stationary rovibrational states. The absorption cross sections of the weakest detected methane transitions are estimated to be as small as ≈10−26cm2molecule−1. The technique is suitable to provide data for the development and verification of new models describing the complex energy structure of highly vibrationally excited molecules.

Calculations of molecular ir spectra in the overtone and combination frequency regions

Frequencies and intensities of fundamental, overtone, and combination absorption bands have been calculated in an anharmonic approximation for substituted benzene and ethene, aldehydes, ketones, and alcohols in the range 400–4000 cm–1. The calculated values agree well with the experimental data. It is shown that such a calculation allows one to study and predict dependences of the frequencies and intensities of the overtone and combination bands on the substance structure.

Estimation of the contributions of absorption bands for overtones and composite frequencies in infrared spectra of brominated and oxygen-containing organic compounds

Intensities of fundamental, overtone, and composite absorption bands for 27 brominated hydrocarbons and 20 oxygen-containing organic compounds are calculated in an anharmonic approach. The first and second derivatives of the electric dipole moment of the molecule with respect to normal coordinates are determined using ab initio quantum-chemical MP2/6-31G(1d) calculations. For the studied compounds, the average contributions of overtones and composite frequencies to absorption in the region 100–4000 cm−1 is 4.8% for brominated hydrocarbons and 3.2% for oxygen-containing compounds. The major part of the contribution of overtones and composite frequencies falls into the regions (mainly from 1600 to 2800 cm−1) where fundamental transitions are observed rarely. The calculation performed well describe the positions of maxima and the intensities of fundamental, overtone, and composite absorption bands and can be used for the standardless spectrochemical analysis of compounds by their overtone spectra.

Estimation of the contribution of absorption bands for overtones and composite frequencies in IR spectra of olefins and heteroatomic organic compounds

Intensities of fundamental, overtone, and composite absorption bands for 11 olefins, 17 nitrogen- and oxygen-containing organic compounds, and 12 sulfur-containing organic compounds are calculated in the anharmonic approximation. The first and second derivatives of the electric dipole moment of a molecule were calculated by the quantum-chemical ab initio MP2/6-31G(1d) approach. It is shown that, for the studied compounds, the average contribution of overtones and composite frequencies to absorption in the region from 100 to 4000 cm−1 is of about 10%. The major contribution (on the average 80%) of overtones and composite frequencies falls in the regions (mainly from 1600 to 2800 cm−1) where fundamental transitions are rarely observed. The calculations well describe the centers and intensities of the fundamental, overtone, and composite absorption bands and can be used for the standard-free spectrochemical determination of the compounds of interest by their overtone spectra.

Quantum chemical estimation of the overtone contribution to the IR spectra of hydrocarbon halogen derivatives

The problem of a comparison of quantum chemical calculation results with the experimental characteristics of molecules is discussed, and it is noted that the accuracy of quantum computations can be most fully evaluated by a comparison of the theoretical results with the experimental spectra. By means of calculations, the contribution of anharmonism to the theoretical values of absorption band intensities in the IR spectra of 15 fluoro and 36 chloro derivatives of hydrocarbons is estimated. The first and second derivatives of the molecular dipole moment are obtained by ab initio quantum chemical calculations with the use of the 6–31G(1d) basis set and allowance for electron correlations by the second order Moller-Plesset method. The performed calculation well reproduces the positions of maxima and the intensity ratio of fundamental, overtone, and complex absorption bands.

Estimation of absorption band contribution for overtones and combined frequencies in the fingerprint region for alkanes, nitriles, amines, and nitroalkanes

In the anharmonic approximation the intensities of fundamental, overtone, and combined absorption bands are calculated in the range from 100 cm-1 to 4000 cm-1 for a series of alkanes, nitriles, amines, and nitroalkanes. The first and second derivatives of the molecular dipole moment are calculated by quantum chemical methods in the ab initio approximation using the MP2/6-31G(1d) basis set. Overtones and combined frequencies are found to make a significant contribution (5–20%) to the total absorption. The spectral distribution of this contribution depends on the molecular structure. For nitriles and amines significant absorption of the overtones and combined frequencies is observed in the same regions where fundamental absorption bands characteristic of these compounds are located.

Spectral reflectance properties of minerals exposed to simulated Mars surface conditions

A number of mineral species were exposed to martian surface conditions of atmospheric pressure and composition, temperature, and UV light regime, and their evolution was monitored using reflectance spectroscopy. The stabilities for different groups varied widely. Phyllosilicate spectra all showed measurable losses of interlayer H 2O, with some structural groups showing more rapid H 2O loss than others. Loss of OH from the phyllosilicates is not always accompanied by a change in metal–OH overtone absorption bands. OH-bearing sulfates, such as jarosite and alunite, show no measurable change in spectral properties, suggesting that they should be spectrally detectable on Mars on the basis of diagnostic absorption bands in the 0.4–2.5 μm region. Fe 3+- and H 2O-bearing sulfates all showed changes in the appearance and/or reduction in depths of hydroxo-bridged Fe 3+ absorption bands, particularly at 0.43 μm. The spectral changes were often accompanied by visible color changes, suggesting that subsurface sulfates exposed to the martian surface environment may undergo measurable changes in reflectance spectra and color over short periods of time (days to weeks). Organic-bearing geological materials showed no measurable change in C H related absorption bands, while carbonates and hydroxides also showed no systematic changes in spectral properties. The addition of ultraviolet irradiation did not seem to affect mineral stability or rate of spectral change, with one exception (hexahydrite). In some cases, spectral changes could be related to the formation of specific new phases. The data also suggest that hydrated minerals detected on Mars to date retain their diagnostic spectral properties that allow their unique identification.

