Ν3 Fundamental Band Research Articles

Compact vertical emitting ring interband cascade lasers for isotope-resolved CO2 sensing

We present a compact vertically emitting ring interband cascade laser (ICL) with low power consumption and the possibility for seamless integration into various CO2 sensing applications. Our devices exhibit desirable performance characteristics in battery-driven handheld devices, including room temperature (20 °C) threshold currents as low as 15 mA, small footprints, and stable single-mode emission, suitable for rapid isotope-resolved CO2 detection. Through epi-down bonding with sub-micron accuracy, we achieved robust integration of substrate-emitting ring ICLs, ensuring reliability and scalability that would be required for mass production. We present comprehensive experimental results validating the efficacy of our approach, including spectral analysis and CO2 sensing capabilities with limits of detection of 24 and 13 ppmv utilizing the 12CO2 P(60) and 13CO2 R(10) transitions in the ν3 fundamental band, respectively. The demonstrated devices hold great promise for a wide range of industrial applications, including environmental monitoring, process control, and atmospheric research, where compact low-power sensors are essential.

Real-time monitoring of CH4 and N2O emissions from livestock using mid-infrared external cavity quantum cascade laser absorption spectroscopy

The largest share of greenhouse gas (GHG) emissions related to livestock originates from methane (CH4) and nitrous oxide (N2O) which have a far higher influence on global warming, it is therefore necessary to accurately monitor CH4 and N2O emissions to provide theoretical and practical basis for further estimating and regulating GHG emissions from livestock and improving livestock production performance. For the purpose of sensing CH4 and N2O emissions during livestock living process in real time, an optical sensor based on continuous-wave (CW) external cavity quantum cascade laser (EC-QCL) operating at room temperature was developed. CH4 and N2O absorption lines, located around 8 μm, of the ν4 and ν1 fundamental vibrational bands, respectively, were chosen for direct absorption spectroscopy, which allows for sensitive, selective and simultaneous measurement of CH4 and N2O concentrations. Use of a Herriot multi-pass cell with an effective path-length of 100 m, 1σ (SNR = 1) limits of detection of 26.8 ppbv, 20.3 ppbv and 0.01 % for CH4, N2O and H2O vapor were achieved, respectively. Field measurement of CH4 and N2O emissions from horses has been carried out in a stable over two weeks at the Vernaelde farm in Couderkerque Branche city, France. Concentrations of CH4 and N2O up to 10 times and 1.5 times higher than their levels in the local ambient air (∼ 2.12 ppmv and ∼ 427 ppbv) were observed, respectively.

Spectroscopic observation and ab initio calculations of a new isomer of the CS2 trimer

We report spectroscopic observation and theoretical calculations of a new isomer of (CS2)3 as observed in the regions of the ν3 fundamental band of CS2 (6.5 μm) and the ν1 + ν3 combination band (4.5 μm), using tunable laser sources and a pulsed supersonic slit-jet. The previously observed CS2 trimer has a barrel-shaped structure with three equivalent monomers and D3 symmetry. The new isomer consists of a staggered parallel “dimer pair” of equivalent CS2 monomers with a third CS2 monomer sitting “on top”, similar to the known non-cyclic CO2 trimer. This structure has C2 rotational symmetry corresponding to the b inertial axis of the trimer, as proven by observed nuclear spin statistics. Ab initio calculations correctly give the two observed isomer structures and indicate that they lie very close in binding energy.

High-temperature line strength and line shape parameters measurements of Ar- and N2-perturbed CO2 lines near 4.18 µm in a shock tube

Line strengths and line shape parameters of Ar- and N2-perturbed CO2 R-branch transitions (82 ≤J″≤ 90) in the ν3 fundamental band were measured in a shock tube from 730 K to 2500 K and pressure below 1.13 atm using laser absorption spectroscopy (LAS). The retrieved absorption curves were fitted with the Voigt and the quadratic speed-dependent Voigt (qSDV) profiles to obtain the line strengths, broadening, and shift coefficients. Line strengths were compared to values in HITEMP, HITRAN2020, and Ames2021 databases, and the HITEMP shows the best agreement with the measured results. Ar- and N2-broadening and shift coefficients were regressed into the power law and the double power law form to study the temperature dependence of the targeted lines. The speed-dependence parameter aw was temperature-dependent. The J-dependence of the derived broadening and shift coefficients was discussed. The results of this work will aid the development of the absorption model in the far-wing region and the design of CO2 LAS sensors for high-temperature measurement.

