Pulsed Cavity Ring-down Spectroscopy Research Articles

Pulsed-cavity ring down spectroscopic study of NO2 in 501–506 nm spectral region

Cavity ring-down spectroscopy (CRDS) is a highly sensitive laser absorption spectroscopic technique. In this paper, we report the pulsed CRDS study of nitrogen dioxide (NO 2 ) using an home-built CRDS set-up. Pulsed CRDS measurements of diluted NO 2 gas (0.091% in N 2 ) at various low pressures have been carried out in 501–506 nm spectral range. From the CRDS measurements, molecular spectral features of NO 2 at very low partial pressure, ∼10 mTorr (3.42 × 10 14 molecules/cm 3 ), was evidently observed. CRDS ring-down decay measurements carried out at wavelength of 504.76 nm for various molecular concentrations of NO 2 demonstrate a minimum detectable number density of NO 2 of 8.65 × 10 12 molecules/cm 3 , which would correspond to 350 ppbv under atmospheric condition. 40 ppbv of detection limit of NO 2 is estimated from the standard deviation (1σ) of ring-down decay time measurements at 504.76 nm wavelength. Absorption and integrated absorption cross section of NO 2 are also estimated.

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.

High-resolution laser spectroscopy of the linear SiC2Si molecule and its astrophysical implications

Small silicon-carbon clusters are important gas-phase constituents of stellar atmospheres, and are thought to play a role as potential seeds of the interstellar dusts formed in the envelopes of evolved carbon stars. Here we present the high-resolution optical spectra of the linear SiC2Si molecule (l-Si2C2) studied via laboratory experiments. The l-Si2C2 molecules are generated in a supersonically expanding planar plasma by discharging a silane-acetylene-argon gas mixture. The optical absorption spectra in the 5000−5300 Å region are recorded using sensitive pulsed cavity ring-down spectroscopy. In total, five optical absorption bands belonging to the $ \tilde{C} ^{3}\Sigma_{u} ^{-} $ – $ \tilde{X} ^{3}\Sigma_{g} ^{-} $ electronic transition system of l-Si2C2 are measured with fully resolved spin splitting fine structures in individual rotational transitions. Accurate spectroscopic constants for both $ \tilde{X} ^{3}\Sigma_{g} ^{-} $ and $ \tilde{C} ^{3}\Sigma_{u} ^{-} $ states of l-Si2C2, including the spin-spin interaction constants and spin-rotation interaction constants, are determined from the experimental spectra, which can be used to simulate these optical bands with different temperatures. Using the determined spectroscopic constants, optical spectra of l-Si2C2 simulated with different rotational excitation temperatures are compared to the stellar spectra of evolved carbon stars V Hya and IRAS 12311−23509, where the triatomic SiC2 are known to be abundant. Tentative assignments of the l-Si2C2 spectral features in the stellar spectra are discussed.

A widely-tunable and sensitive optical sensor for multi-species detection in the mid-IR

Pulsed cavity ringdown spectroscopy (CRDS) technique was used to develop a novel widely-tunable laser-based sensor for sensitive measurements of ethylene, propene, 1-butene and allene in the mid-IR. The use of an external-cavity quantum cascade laser (EC-QCL) enabled the sensor to cover a wide wavelength range from 10 to 11.1 µm (900–1000 cm−1) to detect multiple gases relevant to combustion and environment. The sensor operation was validated in a room-temperature static cell using well-characterized absorption lines of carbon dioxide near 938.69 cm−1 and 974.62 cm−1. Detection limits for ethylene, propene, 1-butene, and allene were measured to be 17, 134, 754 and 378 ppb, respectively, at 296 K and 760 Torr for a single-pass path-length of 70 cm with averaging time of 4 ms. The excellent sensitivity of the optical sensor enabled it to measure the aforementioned gases at levels smaller than 1% of their recommended exposure limits. To the best of our knowledge, this is one of the first successful applications of the pulsed CRDS technique to measure trace levels of multiple gases in the 10–11 µm wavelength region.

