Mid-infrared Absorption Spectroscopy Research Articles

CO2-broadening coefficients for the NO2 transitions at 6.2 µm measured by mid-infrared absorption spectroscopy

A continuous-wave (CW) quantum cascade laser (QCL)-based spectroscopic sensor is demonstrated to investigate the high-resolution spectra of 35 spectral lines of NO2 ranging from 1630.2 cm−1 to 1632.5 cm−1. The air-broadened half widths of two NO2 lines are compared with the HITRAN database to assess the performance of the system. The absorption spectra of the NO2 and CO2 mixtures at pressures of 10–90 mbar are measured to obtain the CO2-broadening coefficients. To the best of our knowledge, no measured CO2-broadening coefficients for the NO2 transitions in this region have been published previously. For signal processing of unresolved lines, a self-established spectral analysis algorithm integrated with the continuous wavelet transform is developed to calculate the second derivative for determining the number and position of individual peaks. The measurement results are useful for the design of a spectroscopic sensor for monitoring carbonated beverages production, tobacco puffing treatment, and food preservation.

Non-differential dual-channel mid-infrared absorption spectroscopy for CH4 detection using Wiener-Homomorphic filtering technique

A non-differential dual-channel mid-infrared methane (CH4) sensor system was developed by employing a dual-source dual-detector structure. The reference channel operating at the same wavelength of 3.31 μm as the detection channel was used to derive the noise in the electrical and optical domain. The obtained noise was supplied to a Wiener-Homomorphic filter as a reference for suppressing the additive and multiplicative noise in the polluted signal output from the detection channel. Numerical simulation and experiments were performed to verify the function of the Wiener-Homomorphic filtering algorithm by directly applying noise into the sensor system. Under the intrinsic noise level of the sensor, the measurement precision was improved from ~75 parts-per-million (ppm) to ~50 ppm with a ~3 s averaging time due to the use of the technique; with external applied optical interference at 3.31 μm into the system, the technique can well correct the sensor drift and improve the precision from ~150 ppm to ~100 ppm. The proposed technique has the ability of suppressing additive/multiplicative noise in the electrical and optical domain and can be effective in noisy environment for CH4 detection.

Time-resolved characterization of non-thermal plasma-assisted photocatalytic removal of nitric oxide

We performed time-resolved characterization of plasma-photocatalytic removal of nitric oxide (NO) in a ceramic foam packing double dielectric barrier discharge reactor using mid-infrared laser absorption spectroscopy. Non-thermal plasma (NTP) was generated in the porous ceramic foam coated with mesoporous TiO2 thin film. Real-time NO monitoring was performed using a quantum cascade laser at 5.26 µm combined with a custom-designed multipass gas cell, leading to a sub-ppm detection limit at 0.1 s measurement time. By analyzing the feed gas of 90 ppm NO/1% O2 balanced in N2, the plasma treatment without TiO2 photocatalyst had no effect on NO removal when the discharge power was below 45 W. In comparison, the NTP-TiO2 system achieved an efficiency of 49% for NO removal even at the relatively low discharge power of 18 W. The synergy between NTP and TiO2 photocatalyst effectively promotes the NO removal at low plasma discharge powers. However, we also observed a negative effect of TiO2 photocatalyst on NO removal at a relatively high discharge power of 135 W.

Quantitative measurements of formaldehyde in the low-temperature oxidation of iso-octane using mid-infrared absorption spectroscopy

Time-resolved quantitative measurements of formaldehyde (HCHO) in the low-temperature oxidation of iso-octane using a rapid compression machine have been performed with mid-infrared laser absorption spectroscopy. Due to the weak interference of the broadband absorption of iso-octane, a two-color detection scheme was applied to HCHO detection. The cross-sections of HCHO and iso-octane in two colors were measured using the rapid compression machine in the temperature range of 450–737 K and pressure range of 100–700 kPa. The time-resolved quantitative HCHO profiles in the low-temperature oxidation of iso-octane at 0.77 MPa, 645 K, and an equivalence ratio of 1.0 were successfully obtained. The calculated HCHO profiles using the latest chemical kinetic model of iso-octane show the same tendency as the experimental profiles.