Measurements of absolute line strength of the ν1 fundamental transitions of OH radical and rate coefficient of the reaction OH + H2O2 with mid-infrared two-color time-resolved dual-comb spectroscopy

Absolute line strengths of several transitions in the ν1 fundamental band of the hydroxyl radical (OH) have been measured by simultaneous determination of hydrogen peroxide (H2O2) and OH upon laser photolysis of H2O2. Based on the well-known quantum yield for the generation of OH radicals in the 248-nm photolysis of H2O2, the line strength of the OH radicals can be accurately derived by adopting the line strength of the well-characterized transitions of H2O2 and analyzing the difference absorbance time traces of H2O2 and OH obtained upon laser photolysis. Employing a synchronized two-color dual-comb spectrometer, we measured high-resolution time-resolved absorption spectra of H2O2 near 7.9 µm and the OH radical near 2.9 µm, simultaneously, under varied conditions. In addition to the studies of the line strengths of the selected H2O2 and OH transitions, the kinetics of the reaction between OH and H2O2 were investigated. A pressure-independent rate coefficient kOH+H2O2 was determined to be [1.97 (+0.10/−0.15)] × 10−12 cm3 molecule−1 s−1 at 296 K and compared with other experimental results. By carefully analyzing both high-resolution spectra and temporal absorbance profiles of H2O2 and OH, the uncertainty of the obtained OH line strengths can be achieved down to <10% in this work. Moreover, the proposed two-color time-resolved dual-comb spectroscopy provides a new approach for directly determining the line strengths of transient free radicals and holds promise for investigations on their self-reaction kinetics as well as radical–radical reactions.

Line positions and effective line strengths of trans-HONO near 1280 cm−1

The measurement of the line positions and effective line strengths of the ν3 fundamental band of trans-nitrous acid (trans-HONO) near 1280 cm−1 (7.8 µm) by tunable laser absorption spectroscopy (TLAS) utilizing a room temperature continuous-wave quantum cascade laser (cw-QCL) was reported. The effective line strengths of 30 well-resolved trans-HONO absorption lines in the range of 1279.8–1282.2 cm−1 were determined using the HONO line strength at 1280.3841 cm−1 as a scale. The maximum measurement uncertainty of 7.64% in the line strengths is mainly determined by the uncertainty of the referenced line strength, while the measurement precision of the line positions is better than 5.56 * 10−3 cm−1. The line positions and strengths of the trans-HONO absorption lines obtained in this work provide a reference for continuous gas monitoring and analysis of the sources and sinks of atmospheric HONO.

Elucidation of rotational-interaction coupling and collision-induced effects in rovibrational transitions of β-N2O isotopologue at 7.8 µm mid-infrared region by cavity ring-down spectroscopy

High-sensitive cavity ring-down spectroscopy (CRDS) coupled with a cw-quantum cascade laser operating at 7.8 μm mid-infrared (MIR) region has been employed to investigate the rovibrational interaction coupling for various MIR rotational transitions of the P-and R-branches in the ν1 fundamental vibrational band (1000 ← 0000) of the β-site-specific isotopologue of nitrous oxide (β-N2O) i.e., 15N14N16O. The coupled rotational interaction was experimentally quantified from the high-resolution CRDS data and a data-driven approach was taken for the approximation of the Herman-Wallis series. Moreover, J-dependent pressure-broadened absorptions and their collision lifetimes in response to foreign-perturbing gas molecules were investigated to understand the collision-induced rotational quantum effect in β-N2O molecule. This study reveals an experimental elucidation of the fundamental infrared physics underlying rovibrational interaction and its coupling to the vibrational and rotational degrees of freedom for a non-centrosymmetric linear polyatomic molecule possessing a permanent dipole moment. Moreover, the experimentally probed interference-free spectral lines of β-N2O molecule attributing different spectroscopic signatures in the MIR region may drive new applications in environmental monitoring, biological and biomedical diagnostics in the future.

