Long-path Cell Research Articles

IMPMH, direct tunable diode laser absorption spectroscopy for detection of water vapour under “Optically thick Condition”

Water vapour plays a crucial role in atmospheric processes. Hence, monitoring the altitude-related variations in water vapour properties is important to decipher atmospheric processes. Direct tunable diode laser absorption spectroscopy (dTDLAS) measures the concentration and temperature of gas molecules by scanning the rotation-vibration absorption lines using a high-spectral-resolution laser. In this study, we devised an integrated measurement and data processing method (integrative measurement and processing method for hygrometry, IMPMH) to enhance the in-situ airborne measurement capability of dTDLAS. We measured a wide range (240–18,000 ppm) of water vapour concentrations, aiming for atmospheric measurements in a highly water-saturated regime, called the “optically thick condition”. For recovering the full absorption spectra, the “integrative area” was defined and a difference factor D, which is the distance between two spectral regions with width corresponding to the half width of half maximum of the Voigt profile, was used to calculate the area. From the data, the low-bound concentration was measured to be 244 ppm. At D = 1.8, the transition concentration to the “optically thick condition” was measured to be 5,800 ppm. By increasing D from 1.8 to 2.8, the measurable upper-bound concentration increased to 17,993 ppm. IMPMH was applied to the measured data to estimate the final absorber density or water vapour concentration. The estimation was well-fitted with the measured detector signal with signal-to-noise ratio (SNR) of ∼ 300 of the residual spectrum, promising its applicability to in-situ airborne measurements. To validate IMPMH, the water vapour concentration range was divided into two regimes: (1) optically thick (5,800 < c < 18,000 ppm) and (2) optically thin (c < 5,800 ppm) conditions. Under the optically thick condition, IMPMH was validated by comparing the results between the short and long-path cells. In the optically thin condition, IMPMH was validated through comparison with the general dTDLAS method. Lastly, long-term stability of the dTDLAS system was validated by measuring 10 different concentrations (240–18,000 ppm) for 1000 s by characterising the precision and SNRs of the residual. The results demonstrate that IMPMH significantly enhances the in-situ airborne measurement capability of dTDLAS under both optically thick and thin conditions. Furthermore, requirements for the implementation of IMPMH in airborne measurement were investigated considering four aspects—sampling, low-pressure measurement, accuracy and precision, and multiplex detection. The results were examined with regard to atmospheric implications.

Line parameters of 12CH4 around 2008 μm studied by tunable diode laser spectroscopy with a long-path White cell

The absorption spectrum of CH412 at 2.008 μm has been recorded at 296 K, using a high-resolution tunable diode laser absorption spectrometer combined with a long-path, multi-pass White cell. The line positions, intensities, and self-broadening coefficients for 32 transitions of the lower part of the Tetradecad in the 4975–4985 cm−1 spectral range of CH412 have been measured. To our knowledge, no experimental information has been obtained for the self-broadening coefficients of these lines before. Comparisons of the line parameters determined in the present work with those from HITRAN2016 line lists are provided.

Analysis of Random Noise and Long-Term Drift for Tunable Diode Laser Absorption Spectroscopy System at Atmospheric Pressure

The random noise and the slow drift, which respectively limit the signal-to-noise ratio and the long-term stability, of a wavelength modulated tunable diode laser absorption spectroscopy system, are critically analyzed and evaluated experimentally. The noise sources considered in detail include the noise contributions from the lock-in amplifier, detector-preamplifier combination noise, excess noise from the DFB laser, and detection cell instabilities, especially in multiple reflection long-path cells. We also analyze contributions to long-term drift and find that the drift may be significantly compensated by the use of an optical reference path. We find experimentally that a white cell with a path length of 17.5 m introduces excess noise resulting in its having a performance equivalent to 6-m single pass absorption path. The experimental system optimized using the results of this analysis and using the 17.5-m white cell is found to operate at a sensitivity of 1.5-ppmv carbon monoxide in a bandwidth of 4 Hz. The long-term zero offset drift levels measured over 8 h are found to be $\sim 120$ ppmv without the reference optical path and are reduced to 20 ppmv using the balance path technique.

