Mid-infrared Absorption Spectroscopy Research Articles

Mid-infrared Doppler-free saturation absorption spectroscopy of the Q branch of CH4ν3 = 1 band using a rapid-scanning continuous-wave optical parametric oscillator.

We have developed a mid-infrared Doppler-free saturation absorption spectroscopy apparatus that employs a commercial continuous-wave optical parametric oscillator (CW OPO), complemented by a home-built automation and wavelength scanning system. Here, we report a comprehensive spectral scan of the Q branch transitions of the ν3 = 1 band of methane (CH4) with an average linewidth (FWHM) of 4.5 MHz. The absolute frequency calibration was achieved using previously reported transition frequencies determined using optical frequency combs, while a Fabry-Perot etalon was used for the relative frequency calibration. We report 15 transitions with improved accuracies of 1.13 MHz (3.76 × 10-5 cm-1).

Background-free mid-infrared absorption spectroscopy using sub-cycle pulses

We have demonstrated highly sensitive single-shot based background-free mid-infrared (MIR) absorption spectroscopy using sub-cycle MIR pulses generated through filamentation. The MIR pulse transmitted through a sample was upconverted with a fast rising and long tailing gate pulse through four-wave difference frequency generation in a silicon membrane. By recording the upconverted spectrum of the free induction decay alone, we successfully measured the absorption spectrum as a positive signal in the wavenumber range from 500 to 4500 cm−1, which covers both the fingerprint and functional group regions. We obtained an absorption spectrum of ∼50 mM of aqueous glucose, which is not detectable with a standard Fourier transform infrared spectrometer.

Behavioral and physiological responses of Girardia tigrina exposed to polyethylene microplastics.

Microplastic particles appear in great abundance and variety in freshwater ecosystems across the globe, spanning lakes and rivers, with increasingly frequent exposure of aquatic organisms. Studies on the mechanisms of any effects of plastic particles are still scarce, particularly in relation to the regenerative capacity of biota, for which there is no established model organism; however, planaria have shown sensitivity for assessing these risks to the aquatic environment. Thus, the present study aimed to investigate the behavioral and regeneration responses of the freshwater planaria Girardia tigrina exposed to polyethylene (PE) microplastics (MPs) incorporated into their food source. The greatest effect was observed on planarian regeneration, which was manifested at 10μg/mg liver. Planaria reproduction and fertility were affected at 50μg/mg liver; however, planaria locomotion was not affected at the concentrations evaluated. Mid-infrared absorption spectroscopy (FT-IR) was used to identify the constituent polymers, and ingestion of the polyethylene microplastic by the planaria was confirmed by infrared spectroscopy. The results highlight the potential adverse effects of exposure to polyethylene microplastic and show that the reproductive behavior and regeneration of a freshwater organism can be indicators of toxicity resulting from environmental pollution.

Modifications of astrophysical ices induced by cosmic rays

Aims. Astrophysical ices on dust grain mantles in the interstellar medium (ISM) and dense circumstellar envelopes (CSEs) are continuously exposed to galactic cosmic rays (GCRs). In a laboratory setting, we studied the physical and chemical modifications of ice layers induced by energetic heavy ions as GCR analogues. The ice layers used have a molecular composition similar to that of icy grain mantles. Methods. Mixtures of H2O:CO:CH3OH molecules (percentages 73:24:3, 68:30:3, and 58:38:3) were condensed on a substrate at 15 K and irradiated with 40 MeV 58Ni11+ ion beams. Irradiation-induced modifications were followed using the mid-infrared absorption spectroscopy technique. Results. We observed the evolution of infrared bands of CO2, HCO, HCOOH, CH4, H2CO,H2O2, and more complex synthesised molecules. From the molecular column densities, cross-sections and sputtering yields were determined and compared to published results of water and carbon monoxide. Analysis of the chemical modifications reveals that the precursors are easily destroyed when they are in a molecular mixture, while others are desorbed. Conclusions. The main radiolitic modifications induced by GCR irradiations are molecular decomposition and sputtering. Extrapolation to astrophysical radiation conditions shows a strong dependence on the intensity of the GCR distributions at low energies, which allows the analysis of the ice evolution at timescales comparable to those of the ISM and CSE.