Influence of Light Coupling Configuration and Alignment on the Stability of HWG-Based Gas Sensor System for Real-Time Detection of Exhaled Carbon Dioxide

A mid-infrared tunable diode laser absorption spectroscopy (TDLAS) gas sensor based on hollow waveguide (HWG) gas cell for real-time exhaled carbon dioxide (eCO2) detection is reported. A $2.73~\mu \text{m}$ distributed feedback (DFB) laser was used to target a strong CO2 absorption line, and wavelength modulation spectroscopy (WMS) with the second harmonic (WMS- $2f$ ) was used to retrieve the CO2 concentration with high sensitivity. The influence of different parameters, including coupling configuration of HWG, laser-to-HWG and HWG-to-detector coupling alignment on the stability of the HWG sensor is systematically studied. The HWG eCO2 sensor showed a fast response time of 2.7s, detection limit of 17 ppmv, and measurement precision of 20.9 ppmv with a 0.54 s temporal resolution. The eCO2 concentrations changed in breath cycles were measured in real time. The Allan variance indicated that the detection limit can reach 1.7 ppmv, corresponding to a detection sensitivity of $1.3\times 10^{\mathbf {-8}}$ cm−1 Hz−1/2, as the integration time increases to 26 s. This work demonstrates the performance characteristics and merits of HWG eCO2 sensor for exhaled breath analysis and potential detection for other exhaled gases.

Silver nanoparticles on copper foam as substrate for full range mid-infrared surface enhanced infrared absorption spectroscopy in transmission configuration

The availability of sensitive, reproducible, and broad spectral window range surface enhanced infrared absorption spectroscopy (SEIRAS) substrate is essential to the wide application of SEIRAS technique. Here, a facile method was proposed for the preparation of full mid-infrared range transmission surface-enhanced infrared absorption spectroscopy (T-SEIRAS) substrate, where silver nanoparticles (Ag NPs) was synthesized in situ by immersing copper foam into the silver nitrate solution with the help of hydroxylamine hydrochloride. The T-SEIRAS substrate was characterized with Fourier Transform Infrared Spectroscopy (FT-IR), Scanning Electron Microscope (SEM) and X-ray Photoelectron Spectroscopy (XPS). It was found that the spectral window of the substrate was from 600 to 4000 cm−1. The apparent signal enhancement factor of the T-SEIRAS substrate was found to be about 35 times with the utilization of 11-mercaptoundecanoic acid (MUA) as probe molecule. Finally, the T-SEIRAS spectrum of E. coli was successfully obtained. It is expected that this type of T-SEIRAS substrate could be used in inexpensive IR instruments for field sensing applications.

Mid-infrared laser absorption spectroscopy of the Ne-NO(X2Π) complex.

The rovibrational spectrum of the Ne-NO(X2Π) open-shell complex has been measured in the 5.3 µm region using distributed feed-back quantum cascade lasers to probe the direct absorption in a slit-jet supersonic expansion. Three P-subbands (P' ← P″: 1/2 ← 1/2, 3/2 ← 1/2, and 5/2 ← 3/2) were observed, where P is the projection of the angular momentum J along the inertial a-axis of the complex. The hyperfine structure due to the nuclei spin of 14N (I = 1) was partially resolved in the P' ← P″: 1/2 ← 1/2 and 3/2 ← 1/2 subbands. The observed mid-infrared spectrum of Ne-NO (X2Π) together with the previously reported microwave spectrum was analyzed using a modified semirigid asymmetric rotor Hamiltonian for a planar open-shell complex. The band origin is located at 1876.0606(97) cm-1, which is blue-shifted from that of the NO monomer by only 0.0888 cm-1. The complex shows strong structural relaxation upon excitation of the overall rotation and the internal rotation of the NO subunit.

Broadband FIR absorption spectra of low‐density polyethylene sheets containing six different antioxidants and estimation of their contents by chemometric analysis

Antioxidants are usually added to polymeric insulating materials to suppress oxidative degradation. However, the details of antioxidant added to a polymer are often not disclosed. On the other hand, compared to mid-infrared absorption spectroscopy, terahertz (THz) absorption spectroscopy, and far-infrared (FIR) absorption spectroscopy have not been performed so often. Based on the above backgrounds, six different kinds of antioxidants were added into low-density polyethylene sheets with four different contents, and their THz and FIR absorption spectra were measured. As a result, it has become clear that each antioxidant shows its specific absorption peaks, by which the antioxidants can be identified. Furthermore, the antioxidant content can be estimated by the absorption intensity of the specific peak. A chemometric analysis can make the estimation more accurately than a simple comparison of absorption intensity.