Compact photoacoustic gas sensor based on broadband IR source

We present the realization of a novel, compact IR-optical gas sensing system based on photoacoustic detection. A modular detection cell in combination with a long-path cell allowed the detection of CO2 and ethylene (C2H4) using a broadband IR source (thermal emitter). The system performance was investigated both in simulations and experiments using CO2 and ethylene as target gases. The experimental results were in good accordance with the simulation results. We aimed at the integration of low-cost components and the miniaturization of the complete system to open up the possibility for sensor networks. Furthermore, the modular detection cell allows the optimization of the optical path length within the measurement cell and thus the adaptation to different target gases.

The tunable diode laser absorption spectroscoty for measurement of NH3 with particles

Tunable diode laser absorption spectroscopy (TDLAS) with wavelength modulation is used to measure the NH3 concentration at normal temperature and pressure near 1531.7nm in near-infrared region. The concentration of 25ppm can be obtained by 10m long-path cell，and concentration from 25ppm to 400ppm has good linear relationship with 2f signal. The effect of particles on 2f signal is discussed in this paper，and a solution of the problem using laser intensity linear fit to make the concentration measurement immune to particle disturbance is suggested.

Improved wave-number tables of the carbonyl sulfide ν_2 and 2ν_2 bands and guides for accurate measurement

By use of the ν2 and 2ν2 bands of carbonyl sulfide (OCS), the high accuracy of the CO2 laser lines is transferred from 10 μm to the far-infrared region near 20 μm. Measurements were performed with a slightly modified Bruker IFS 120 HR spectrometer at the infrared laboratory of the University of Oulu. The effect of the pressure shift was reduced by measurement of the 2ν2 band with the CO2 laser band in a long-path cell with low sample pressures. With highly sensitive measurement, the absolute accuracy of the 2ν2 band center was reduced to 1.4×10−6 cm−1. Together with the accurate result for the 2ν2 band, measurement of the ν2 band with a resolution of 0.001 cm−1 and a signal-to-noise ratio of ∼150, accurate wave numbers in the region of the lower band can also be obtained. To the best of the author’s knowledge, the absolute accuracy of 3.9× 10−6 cm−1 for the ν2 band center is the best ever reached in the region near 500 cm−1. New accurate wave number tables are presented for the bands 0200–0000 near 1047 cm−1 and 0200–0110 and 0110–0000 in the region from 495 to 555 cm−1. The most important factors that limit the performance of Fourier spectrometers in high-absolute-accuracy measurements are discussed. With systematic alignment methods these barriers have been overcome with the spectrometer used. An easy method to confirm the quality of a measured spectrum is presented.

MOLTEN SALT OXIDATION OF CHLOROBENZENE

This investigation focuses on the oxidation of chlorobenzene izn a laboratory-scale molten salt reactor. A Fourier transform infrared spectrometer with a long-path cell is used to monitor the organic waste oxidation process, including the formation of by-products as a function of temperature. The measured oxidation by-products are compared with results reported in the literature and are obtained in a combustion-driven flow reactor at similar oxidation temperatures; the results are modeled using detailed chemical kinetics. The results outlined in this work provide a direct comparison of the effectiveness of molten salt oxidation compared with conventional combustion oxidation and highlight the advantages and disadvantages associated with the molten salt oxidation process.