Mid-infrared gas absorption spectroscopy using a silicon germanium waveguide based chirped supercontinuum

We report the simulations of coherent supercontinuum generation from 2.63 to 8.04 μm in a silicon germanium photonic waveguide. The influence of input quantum noise pulses on coherence of the generated spectra was investigated. A high value of first order degree of coherence (i.e. 0.98) on supercontinuum spectra was predicted numerically. Our mid-infrared simulated coherent chirped supercontinuum source was then used as the input light source in absorption spectroscopy of carbon dioxide and methane gases. The simulated absorbance spectra for these greenhouse gases have high molecular contrast, thanks to the intense, chirped supercontinuum used.

Laboratory Analogs of Thermally Processed Ices Containing H2O, N2, NH3, CO2, and C2H3N Relevant to Astrophysical Environments

Introduction: Laboratory simulations can benefit ground- and space-based observations of icy bodies in outer space. It is well-known that NH3 and CO2 can interact, forming ammonium carbamate (CH6N2O2). Method: This study examines NH3 and CO2 in thermally processed H2O-rich ices in the laboratory via mid-infrared absorption spectroscopy. In particular, the presence of CO2 in NH3- ice mixtures thermally annealed at 150 K for more than four hours in systematic experiments suggested that ammonium carbamate could potentially trap volatiles within the ice matrix. Result: Additional studies with acetonitrile (C2H3N) in ice mixtures containing H2O, CO2, and NH3 were also performed. Absorption peak position changes were recorded when the temperature was slowly increased (≤ 5K/min) and also annealed at temperatures up to 150 K. Conclusion: These studies will hopefully be useful in interpreting pre-biotic chemistry in the Solar System.

Sensitive Discrete Frequency Mid-Infrared Absorption Spectroscopy Using Digitally Referenced Detection.

Broadly tunable mid-infrared (IR) lasers, including quantum cascade lasers (QCL), are an asset for vibrational spectroscopy wherein high-intensity, coherent illumination can target specific spectral bands for rapid, direct chemical detection with microscopic localization. These emerging spectrometers are capable of high measurement throughputs with large detector signals from the high-intensity lasers and fast detection speeds as short as a single laser pulse, challenging the decades old benchmarks of Fourier transform infrared spectroscopy. However, noise in QCL emissions limits the feasible acquisition time for high signal-to-noise ratio (SNR) data. Here, we present an implementation that is broadly compatible with many laser-based spectrometer and microscope designs to address these limitations by leveraging high-speed digitizers and dual detectors to digitally reference each pulse individually. Digitally referenced detection (DRD) is shown to improve measurement sensitivity, with broad spectral indifference, regardless of imbalance due to dissimilarities among system designs or component manufacturers. We incorporated DRD into existing instruments and demonstrated its generalizability: a spectrometer with a 10-fold reduction in spectral noise, a microscope with reduced pixel dwell times to as low as 1 pulse while maintaining SNR normally achieved when operating 8-fold slower, and finally, a spectrometer to measure vibrational circular dichroism (VCD) with a ∼ 4-fold reduction in scan times. The approach not only proves versatile and effective but can also be tailored for specific applications with minimal hardware changes, positioning it as a simple and promising module for spectrometer designs using lasers.

Simultaneous measurement of NH3 and NO by mid-infrared tunable diode laser absorption spectroscopy based on machine-learning algorithms

Accurate measurements of NH3 and NO are essential for controlling NOx emissions from coal-fired power plants, reducing ammonia slip, and improving the efficiency of selective catalytic reduction (SCR) operation. We propose a method for measuring NH3 and NO concentrations based on machine learning algorithms to address the challenges posed by overlapping interference of CO2 and H2O spectral lines in flue gas. Our approach introduces a novel method for acquiring mixture spectra for the model. Initially, we measure the spectra of individual components under different concentrations and temperatures. Subsequently, mixed spectral samples are generated by combining the measured spectra of the individual components. This approach simplifies the spectral measurement process while preserving accuracy. The particle swarm optimization support vector machine (PSO-SVM) algorithm is leveraged, providing a reliable foundation for the continuous and synchronous measurement of NH3 and NO. Upon testing, the PSO-SVM demonstrates average relative errors of 4.0 % and 0.8 % for NH3 and NO concentrations, respectively. The corresponding measurement precision is 0.05 ppm for NH3 and 0.42 ppm for NO, better than the conventional integral absorbance method. The minimum detection limit (MDL) for NH3 is 16.1 ppb at an average time of 50 s, while for NO, it is 38.4 ppb at an average time of 71 s. The methodology of this paper is expected to play an important role in reducing the influence of interfering components and improving the accuracy of field measurements.