Waveguide Absorption Spectroscopy of Bovine Serum Albumin in the Mid-Infrared Fingerprint Region

Protein sensing in biological fluids provides important information to diagnose many clinically relevant diseases. Mid-infrared (MIR) absorption spectroscopy of bovine serum albumin (BSA) is experimentally demonstrated on a germanium on silicon (GOS) waveguide in the 1900-1000 cm-1 (5.3-10.0 μm) region of the MIR. GOS waveguides were shown to guide light up to a wavelength of 12.9 μm. The waveguide absorption spectrum of water, showing molecular bending vibrations, was obtained experimentally and compared with a theoretical model showing good agreement. Measurement of a concentration series of BSA protein in phosphate buffered saline (PBS) from 0.1 mg/mL to 100 mg/mL was performed on the waveguide using filter paper as a flow strip, and the amide I, II, and III peaks were observed and quantified.

A benchtop shock physics laboratory: Ultrafast laser driven shock spectroscopy and interferometry methods.

Common Ti:sapphire chirped pulse amplified laser systems can be readily adapted to be both a generator of adjustable pressure shock waves and a source for multiple probes of the ensuing ultrafast shock dynamics. In this paper, we detail experimental considerations for optimizing the shock generation, interferometric characterization, and spectroscopic probing of shock dynamics with visible and mid-infrared transient absorption. While we have reported results using these techniques elsewhere, here we detail how the spectroscopies are integrated with the shock and interferometry experiment. The interferometric characterization uses information from beams at multiple polarizations and angles of incidence combined with thin film equations and shock dynamics to determine the shock velocity, particle velocity, and shocked refractive index. Visible transient absorption spectroscopy uses a white light supercontinuum in a reflection geometry, synchronized to the shock wave, to time resolve shock-induced changes in visible absorption such as changes to electronic structure or strongly absorbing products and intermediates due to reaction. Mid-infrared transient absorption spectroscopy uses two color filamentation supercontinuum generation combined with a simple thermal imaging microbolometer spectrometer to enable broadband single shot detection of changes in the vibrational spectra. These methods are demonstrated here in the study of shock dynamics at stresses from 5 to 30 GPa in organic materials and from a few GPa to >70 GPa in metals with spatial resolution of a few micrometers and temporal resolution of a few picoseconds. This experiment would be possible to replicate in any ultrafast laser laboratory containing a single bench top commercial chirped pulse amplification laser system.

Probing ultrafast shock-induced chemistry in liquids using broad-band mid-infrared absorption spectroscopy.

We probe shock-induced chemistry in two organic liquids by measuring broadband, midinfrared absorption in the 800-1400 cm-1 frequency range. To test this new method and understand the signatures of chemical reactions in time resolved vibrational spectra, we compared liquid benzene shocked to unreactive conditions (shocked to a pressure of 18 GPa for a duration of 300 ps) to nitromethane under reactive conditions (25 GPa). We see clear signatures of shock-induced chemistry that are distinguishable from the pressure- and temperature-induced changes in vibrational mode shapes. While shocked benzene shows primarily a broadening and shifting of the vibrational modes, the nitromethane vibrational modes vanish once the shock wave enters the liquid and simultaneously, a spectrally broad feature appears that we interpret as the infrared spectrum of the complex mixture of product and intermediate species. To further interpret these measurements, we compare them to reactive quantum molecular dynamics simulations, which gives qualitatively consistent results. This work demonstrates a promising method for time resolving shock-induced chemistry, illustrating that chemical reactions produce distinct changes in the vibrational spectra.

Towards Integrated Mid-Infrared Gas Sensors

Optical gas sensors play an increasingly important role in many applications. Sensing techniques based on mid-infrared absorption spectroscopy offer excellent stability, selectivity and sensitivity, for numerous possibilities expected for sensors integrated into mobile and wearable devices. Here we review recent progress towards the miniaturization and integration of optical gas sensors, with a focus on low-cost and low-power consumption devices.

Quantitative analysis of ammonia adsorption in Ag/AgI-coated hollow waveguide by mid-infrared laser absorption spectroscopy