Dual mid-infrared wavelength Faraday rotation spectroscopy NOx sensor based on NdFeB ring magnet array

Faraday rotation spectroscopy (FRS) exploits the magneto-optical Faraday rotation effect to achieve highly sensitive detection of paramagnetic molecules. Most of the currently reported FRS sensors suffer from high power consumption, while enabling detection of only a single component. In this work, a novel dual mid-infrared wavelength FRS sensor is proposed that enables the simultaneous detection of two paramagnetic molecules (nitrogen oxides, NOx = NO + NO2) in a single absorption cell. A neodymium-iron-boron (NdFeB) ring magnet array was designed to replace the solenoid coil to provide a stable, power-free magnetic field. The magnetic induction line distribution characteristics around the permanent magnets were analyzed based on the finite element method. Two mid-infrared quantum cascade lasers were used to detect the strongest fundamental ν2 rotational-vibrational band of NO (1875.81 cm−1) and the ν3 fundamental band of NO2 (1613.25 cm−1), respectively. A dual-wavelength Herriott cell (DWHC) is coaxially combined with a NdFeB ring magnet array to amplify the interaction of the laser beam with paramagnetic NOx molecules in an axial magnetic field. This low-power FRS NOx sensor achieves minimum detection limits of 0.58 ppb and 0.95 ppb for NO2 and NO, respectively, over an optical length of 23.7 m.

Spectra of CO2-Rg2 and CO2-Rg-He trimers (Rg = Ne, Ar, Kr, and Xe): Intermolecular CO2 rock, vibrational shifts and three-body effects.

Weakly bound CO2-Rg2 trimers are studied by high-resolution (0.002cm-1) infrared spectroscopy in the region of the CO2 ν3 fundamental band (≈2350cm-1), using a tunable optical parametric oscillator to probe a pulsed supersonic slit jet expansion with an effective rotational temperature of about 2K. CO2-Ar2 spectra have been reported previously, but they are extended here to include Rg = Ne, Kr, and Xe as well as new combination and hot bands. For Kr and Xe, a unified scaled parameter scheme is used to account for the many possible isotopic species. Vibrational shifts of CO2-Rg2 trimers are compared to those of CO2-Rg dimers, and in all cases the trimer shifts are slightly more positive (blue-shifted) than expected on the basis of linear extrapolation from the dimer. Combination bands directly measure an intermolecular vibrational mode (the CO2 rock) and give values of about 32.2, 33.8, and 34.7cm-1 for CO2-Ar2, -Kr2, and -Xe2. Structural parameters derived for CO2-Rg2 trimers are compared with those of CO2-Rg and Rg2 dimers. Spectra of the mixed trimers CO2-Rg-He are also reported.

Measurement of temperature-dependent line parameters of ammonia transitions near 1103 cm−1

Accurate knowledge of the line parameters of ammonia (NH3) transitions is important for developing precise laser-based NH3 sensors. We report the measurement of line-strengths and temperature-dependent broadening coefficients of NH3 with N2, air, Ar, and CO2 for six transitions in the ν2 fundamental band near 1103 cm−1. The absorption feature contributed by the target transitions is of particular interest for combustion emission applications due to its strong absorbance and the minimal spectral interference from other species such as H2O, CO2, and NOX. In this work, we measured NH3 absorption spectra with different collisional partners at the temperature range of 296–573 K using a quantum cascade laser. Spectroscopic parameters were determined by conducting multi-line fitting of the measured spectra using the Galatry lineshape function due to the evident line narrowing effect. Our study will aid the development of mid-infrared NH3 sensors particularly for high-temperature applications.