Determination of Air–Fuel Ratio at 1 kHz via Mid-Infrared Laser Absorption and Fast Flame Ionization Detector Measurements in Engine-Out Vehicle Exhaust

<div>Measurements of air–fuel ratio (AFR) and <i>λ</i> (AFR<sub>actual</sub>/AFR<sub>stoich</sub>) are crucial for understanding internal combustion engine (ICE) performance. However, current <i>λ</i> sensors suffer from long light-off times (on the order of seconds following a cold start) and limited time resolution. In this study, a four-color mid-infrared laser absorption spectroscopy (LAS) sensor was developed to provide 5 kHz measurements of temperature, CO, CO<sub>2</sub>, and NO in engine-out exhaust. This LAS sensor was then combined with 1 kHz hydrocarbon (HC) measurements from a flame ionization detector (FID), and the Spindt exhaust gas analysis method to provide 1 kHz measurements of <i>λ</i>. To the authors’ knowledge, this is the first time-resolved measurement of <i>λ</i> during engine cold starts using the full Spindt method. Three tests with various engine AFR calibrations were conducted and analyzed: (1) 10% lean, (2) stoichiometric, and (3) 10% rich. The measurements were acquired in the exhaust of a light-duty truck with an 8-cylinder gasoline engine. The LAS-FID-based <i>λ</i> sensor results were compared with those obtained from a universal exhaust gas oxygen (UEGO) sensor. The LAS-FID method provided robust <i>λ</i> measurements from the first combustion exhaust event (avoiding the light-off time associated with traditional <i>λ</i> sensors) in addition to enhanced temporal resolution (on the order of 100× increase compared to traditional diffusion-based <i>λ</i> sensors). The insight gained from this novel method could be used to benefit crank, cold start, and open- or closed-loop air–fuel ratio control strategies in gasoline engines for reduced emissions.</div>

Oxy-biodegradable plastics in freshwater environments: degradation and biofouling

Growing concern about the impact of plastic pollution on the environment has led to the creation of global public policies and the consumption of “environmentally friendly” products, such as oxy-biodegradable plastics. In this context, “greenwashing” practices can arise, i.e., the product promises more environmental benefits than it actually offers, which can lead to superfluous use, inappropriate disposal, and the generation of microplastics (MPs). However, the scientific literature lacks studies that evaluate the behavior of oxy-biodegradable plastic when exposed to the freshwater environment. In this respect, the aim of the current study was to evaluate the degradation process of oxy-biodegradable plastic bags made from high-density polyethylene (HDPE) in river water. To this end, the current study aimed to assess whether these bags actually meet the label information (which corresponds to the complete degradation of material) or whether they correspond to greenwashing practices. The physical and chemical alterations, and the formation of biological communities that occurred on the surface of the plastic material when exposed to natural aging and submerged in freshwater were monitored using mid-infrared absorption spectroscopy with attenuated total reflectance (FTIR-ATR) and scanning electron microscopy (SEM). The characterization of the samples after 180 days of exposure showed that the oxy-biodegradable bags were not completely degraded, with only fragmentation of the material and generation of MPs. In addition, it was also observed that microorganisms present in the water easily colonized the plastic surface from the start of the experiment. In this way, the oxy-biodegradable bags analyzed correspond to a greenwashing practice, which is extremely harmful, since it can influence the increase in consumption of these products, generating greater improper disposal of these materials, and consequently the generation of MPs and the formation of biofilms, which can carry pathogenic microorganisms to the aquatic biota and to humans.

High-Temporal-Resolution In Situ Sensor for Oceanic CO2 Isotope Measurement Enabling Multidimensional Isotope Tracing Analysis (R13C, R18O, and R17O) via Laser Absorption Spectroscopy.