The adsorption effect of ammonia gas molecules on the inner surface of silver and silver iodide-coated hollow waveguides (Ag/AgI-HWGs) was studied by using a direct absorption spectrometry system with a 9.56-µm quantum cascade laser as the light source. The dynamic changes in ammonia concentration in the Ag/AgI-HWG during the process of NH3 influx and N2 purge were measured with a high time resolution. The adsorption and desorption processes of ammonia were quantitatively analyzed. The results indicate that the density of ammonia molecules adsorbed on the inner wall surface of the Ag/AgI-HWG was 1015 molecules/cm2, characterized as multilayer adsorption, and the corresponding F value (a dimensionless number commonly used to characterize influence of the adsorption effect) was about 5.89. The adsorption capacity increased with the pressure and decreased with the temperature, which is consistent with Le Chatelier's principle. The differential heats of adsorption were deduced to be 7.6 and 6.1 kJ/mol for monolayer and multilayer adsorption, respectively, and the Brunauer-Emmett-Teller model was established and agreed well with our measurement results. The adsorption kinetic process also fit well to both the pseudo-first-order and pseudo-second-order kinetic model. This study fills in the gap in the research of Ag/AgI-HWGs adsorption and is meaningful for accurate gas detection using HWG-based spectroscopy systems.

Fitting of single-exhalation profiles using a pulmonary gas exchange model—application to carbon monoxide

Real-time breath gas analysis coupled to gas exchange modeling is emerging as promising strategy to enhance the information gained from breath tests. It is shown for exhaled breath carbon monoxide (eCO), a potential biomarker for oxidative stress and respiratory diseases, that a weighted, nonlinear least-squares fit of simulated to measured expirograms can be used to extract physiological parameters, such as airway and alveolar concentrations and diffusing capacities. Experimental CO exhalation profiles are acquired with high time-resolution and precision using mid-infrared tunable diode laser absorption spectroscopy and online breath sampling. A trumpet model with axial diffusion is employed to generate eCO profiles based on measured exhalation flow rates and volumes. The concept is demonstrated on two healthy non-smokers exhaling at a flow rate of 250 ml s−1 during normal breathing and at 120 ml s−1 after 10 s of breath-holding. The obtained gas exchange parameters of the two subjects are in a similar range, but clearly distinguishable. Over a series of twenty consecutive expirograms, the intra-individual variation in the alveolar parameters is less than 6%. After a 2 h exposure to 10 ± 2 ppm CO, end-tidal and alveolar CO concentrations are significantly increased (by factors of 2.7 and 4.9 for the two subjects) and the airway CO concentration is slightly higher, while the alveolar diffusing capacity is unchanged compared to before exposure. Using model simulations, it is found that a three-fold increase in maximum airway CO flux and a reduction in alveolar diffusing capacity by 60% lead to clearly distinguishable changes in the exhalation profile shape. This suggests that extended breath CO analysis has clinical relevance in assessing airway inflammation and chronic obstructive pulmonary disease. Moreover, the novel methodology contributes to the standardization of real-time breath gas analysis.

Improvement of photocatalytic activity under visible-light irradiation by heterojunction of Cu ion loaded WO3 and Cu ion loaded N-TiO2

Visible-light-sensitive photocatalysts have frequently been used for indoor environmental purification. In order to further expand the applications, improvements in photocatalytic performance are desired. In this study, we have prepared new composite photocatalysts with copper ion loaded tungsten trioxide (Cu/WO3) and copper ion loaded nitrogen-doped titanium dioxide (Cu/N-TiO2) as a function of the composition ratio. Acetaldehyde decomposition activities under visible-light irradiation have been significantly enhanced by the heterojunction photocatalysts at a composition ratio of 1:9 against Cu/WO3 and Cu/N-TiO2. We have also investigated the enhanced mechanisms of the photocatalytic activity using time-resolved visible to mid-infrared (mid-IR) absorption spectroscopy. It has been revealed that there are longer hole lifetimes via electron transfer from the conduction bands of N-TiO2 and WO3 to Cu ions, and hole transfer from the valence band of WO3 to that of N-TiO2.

Background Infrared Sources for Studying the Galactic Center’s Interstellar Gas

Abstract We briefly describe the results of a K-band spectroscopic survey of over 500 highly reddened point-like objects on sightlines toward the Central Molecular Zone (CMZ) of the Galaxy. The goal was to find stars with featureless or nearly featureless spectra suitable for near- and mid-infrared absorption spectroscopy of the Galactic center’s interstellar gas on sightlines spread across the CMZ. Until recently only a few such stars have been known outside of very localized sightlines in the vicinity of the Quintuplet and Central clusters. We have used Spitzer Space Telescope Galactic Legacy Infrared Midplane Survey Extraordinaire and Two-Micron All-Sky Survey photometry to select promising candidates, and over the last 10 years have been acquiring low-resolution K-band spectra of them. As expected, the vast majority are cool and/or highly reddened red giants with complex photospheric spectra unsuitable for measuring faint interstellar lines. Approximately 10% of them, whose observations are reported here, have featureless or nearly featureless spectra. Although not evenly distributed in Galactic longitude, these stars are scattered across the CMZ. Many of them are luminous stars that are deeply embedded in warm dust cocoons, and have K-band continua rising steeply to longer wavelengths. A significant fraction of them are hot stars of a variety of spectral types, including at least five newly discovered Wolf–Rayet stars. All of them should be suitable for spectroscopy of interstellar absorption lines at infrared wavelengths greater than 3 μm and many are also suitable at shorter wavelengths.