A methane line list with sub-MHz accuracy in the 1250 to 1380 cm−1 range from optical frequency comb Fourier transform spectroscopy

We use a Fourier transform spectrometer based on a difference frequency generation optical frequency comb to measure high-resolution, low-pressure, room-temperature spectra of methane in the 1250 – 1380-cm−1 range. From these spectra, we retrieve line positions and intensities of 678 lines of two isotopologues: 157 lines from the 12CH4 ν4 fundamental band, 131 lines from the 13CH4 ν4 fundamental band, as well as 390 lines from two 12CH4 hot bands, ν2 + ν4 – ν2 and 2ν4 – ν4. For another 165 lines from the 12CH4 ν4 fundamental band we retrieve line positions only. The uncertainties of the line positions range from 0.19 to 2.3 MHz, and their median value is reduced by a factor of 18 and 59 compared to the previously available data for the 12CH4 fundamental and hot bands, respectively, obtained from conventional FTIR absorption measurements. The new line positions are included in the global models of the spectrum of both methane isotopologues, and the fit residuals are reduced by a factor of 8 compared to previous absorption data, and 20 compared to emission data. The experimental line intensities have relative uncertainties in the range of 1.5 – 7.7%, similar to those in the previously available data; 235 new 12CH4 line intensities are included in the global model.

N2O–Ar and N2O–Kr: Intermolecular vibrations of N2O–Kr and symmetry breaking of the N2O bending mode in the presence of a rare gas

Rotationally-resolved infrared spectra of N2O–Ar and N2O–Kr are studied in the region of the N2O ν1 vibration (≈2224 cm−1) using a tunable Quantum Cascade laser source to probe a pulsed supersonic jet. The N2O–Kr ν1 fundamental band is re-analyzed, together with previous ν3 band data, using a unified scheme to fit the (small) observed Kr isotope splittings. This scheme is then transferred to analyze the bending combination band of N2O–Kr near 2257 cm−1 where isotope effects are much larger due to stretch-bend interactions. As a result, N2O–Kr intermolecular bend (33.29 cm−1) and stretch (34.48 cm−1) frequencies are directly determined for the first time. Weak spectra are detected for both N2O–Ar and –Kr corresponding to the (ν1, ν2l2, ν3) = (1110) ← (0110) hot band of N2O. Their analysis yields the magnitude of the splitting of the N2O ν2 bending mode into in-plane and out-of-plane components. These splittings are found to be significantly smaller than those observed in the analogous CO2-containing dimers.

Line-strengths, collisional coefficients and narrowing parameters in the ν3 band of methane: H2, He, N2, O2, Ar and CO2 collider effects

The ν3 fundamental absorption band of methane (12CH4) dominates the 3.3–3.4 μm infrared spectral window and has been extensively used for atmospheric, planetary and astrophysical investigations. Spectroscopic data, such as line positions, line-strengths and foreign-collisional coefficients, are needed to analyze the infrared spectra of planetary atmospheres for the understanding of relevant physical chemistry. In this study, high-resolution infrared spectra of 12CH4 belonging to the ν3 fundamental vibrational band are recorded over 2884–2969 cm−1 with a difference–frequency–generation (DFG) laser having a linewidth of Δν ≈ 0.0002 cm−1. Line-strengths as well as H2-, He-, N2-, O2-, Ar- and CO2- broadening coefficients are determined for 49 methane transitions at room temperature. Data are retrieved using Voigt and Galatry profiles to compute the measured line shape of each individual transition at various pressures. Narrowing coefficients due to Dicke effect are also determined. The new data extend and supplement previous measurements over the target spectral region (2884–2969 cm−1) where very fewer studies are reported. In particular, the present work is the first known complete determination of broadening data and narrowing parameters at high rotational quantum numbers of the P-branch of the ν3-CH4 band.