Fluorometric Characteristics of a Wave‐Guide Cell with Low Refractive Index

Recently, Teflon AF2400 (AF 1600, and AF 1601) was commercialized (DuPont Fluoroproducts, Wilmington, DE, U.S.A), that has a refractive index (1.29) lower than water (1.33), and which means that the wave‐guiding of light is possible in water. In this study, we used Teflon AF 2400 as a waveguide capillary longpath cell for fluorometry. He‐Cd and Ar+ lasers were used as the excitation source, at 325 nm and 514.5 nm respectively. The length of the capillary wave‐guide cell was 18.70 cm. The was wound twice on a flat surface (loop diameters: 2 cm and 3 cm). The excitation was executed through the wave‐guide cell and the fluorescence from the wound capillary cell wall was collected in a perpendicular direction to the loop. With excitation at 325 nm, the fluorescence intensity at 450 nm emitted from the cell wall decreased along with the increase in the refractive index of the solvent. This can be caused by attenuation of the source light due to absorption by the solvent. In our experiment, the solvent of higher refractive index has the higher absorption at 325 nm. On the other hand, the fluorescence intensity at 590 nm, with excitation at 514.5 nm, increases with increased refractive index of the solvent. This result shows that an increase in the refractive index of the solvent is preferable for maintaining the wave‐guiding of the source light. Here, the characteristics of fluorescence spectrometry are discussed in terms of the collection of fluorescence from the wave‐guide capillary cell wall.

Diode laser spectroscopy of the weakly bound complex Ne–CH4

The infrared absorption spectrum of the weakly bound rare-gas–spherical-top complex Ne–CH4 was discovered and analyzed for the first time. Measurements were made with tunable diode laser spectrometers using a pulsed supersonic jet and a long-path low-temperature absorption cell. Close to the R(0) transition of the methane ν4 fundamental band at 1311.430 cm–1, the Ne–CH4 spectrum was recorded as a very compact absorption pattern. Within a total wave-number range of about 0.1 cm–1, P-, Q-, and R-branches are located. As the first step, the Ne–CH4 spectrum was recorded and analyzed in a supersonic jet at low rotational temperature of about 5 K. Three branches were identified, of which the P- and R-branches were partially resolved and the Q-branch remained unresolved. Compared with the previously measured spectra of Ar–CH4 and Kr–CH4 [Z. Naturforsch. A, 53, 725 (1998).], the absorption pattern in the spectrum of Ne–CH4 is much denser and considerably more compact. However, by analogy with the spectra of Ar–CH4 and Kr–CH4, assignment and analysis were carried out using a Hamiltonian model that incorporates a Coriolis interaction between the total angular momentum of the complex and the angular momentum of the methane monomer. This analysis then allowed us to assign the same spectrum as recorded in a long-path (160 m) cell at a higher temperature of 62 K. The observed rotational constant for Ne–CH4, B"= 0.129(9) cm–1, corresponds to an effective intermolecular separation of 3.8 Å. PACS Nos.: 33.20E, 34.25, 35.20P, 36.40

The infrared spectrum of the Ar-CO complex Comprehensive analysis including van der Waals stretching and bending states

The infrared spectrum of the weakly-bound Ar-CO van der Waals complex in the 2150 cm-1 region of the CO stretch has been studied using a continuous supersonic slit jet expansion coupled with a tunable laser spectrometer, and also with a cooled (57 K) long-path cell coupled with a Fourier transform spectrometer. Seven new subbands were observed, and previously observed bands were observed in greater detail. Four substates were thus identified for the first time, including the excited van der Waals stretching state, v 3 = 1, and the excited bend, v 2 = 1, within the ground CO stretching state, v CO = 0. A comprehensive simultaneous analysis was made of all the existing infrared and microwave data on the complex for levels with K < 5, in order to obtain an accurate and self-consistent set of substate origins, rotational parameters, and centrifugal distortion parameters. A strong Coriolis interaction between K = 1 of the excited bend and K = 0 of the excited stretch was analysed in detail.

The Fourier Transform Infrared Spectrum of PN

The infrared absorption spectrum of the PN molecule has been recorded for the first time using an FTIR spectrometer. A flowing discharge of PCl3 and N2 was used to produce short-lived PN molecules at room temperature in a long-path cell. Spectra of the fundamental transition were recorded at a nominal resolution of 0.02 cm−1 and calibrated against accurately known water vapor lines present in the background to give an absolute and relative accuracy on the order of 0.0004 cm−1. The line positions measured in this work were analyzed in terms of effective band constants and combined with previous high-temperature microwave and infrared data to produce accurate spectroscopic parameters for the ground state X1Σ+.