Mid-Infrared Tunable Laser-Based Broadband Fingerprint Absorption Spectroscopy for Trace Gas Sensing: A Review

The vast majority of gaseous chemical substances exhibit fundamental rovibrational absorption bands in the mid-infrared spectral region (2.5–25 μm), and the absorption of light by these fundamental bands provides a nearly universal means for their detection. A main feature of optical techniques is the non-intrusive in situ detection of trace gases. We reviewed primarily mid-infrared tunable laser-based broadband absorption spectroscopy for trace gas detection, focusing on 2008–2018. The scope of this paper is to discuss recent developments of system configuration, tunable lasers, detectors, broadband spectroscopic techniques, and their applications for sensitive, selective, and quantitative trace gas detection.

Tri-carbon defects in carbon doped GaN

Carbon doped GaN crystals grown by hydride vapor phase epitaxy have been investigated using mid-infrared and near-ultraviolet absorption spectroscopy. Two local vibrational modes (LVMs) at 1679 cm−1 and 1718 cm−1 as well as an absorption shoulder in front of the band edge absorption of GaN are discovered, all of which increase in intensity with the carbon concentration. The LVMs are similar in wavenumber position to an LVM formerly observed in carbon-rich AlN crystals and unambiguously assigned to a tri-carbon defect. Together with the polarization dependence of the LVMs, we conclude that in GaN the underlying defects are two crystallographically inequivalent configurations of each three nearest neighbor carbon atoms. The strength of both the infrared and ultraviolet absorption features implies concentrations of the underlying defects of the same order as the total carbon concentration. Hence, the tri-carbon defects contribute to the UV absorption and possess deep energy levels in GaN.

Performance Enhancement of Methane Detection Using a Novel Self-Adaptive Mid-Infrared Absorption Spectroscopy Technique

An electrical-domain self-adaptive mid-infrared absorption spectroscopy for methane detection based on an interband cascade laser was demonstrated. By adding noise into the laser drive signal, denoising and sensing performances were evaluated for the technique. Experiments were made to study the effects of noise level/type on sensor stability, characterized by Allan deviation. High- and low-frequency noise levels have the same functional variation trend on Allan deviation, which differs from white Gaussian noise. Within a noise level range of 0–125 mV for low- and high-frequency noise and 0–62.5 mV for white Gaussian noise in the mercury–cadmium–telluride detector's output (with a pure signal amplitude of ∼300 mV), the sensor stability using self-adaptive denoising was enhanced by a factor of 1.05–20, 1.32–6.25, and 1.15–3.33 times compared to that using no filtering, for the three kinds of noise, respectively. The reported self-adaptive methane sensor system shows enhanced stability compared to the direct laser absorption spectroscopy sensor using traditional sensing architecture and classic filtering method. The sensor was further evaluated through outdoor atmospheric methane measurements using such technique. A second-order self-adaptive direct laser absorption spectroscopy technique was also proposed for noise suppression in both optical and electrical domain as an outlook of the concept of this paper.

Temporally resolved two dimensional temperature field of acoustically excited swirling flames measured by mid-infrared direct absorption spectroscopy.

The detailed understandings of temperature profiles and flow-flame interaction in unsteady premixed swirling flames are crucial for the development of low emission turbine engines. Here, a phase-locked tomographic reconstruction technique measuring the large absorption cross section of CO2 at its mid-infrared fundamental band around 4.2 μm is used to acquire the flame temperature and in situ CO2 volume fraction distribution in a turbulent premixed swirling flame under different levels of external acoustic forcing amplitude. The temporally resolved temperature field variation reveals large temperature fluctuation in unsteady premixed swirling flames produced near the nozzle exit due to vortex-driven mixing of surrounding cold gas. The temperature fluctuation quickly dissipates when moving downstream of the flame with the flow velocity of the burnt gas. The accurate high temporal resolution thermodynamic measurements of the phase-locked tomographic thermometry technique reported in this work can be generally applied to periodic reacting flows.