Combined in situ analysis of oceanic CO2 concentrations and diverse C and O isotope characteristics can offer a unique perspective with multiple isotopic tracing dimensions for identifying marine biogeochemical processes. Applying this strategy in marine environments is urgently required, yet it faces inherent challenges in terms of existing analytical methods and instruments, e.g., a lack of in situ sensors, limited detectable isotope variety, and low-temporal-resolution data. Here, we report an underwater in situ dissolved CO2 isotope sensor based on mid-infrared tunable diode laser absorption spectroscopy (MIR-TDLAS) and membrane extraction technology. Through the proposed targeted strategies, the sensor is capable of providing high-temporal-resolution in situ measurement of all monosubstituted isotopes of dissolved CO2 (16O13C16O, 18O12C16O, and 17O12C16O) at marine background concentrations. The sensor is demonstrated to provide comparable precision to that of isotope ratio mass spectrometry. At 400 ppmv, the precision for R13C, R18O, and R17O could achieve 0.084, 0.042, and 0.013‰, respectively, for a 1 s integration time. By enabling a high-frequency in situ analysis in fixed-point time-series field deployment, a 17O anomaly with strong regularity is observed, which is not obvious in 18O and 13C, and therefore, the superiority of the proposed multidimensional in situ isotope tracing strategy is demonstrated. The developed sensor has great potential to open up new prospects for advancing marine carbon research.

Mid-Infrared Absorption Spectroscopy for NO and NO2 Detection in a Flexible Hollow Core Fiber (Invited)

Mid-Infrared Absorption Spectroscopy for NO and NO2 Detection in a Flexible Hollow Core Fiber (Invited)

Solvation Plays a Key Role in Antioxidant-Mediated Attenuation of Elevated Creatinine Level: An In Vitro Spectroscopic Investigation.

An elevated level of creatinine (CRN) is a mark of kidney ailment, and prolonged retention of such condition could lead to renal failure, associated with severe ischemia. Antioxidants are clinically known to excrete CRN from the body through urine, thereby reducing its level in blood. The molecular mechanism of such an exclusion process is still illusive. As the excretion channel is urine, solvation of the solute is expected to play a pivotal role. Here, we report a detailed time-domain and frequency-domain terahertz (THz) spectroscopic investigation to understand the solvation of CRN in the presence of two model antioxidants, mostly used to treat elevated CRN level: N-Acetyl-l-cysteine (NAC) and ascorbic acid (ASC). FTIR spectroscopy in the mid-infrared region and UV absorption spectroscopy measurements coupled with quantum chemical calculations [at the B3LYP/6-311G++(d,p) level] reveal that both NAC and ASC form HBonded complexes with CRN and rapidly undergo a barrier-less proton transfer process to form creatinium ions. THz measurements provide explicit evidence of the formation of highly solvated complexes compared with bare CRN, which eventually enables its excretion through urine. These observations could provide a foundation for designing more beneficial drugs to resolve kidney diseases..

Mid-infrared absorption spectroscopy measurements and model improvements during the oxidation of ammonia/ethanol and ammonia/diethyl ether blends in a shock tube

Blending ammonia with highly active fuels (hydrogen, hydrocarbons, oxygenated hydrocarbons, etc.) has been identified as an efficient way to improve the mixture's reactivity. In this study, the temperature, CO, CO2, H2O, NH3, and NO time-histories were measured simultaneously during the oxidation of ammonia/ethanol and ammonia/diethyl ether (DEE) blends (equivalence ratio ϕ ∼ 1.2) using mid-infrared laser absorption spectroscopy behind reflected shock waves. Various models (ten for ammonia/ethanol and three for ammonia/DEE) were selected to perform the simulation and compared with the measurements. The predictions are very different for both mixtures, among which the CRECK model was selected to perform discussion and kinetics analyses for the ammonia/ethanol mixture and the Shrestha 2022 model was selected for the ammonia/DEE mixture. The CRECK model performs well in predicting the temperature rise/plateau, H2O formation rate/plateau, CO consumption rate/plateau, and CO2 formation rate. However, it underpredicts the CO formation rate and slightly overpredicts the CO2 plateau and NH3 consumption rate; for the NO time-history, the CRECK model well captures the rapid NO formation but underpredicts the near-equilibrium NO concentration and gives an unnormal initial NO production. The Shrestha 2022 model accurately predicts the plateaus of temperature and H2O, but significantly underpredicts the reactivity of the ammonia/DEE mixture and overpredicts the near-equilibrium NO concentration. According to the kinetics analyses, key reactions affecting the species' time-histories were selected. Fourteen reactions in the CRECK model and eight reactions in the Shrestha 2022 model were updated by the reported data or estimations to improve the predictions. However, discrepancies still exist, which are mainly located in the CO formation and NH3 consumption rates. Further effort should mainly focus on these two aspects, as well as the interactions between oxygenated hydrocarbons and ammonia at high temperatures.