Vibrational moments of fundamental transitions in low-temperature molecular liquids: Determination from the band shape of vibrations in the overtone spectral range

It is shown that the integrated absorption coefficients of strong fundamental bands of some gaseous substances can be determined with an accuracy of 5–10% using experimental data on the spectral moments of bands in the overtone spectral range of these substances in the liquid phase near the melting point. The absorption coefficients are calculated within the framework of the cell model of the liquid state from the contributions of the resonance dipole-dipole interaction to the second spectral moment. The combination and overtone absorption bands of the SF6, CF4, SiF4, NF3, CHF3, CC1F3, CBrF3, OCS, N2O, and CO2 molecules in the liquid state and in solutions in liquefied Xe, Kr, and O2 are recorded. The data in the literature on the intensity of strong IR bands in the gas phase are analyzed. Using an improved experimental technique, additional measurements are performed. It is found that the absorption coefficients determined from the spectral characteristics of the liquids agree well with the coefficients measured in the gas phase.

Overtones of silicate and aluminate minerals and the 5-8 μm ice bands of deeply embedded objects

The distinct patterns, relatively low intensities and peak positions of overtone-combination bands of silicates and oxides suggest that the 5–8 μm spectral region can provide clues for the dust composition when near optically thick conditions exist for the 10-μm silicate feature. We present 1000–2500 cm−1 room-temperature laboratory spectra obtained from powders of silicate, aluminate and nitride minerals and silicate glasses. The spectra exhibit overtone absorption bands with mass absorption coefficients ∼100 times weaker than the fundamentals. These data are compared with the 5–8 μm spectra of deeply embedded young stellar objects observed with the Short Wavelength Spectrometer on the Infrared Space Observatory. Fits of the laboratory data to the observations, after subtraction of the 6.0-μm H2O ice feature and the 6.0-μm feature identified with organic refractory material, indicate that crystalline melilite (a silicate) or metamict hibonite (a radiation-damaged crystalline aluminate) may be responsible for much of the 6.9-μm absorption feature in the observations, with melilite providing the best match. A weaker 6.2-μm absorption in the young stellar object spectra is well matched by the spectra of hydrous crystalline amphibole silicates (actinolite and tremolite). Relative abundances of Si–O in room-temperature amphiboles to low-temperature H2O ice are in the range 0.46–3.9 and in melilite are in the range 2.5–8.6. No astronomical feature was matched by the overtones of amorphous silicates because these bands are too broad and peak at the wrong wavelength. Hence, this analysis is consistent with the 10-μm features of these objects being due to a mixture of crystalline and amorphous silicates, rather than only amorphous silicates.

X‐Ray and Infrared Observations of Embedded Young Stars in L1630

The HH 24-26 star-forming region in the Lynds 1630 (L1630) dark cloud in Orion contains a remarkable collection of rare Class 0 and Class I protostars, collimated molecular and ionized jets, and a luminous but spatially unresolved ASCA X-ray source. To study the X-ray properties of the embedded protostar population of that region, we have obtained a deep X-ray image with the ACIS-S camera on board the Chandra X-Ray Observatory. A number of Hα emission-line objects were detected in the areas surrounding HH 24-26, of which the weak-line T Tauri star SSV 61 was the brightest source, at a steady luminosity of LX(0.3-10 keV) = 1031.9 ergs s-1. Two Class I protostars aligned with optical jets in HH 24, SSV 63E, and SSV 63W, were also detected, as was the continuum radio source SSV 63NE, which is very likely an extreme Class I or Class 0 object. We observed no X rays from the Class 0 protostars HH 24-MMS and HH 25-MMS, nor any from regions of the cloud bounded by HH 25 and HH 26, at a 2 σ upper limit of LX ~ 1030.0 ergs s-1. HH 26-IR, the Class I object thought to be the origin of the HH 26 flow, was not detected. Near-infrared spectroscopy obtained at the NASA IRTF reveals 3 μm ice bands in the spectra of SSV 59, 63E, 63W, and HH 26-IR, and 2.3 μm CO overtone absorption bands for SSV 61. SSV 60, which lies astride one end of the great arc of nebulosity forming HH 25, exhibits a deep infrared ice band and CO absorption but is not an X-ray source and is most likely a distant background giant of late spectral type.

Spectral line parameters in the (2 ← 0) overtone band and the dipole moment function of the HI molecule

We present here new high-resolution experimental data on the linestrengths and pressure-broadened Lorentzian widths for the P 2(13) to R 2(12) vibration–rotation lines in the first overtone absorption band of hydrogen iodide. By combining the measured linestrengths with our previous results for the fundamental band [J. Mol. Spectrosc. 218 (2003) 75] and with other published data for the higher-overtone bands of this molecule, an improved dipole moment expansion as a function of the dimensionless reduced nuclear displacement x is obtained: μ( x)=0.4471(5)−0.0772(2) x+0.542(3) x 2−1.90(2) x 3. This experimental dipole moment function is compared with the results of a few recent non-relativistic and relativistic ab initio calculations. The agreement between theoretical and experimental Herman–Wallis coefficients for the first two vibrational bands of HI is found best as ever reported before.