Studies of Pyrolysis Products of Explosives in Soils using Infrared Tunable Diode Laser Detection

AbstractWe report on laboratory studies of the thermal decomposition of TNT under a variety of conditions. Our goal is a better understanding of the soil contamination detection technique used in site characterization using cone penetrometers, in which hot filament pyrolysis results in small gaseous molecules, such as NO, which can be detected by a variety of methods to indicate the presence of explosives in the soil. Our laboratory studies use a long-path sampling cell and tunable infrared diode laser absorption, which provide sensitive, specific, time-resolved detection of several of the key decomposition products. In this paper, we present both the initial results of laboratory experiments on mixtures of soil and TNT, designed to simulate the operation of a cone penetrometer probe, and also results of a series of heated tube experiments which serve to relate the first type of experiment to previously reported TNT decomposition studies.

High-Resolution Infrared Spectra of Formyl Fluoride, HFCO

Five fundamental infrared bands of HFCO have been studied at high resolution for the first time: ν5 (∼ 663 cm−1), ν6 (∼ 1011 cm−1), ν4 (∼ 1065 cm−1), ν3 (∼ 1342 cm−1), and ν1 (∼ 2981 cm−1). A Bomem Fourier transform spectrometer and an adjustable long-path cell were used. The effects of the strong a- and b-type Coriolis interactions between ν6 and ν4 were analyzed in detail, as was a weaker but still significant interaction between ν3 and 2ν5. On the other hand, perturbations noted in the ν1 band were not analyzed in detail because of the large number of possible interacting vibrational states in the 3000-cm−1 region. Thirteen of the 14 known far-infrared laser transitions of HFCO, resulting from optical pumping of ν4 and ν6 by CO2 lasers, were assigned in detail on the basis of the present analysis.

Long-path equilibrium IR spectra of weakly bound complexes at low temperatures

Infrared spectroscopy of Van der Waals complexes is commonly performed using supersonic jet expansions, with the advantages of generating many complexes and achieving low internal temperatures. However, a lowtemperature equilibrium gas cell has certain complementary advantages, including the ability to achieve very long absorption paths and to interface efficiently with a Fourier-transform spectrometer. The necessarily higher temperatures in a gas cell are useful if the additional spectral lines can be assigned, which may require preliminary analysis of a jet spectrum and/or good theoretical calculations. In our laboratory, infrared spectra of complexes have been studied from 20–9000 cm–1 at high resolution (0.150–0.002 cm–1), using long paths (20–200 m) and low temperatures (20–140 K) with Bomem FTIR spectrometers and special cryogenic absorption cells. Two new, large-aperture, long-path cells will be especially useful for the difficult far-infrared region (20–500 cm–1) which includes important intermolecular vibrations. The work is illustrated here with observations on (H2)2, (D2)2, H2–Ar, CO–H2 and CO–rare-gas complexes. Spectra of the methane-containing complexes CH4–Ar, –Kr, –H2 and –D2 are reported here for the first time. They occur close to the R(0) line of the CH4v3 band, and their interpretation poses an interesting theoretical challenge.

Experimental and numerical study of the thermal destruction of C 2H 5Cl

Gaseous C 2 H 5 Cl (ethyl chloride) was injected into the post-flame zone of a turbulent combustor, with equivalence ratios and residence times in the range of those encountered in hazardous waste incinerators. Temperatures below those normally associated with incineration were selected to simulate incinerator failure modes. Samples were withdrawn from the reactor and analyzed using a Fourier transform infrared spectrometer (FTIR) coupled to a longpath cell (60 cm base path length). For the highest-temperature case (T max =1225 K), destruction of the injected C 2 H 5 Cl was rapid, and the only observable product species were HCl, CO, H 2 O, and CO 2 . For cooler injection temperatures (T max =1012, 932 K), C 2 H 4 , C 2 H 2 , and C 2 H 3 Cl were observed as well, with a C 2 H 3 Cl/C 2 H 4 ratio between 0.25 and 0.5. C 2 H 5 Cl injection was simulated numerically, using the experimental temperature profiles and modeling the reactor as a plug-flow device. The reaction mechanism developed by Karra et al. was expanded to distinguish between chlorinated C 2 isomers. Rates for the modified reactions were chosen so that the combined reaction rate of the isomers was unchanged. This modification greatly improved the agreement between numerical and experimental results for C 2 H 3 Cl, as it opened new channels for its production.