(Invited) Observation of Free Carriers in Non-Fullerene Acceptors with Broadband Mid-Infrared Transient Absorption Spectroscopy

Non-fullerene acceptors (NFAs) are attracting much attention as a material for efficient organic photovoltaics (OPVs). However, the photoinduced dynamics of NFAs has yet to be fully revealed. It is still questionable whether free carriers can be generated in the film consisting of the stacking of small NFA molecules. Transient absorption (TA) spectroscopy is a powerful method to investigate photoexcited states with ultrafast time resolution. TA spectroscopy in the visible and NIR region enables us to observe strongly bound photoexcitation states, but cannot detect free carriers. On the other hand, broadband MIR TA measurement makes it possible to observe free carriers. However, this is not commonly employed for the study of OPV. Herein, we investigated the photoexcited dynamics in the film of ITIC, one of the most well-known NFAs, by using MIR to visible TA measurement. In the MIR TA spectra, a peak around 3100 cm-1 and a broader component extending to lower energy region were observed. The latter broadband component got increased by blending ITIC with a donor, suggesting that this can be attributed to free carriers generated by charge separation. The observation of the vibrational frequency shift of CN group also supported the charge separation in the ITIC film. This study highlights the utility of MIR-TA spectroscopy in the study of OPVs.

Open-path sensor for atmospheric NO detection based on mid-infrared laser absorption spectroscopy

Nitric oxide (NO) is one of the primary air pollutants, and open optical path detection of NO holds immense significance in safeguarding the ecological environment and human health. In this paper, a high precision and high sensitivity NO open optical path sensor based on mid-infrared laser absorption spectroscopy technology is developed. The average gas concentration inversion of the NO path is achieved by collecting the backscattered light from the cooperative target. The gas measurement deploys a wavelength modulation method to remove the effects from light intensity variations. Furthermore, a Kalman filter and a temperature–pressure sensor optimization software algorithm were utilized to reduce the effects of temperature–pressure variations during the measurement process, leading to a 33% improvement in detection accuracy. The performance of the sensor was evaluated in the laboratory environment in terms of accuracy, sensitivity, and detection limit. The atmospheric NO concentration was also measured under varying time periods and weather conditions in the field environment, thus laying a strong foundation for studying the spatial and temporal evolution of NO in the atmosphere.

Development of Mid-Infrared Absorption Spectroscopy for Gemstone Analysis

Absorption spectroscopy has been widely used in gemstone societies as it enables the identification of the origin and post-treatment of gemstones in a non-destructive way. The infrared (IR) range is critical for studies of gemstones such as corundum, emerald, and diamond. Regarding the corundum sample, absorption peaks related to the -OH bond could be used as an index for heat-treatment detection, and different types of inclusions could be identified by analyzing the IR absorption spectrum. The most widely used method for measuring IR absorptions is Fourier-transform infrared spectroscopy (FTIR), which was designed based on the working of the Michelson interferometer. However, FTIR has a few limitations, such as a long measurement time and difficulty in sample placement, which limits its full automation capability. In this study, a mid-infrared (MIR) spectrometer relying on the upconversion phenomenon of non-linear crystals was used to measure the absorption spectra. Corundum with heat treatment features and/or with different common types of inclusions were measured by both transmission and reflection modes. After comparison with the FTIR spectra captured on the same sample set, the same target peaks could be captured with a shorter measurement time and easier operation. The developed MIR spectrometer could directly measure the absorption spectrum in the 2–4.5 µm (2200–5000 cm−1) range within a few seconds. Meanwhile, as both transmission and reflection modes were available, both loose and mounted gemstones could be measured, supporting the widespread use of this device in large-scale production and its ability to achieve full automation.