Optical frequency comb Fourier transform spectroscopy of 14N216O at 7.8 µm

We use a Fourier transform spectrometer based on a compact mid-infrared difference frequency generation comb source to perform broadband high-resolution measurements of nitrous oxide, 14N216O, and retrieve line center frequencies of the ν1 fundamental band and the ν1 + ν2 – ν2 hot band. The spectrum spans 90 cm−1 around 1285 cm−1 with a sample point spacing of 3 × 10−4 cm−1 (9 MHz). We report line positions of 72 lines in the ν1 fundamental band between P(37) and R(38), and 112 lines in the ν1 + ν2 – ν2 hot band (split into two components with e/f rotationless parity) between P(34) and R(33), with uncertainties in the range of 90-600 kHz. We derive upper state constants of both bands from a fit of the effective ro-vibrational Hamiltonian to the line center positions. For the fundamental band, we observe excellent agreement in the retrieved line positions and upper state constants with those reported in a recent study by AlSaif et al. using a comb-referenced quantum cascade laser [J Quant Spectrosc Radiat Transf, 2018;211:172-178]. We determine the origin of the hot band with precision one order of magnitude better than previous work based on FTIR measurements by Toth [http://mark4sun.jpl.nasa.gov/n2o.html], which is the source of the HITRAN2016 data for these bands.

Infrared laser spectroscopy of the ν3-ν4 difference band and detection of the ν3 band of NO3

The infrared diode laser spectrum of the ν3-ν4 difference band of 14NO3 has been observed in the 690 cm−1 region. By fixing the lower state (v4 = 1) constants to the previously determined values, molecular constants in v3 = 1 have been determined for the first time : the vibrational energy E3 = 1054.13679(69) cm−1, the rotational constants B3 = 0.4562291(41), C3 = 0.2274567(53) cm−1, the first-order Coriolis coupling constant Cζ3 = 0.0032073(99) cm−1, and the spin–orbit interaction constant aeff = –0.03979(70) cm−1, with one standard deviation in parentheses. The ν3 fundamental band has been detected with weaker intensity than the ν3 + ν4 band by a factor of ~ 2000.

Cavity Ring-Down Spectroscopy for Molecular Trace Gas Detection Using A Pulsed DFB QCL Emitting at 6.8 µm

A trace gas sensor based on pulsed cavity ring-down spectroscopy (CRDS) was developed for measurement of the ν4 fundamental vibrational band of ammonia (NH3) centered at 1468.898 cm−1. A pulsed distributed feedback quantum cascade laser (DFB-QCL) operating at 6.8 µm (1470.58 cm−1) quite well covered the absorption band of the ammonia and strong fundamental vibrational absorption bands of different molecular gases in this unexplored region. The cavity was partially evacuated down to 0.4 Atm by a turbo-molecular pump to reduce the partial interference between the NH3 spectra and water near the absorption peak of ammonia. A sensitivity of nine parts per billion was reached for a measurement time of 120 s as well as an optical path length of 226 m. The device demonstrated high spectral performance and versatility due to its wide tuning range, narrow linewidth, and comparatively high-energy mid-IR radiation in the relatively unexplored 6.8 µm region, which is very important for high-resolution spectroscopy of a variety of gases.

New analysis of line positions of the ν3 bands of 35ClNO2 and 37ClNO2 around 370 cm−1

A new investigation of the ν3 bands of 35ClNO2 and37ClNO2, located around 370 cm−1 has been performed using a high resolution (0.00102 cm−1) Fourier transform spectrum recorded at SOLEIL with highly improved experimental conditions as compared to previous work [Orphal J, Morillon-Chapey M, Klee S, Mellau GC, Winnewisser M. J Mol Spectrosc 1998;190:101–6]: (i) the use of synchrotron radiation which resulted in a better signal-to-noise ratio; (ii) a resolution twice better; (iii) a low temperature (221 K) with an optical path length of 8.16 m, allowing low pressure (0.025 hPa) in the sample leading to a well resolved spectrum. As a consequence, significantly better results than previously were obtained. Thanks to the new experimental conditions, the line assignments were pursued up to higher J and Ka quantum number values, J = 83 and Ka = 44. For both isotopomers, a total of 6331 transitions were reproduced with a root-mean-square deviation of 2 × 10−4 cm−1 using a Watson-type A-reduced Hamiltonian. Improved rotational and centrifugal distortion constants for the ν3 fundamental bands of 35ClNO2 and37ClNO2 have been determined. The band centers are 370.1510773(92) cm−1 for the ν3 fundamental band of 35ClNO2 and 364.5218094(96) cm−1 for the ν3 fundamental band of 37ClNO2. The synthetic line list obtained in this study could be interesting for future measurements of ClNO2 in the atmosphere, e.g. using the new space mission FORUM which is one of the concepts chosen by ESA to be developed further and which opens up a new window (150–1400 cm−1) for understanding and quantifying the radiative processes, as well as air quality and pollution effects.