Mirror-Coated Long-Path Cells for Colorimetry with the Use of a Bifurcated Optical Guide

For sensitive measurement of the absorbance of aqueous solution, mirror-coated long-path cells have been constructed, in which introduction of light from a laser diode and collection of the reflected light passing through the analyte solution are done through the same end of the cells with the use of a bifurcated fiber. The absorbance of the solution has been enhanced about 20 to 30 times by using the present system. Also, the present mirror-coated long-path cells show a sharp response in turbidity with respect to the change toward smaller particle sizes in Latex gel. The present detection system has been coupled with the flow injection analysis of phosphate in molybdenum blue colorimetry. Detection of phosphate at 1 ng P/mL is possible with this measurement system—a result which is rather difficult to achieve with the use of a commercial auto-detection system.

Development of an infrared laser long-path absorption system using a Raman shifter.

An infrared laser long path absorption experimental system has been developed using a dyelaser pumped by an Nd: YAG laser and a hydrogen Raman shifter. The accuracy of the measurement system was evaluated by CO2 absorption experiment using an open path in the laboratory and a long-path cell. The measurement error was estimated at less than 0.5 percent in a 10-minute measurement, indicating that the accuracy was high enough for observing diurnaland seasonal variations in CO2 concentration. From the systematic analysis of the measurement errors, the kind of error limiting the system performance was identified. A method for further improvement of the measurement accuracy was investigated.

Pressure broadening of UF 6 by argon

Pressure broadening of the R(6) manifold in ν 3 of UF 6 at 628.32 cm −1 has been measured at pressures of 0–30 torr of Ar in a long-path cell at 199 ± 1 K. We obtain a pressure-broadening coefficient of 9.0 ± 1.5 MHz/torr (FWHM), which corresponds to 7.3 ± 1.2 MHz/torr at 300 K. The UF 6-Ar optical collision diameter is 7.4 ± 0.6 Å; this is larger than the hard-sphere kinetic theory collision diameter of 5.2 Å obtained from the diffusion coefficient and implies rapid rotational relaxation.

The ν1 + ν3 combination band of 238UF 6

The ν 1 + ν 3 band of 238UF 6 has been observed at Doppler-limited resolution between 1291.5 and 1297.2 cm −1 in a statically cooled longpath cell at 226 K. The frequencies of 21 transitions belonging to P(22, 28, 30), 14 belonging to R(12, 30, 31), and 28 belonging to the Q branch with 41 ≤ J ≤ 71, were measured with an accuracy of ±0.002 cm −1. In addition, 53 frequency differences were measured in the Q branch between lines having different J assignments from Q(14) to Q(76). Five spectroscopic constants were fitted to these 116 data points with a standard deviation of 0.0006 cm −1. The rotational constants have nearly the values expected from a consideration of ν 1 and ν 3 themselves, and there is little evidence for interaction of ν 1 + ν 3 with other levels.

Line strengths and positions of the submillimeter magnetic dipole transitions of O 2

Strengths and positions of lines between 14 and 160 cm -1 from pure rotational magnetic dipole transitions of the homonuclear molecule O 2 have been measured under carefully controlled conditions over a range of pressures from 52 to 672 torr in a long-path cell using a Fourier transform spectrometer. Correction methods have been devised to alleviate the difficult problem of baseline determination in the calculation of equivalent width for each measured line. These data show excellent agreement with the theory of Tinkham and Strandberg over the above range of measurement. The possible implication of this work to the calibration of balloon-borne measurements of the cosmologically significant cosmic microwave background is also briefly discussed.