Nitromethane as a nitric oxide precursor for studying high-temperature interactions between ammonia and nitric oxide in a shock tube

Ammonia (NH3) is a promising carbon-free fuel for carbon neutrality that has recently received increasing attention. However, NOx emissions are one of the main problems for NH3 or NH3-blended combustion. This study used nitromethane (CH3NO2) as a nitric oxide precursor to study NH3/NO interactions at high temperatures. The temperature, CO, CO2, NO, and NH3 time-histories were measured simultaneously during the pyrolysis and oxidation (equivalence ratios ϕ = 2.0 and 1.0) of CH3NO2 and CH3NO2/NH3 mixtures behind reflected shock waves using mid-infrared laser absorption spectroscopy to analyze the NH3/NO interactions. The measurements were used to validate and evaluate seven kinetic models which gave very different predictions for the different mixtures, among which the Glarborg et al. (2018) and Shang et al. (2020) models agreed best with the temperature, NO, and NH3 measurements. The measured CO and CO2 time-histories are shown in the Supplementary Material. Both mechanisms accurately capture the temperature plateau while the main difference is the temperature increase during oxidation. The NO formation rate and plateau concentrations during CH3NO2 pyrolysis are better predicted by the Shang model than by the Glarborg model. Both mechanisms capture the NO formation rate for CH3NO2 oxidation with ϕ = 2.0 but underestimate it for ϕ = 1.0. With NH3, the Glarborg model better predicts the NO removal during CH3NO2/NH3 pyrolysis. However, both mechanisms underestimate the NO formation rate and maximum but overestimate the NO removal rate and NH3 consumption for CH3NO2/NH3 oxidation. Rate-of-production and sensitivity analyses show that: for the NO formation, CH3 + NO2 = CH3O + NO and NO2 + H = NO + OH are the primary pathways and CH3NO2 (+ M) = CH3 + NO2 (+ M) is the most sensitive reaction. NH3 + OH = NH2 + H2O contributes the most to the NH3 consumption during CH3NO2/NH3 pyrolysis, while NH3 + H = NH2 + H2 and NH3 + O = NH2 + OH are increasingly important at oxidation condition. NH2 + NO = N2 + H2O and NH2 + NO = NNH + OH are the most important reactions for the NO reduction with NH3 existing. The rate constants of some key reactions were revised to improve the predictions.

In situ probing of atmospheric-pressure warm air glow discharge for nitrogen fixation by multiple laser spectroscopies

The fixation of atmospheric nitrogen into valuable compounds through reactive plasma processes has attracted intense interests due to its easy operation and compatibility with distributed renewable energy sources. However, practical implementation of plasma-assisted nitrogen fixation is hampered because of its relatively low throughput, which is dominantly limited by the unclear underlying mechanisms. In this study, effort was focused on the in situ production of key species in a DC-driven warm air glow discharge at atmospheric pressure with the help of advanced laser spectroscopic diagnostics. Laser Rayleigh scattering was applied to determine the gas temperature distribution in the discharge column. And mid-infrared quantum cascade laser absorption spectroscopy and one/two-photon absorption laser-induced fluorescence were performed on molecular nitric oxide (NO), atomic oxygen and nitrogen (O, N) for their absolute densities in the discharge. It is found that the spatial distributions of gas temperature, O and N atoms show peaks in the hot discharge center. In contrast, a hollow ‘doughnut’ shape characterized by the NO molecule was observed, particularly under conditions of high discharge current but low airflow rate. The steady-state simulation shows that the hollow pattern of NO is dominantly induced by the radial diffusion of species due to the steep spatial gradient of gas temperature in the discharge cross-section. Moreover, the reverse conversion by atomic N leads to a negative effect on the NO synthesis, especially at the discharge center where the N density and gas temperature are high. From the steady-state modeling, a similar hollow distribution of NO2 was depicted in the air glow discharge. These results demonstrate the strong dependence on atomic O for the major formation process of NO, and the importance of suppressing the reverse paths dominated by atomic N for higher NO production in the studied warm air plasma.

A Comparison of CNN and PLSR for Glucose Monitoring Using Mid-Infrared Absorption Spectroscopy

A Comparison of CNN and PLSR for Glucose Monitoring Using Mid-Infrared Absorption Spectroscopy