High-resolution spectroscopy and analysis of the ν3, ν4 and 2ν4 bands of SiF4 in natural isotopic abundance

Silicon tetrafluoride (SiF4) is a trace component of volcanic gases. However, a better knowledge of spectroscopic parameters is needed for this molecule in order to derive accurate concentrations. This motivated FTIR measurements with high-spectral resolution (0.001 cm−1) and an extensive study of its infrared absorption bands, including the fundamentals and overtone and combinations. We present here a detailed analysis and modeling of the strongly absorbing ν3and ν4 fundamental bands, for the three isotopologues in natural abundance: 28SiF4 (92.23 %), 29SiF4 (4.67 %) and 30SiF4 (3.10 %). It includes a global fit with consistent parameter sets for the ground and excited states. In particular, all existing rotational line data have been included. The 2ν4 band of 28SiF4 could also be analyzed in detail. A first fit of the dipole moment derivative for the ν3 band for 28SiF4 has been performed, along with two independent estimates of the integrated band intensity; the results are consistent with literature values, around 690 km/mol. The isotopic dependence of band centers and Coriolis parameters has also been studied. TFSiCaSDa, a new database of cross sections and calculated lines for the ν3 band of SiF4, has been set up.

The fundamental [formula omitted] band of DTO and the [formula omitted] overtone band of HTO from the analysis of a high-resolution spectrum of tritiated water vapour

The ν3 fundamental band of DTO and the 2ν1 overtone band of HTO have been analysed for the first time from a high-resolution infrared spectrum of tritiated water vapour recorded at room temperature using a Bruker IFS 125HR Fourier transform spectrometer applying a resolution of 0.0075cm-1.For DT16O, we report 436 new experimental line positions in the 2500–2900cm-1 range that were assigned to the ν3 band, with rotational quantum numbers up to J=15 and Ka=8. For HT16O, we report 361 new experimental line positions in the 4300–4700cm-1 range that were assigned to the 2ν1 band, with rotational quantum numbers up to J=15 and Ka=7.For both species the assigned lines were used to determine the spectroscopic constants in the A-reduced Watson Hamiltonian. No perturbations were observed.The comparison with variational calculations of line positions and intensities from the Tomsk database shows differences of up to several 0.1cm-1 concerning the line positions, while the relative line intensities agree with the observed values within a few percent.The line list from this study will be useful for detection of gaseous traces of HTO using laser-based methods in the future and further serve the completion of experimental line lists.

High-resolution investigation of temperature and pressure-induced spectroscopic parameters of 13C-isotopomer of CH4 in the ν4 band using cavity ring-down spectroscopy

Measurements of high-resolution spectroscopic line parameters in the ν4 fundamental band of 13CH4 isotopomer of methane (CH4) occurring at 7.5 µm mid-IR spectral region are important for the studies of remote sensing of the Earth and planetary atmospheres at different conditions. Here, a continuous wave (cw) external-cavity quantum cascade laser (EC-QCL) coupled with cavity ring-down spectroscopy (CRDS) technique has been employed to probe the ν4 (F2) fundamental band of 13CH4 isotopomer. We recorded the high-resolution CRD spectra of 22 interference-free transition lines in the P, Q and R-branches of 13CH4 and subsequently determined the symmetry-specific line parameters such as line-intensity, pressure broadening coefficients along with temperature dependence of broadening coefficients of the transition lines. Our study thus provides more accurate experimental evidences of the high-resolution spectral features of 13CH4 isotopomer with rotational symmetry (A, E, F) for the ν4 (F2) band at wavelengths near 7.5 µm for a range of temperatures.