Time-resolved measurements of NO2 concentration in pulsed discharges by high-sensitivity cavity ring-down spectroscopy

To describe the complex kinetics of formation and destruction mechanism of nitrogen dioxide (NO2), there is an increasing demand for real-time and in situ analysis of NO2 in the discharge region. Pulsed cavity ring-down spectroscopy (CRDS) provides an excellent diagnostic approach. In the present paper, CRDS has been applied in situ for time evolution measurement of NO2 concentration which is rarely investigated in gas discharges. In pulsed direct current discharge of NO2/Ar mixture at a pressure of 500 Pa, a peak voltage of −1300 V and a frequency of 30 Hz, for higher initial NO2 concentration (3.05 × 1014 cm−3, 8.88 × 1013 cm−3), the NO2 concentration sharply decreases at the beginning of the discharge afterglow and then becomes almost constant, and the pace of decline increases with pulse duration; however, for lower initial NO2 concentration of 1.69 × 1013 cm−3, the NO2 concentration also decreases at the beginning of the discharge afterglow for 200 ns and 1 μs pulse durations, while it slightly increases and then declines for 2 μs pulse duration. Thus, the removal of low-level NO2 could not be promoted by a higher mean energy input.

Characterization of single airborne particle extinction using the tunable optical trap-cavity ringdown spectroscopy (OT-CRDS) in the UV.

We integrated a rigid optical trap into a tunable pulsed cavity ringdown spectroscopy (OT-CRDS) system to characterize the extinction of single airborne particles in the UV spectral region (306-315 nm). Single solid particles from a multi-walled carbon nanotube (MWCNT), Bermuda grass smut spore, carbon microsphere, and blackened polyethylene microsphere were trapped in air based on the photophoretic force. The improved OT-CRDS system was highly sensitive and able to resolve extinctions of single particles from different materials and sizes at a given wavelength. Further, we successfully manipulated the number of particles, e.g., 1, 2 or more particles, in the trap and measured their distinguishable extinctions using the OT-CRDS. We also show that the particle size and extinction have a good linear correlation from the measurements of 24 single MWCNT particles. Material- and wavelength-dependent extinctions of the four types of airborne particles were also characterized. Results reveal that single airborne particles regardless of their differences in material and size, due to their heterogeneous morphology, have individual-particle dependent extinctions and that dependence can be resolved and characterized using the OT-CRDS technique.

光腔衰荡光谱技术用于OH浓度测量的误差分析

Cavity ring down spectroscopy(CRDS) is an absorption spectroscopy technique with high sensitivity, which is a very important quantitative concentration measurement approach in combustion spectral diagnosis. The principle analysis of CRDS was presented to apply to absolute concentrations of intermediate species in flames, and then an experimental setup of pulsed CRDS was established based on this principle. The absolute OH concentration was measured by using absorption on the P1(2) rotational line in the A2+-X2(0,0) band of OH on a plat flame at atmospheric pressure. Influence of several major factors on the measurement uncertainty was analyzed, such as laser line shape and linewidth, frequency stability of the laser, flame temperature, and cavity parameters. This uncertainty analysis provides several guidelines to improve the measurement accuracy of CRDS.

光腔衰荡光谱技术用于OH浓度测量的误差分析

Cavity ring down spectroscopy(CRDS) is an absorption spectroscopy technique with high sensitivity, which is a very important quantitative concentration measurement approach in combustion spectral diagnosis. The principle analysis of CRDS was presented to apply to absolute concentrations of intermediate species in flames, and then an experimental setup of pulsed CRDS was established based on this principle. The absolute OH concentration was measured by using absorption on the P1(2) rotational line in the A2+-X2(0,0) band of OH on a plat flame at atmospheric pressure. Influence of several major factors on the measurement uncertainty was analyzed, such as laser line shape and linewidth, frequency stability of the laser, flame temperature, and cavity parameters. This uncertainty analysis provides several guidelines to improve the measurement accuracy of CRDS.

Pulsed cavity ring-down spectrometer at 3 µm based on difference frequency generation for high-sensitivity CH4 detection

We present a mid-infrared pulsed cavity ring-down spectroscopy (CRDS) setup based on difference frequency mixing of a tunable dye laser pumped by a Nd:YAG laser and the residual 1064 nm pulse in a temperature-stabilized lithium niobate (LiNbO3) crystal. The performances of such a spectrometer have been investigated by studying the absorption spectrum of methane around 3 µm at room temperature. Our results show a minimum detectable absorption coefficient of 5 × 10−8 cm−1. Our approach combines the excellent sensitivity of CRDS with the wide tunable range of difference frequency generation (DFG) with a dye laser. However, the relatively low resolution of this spectrometer prevents the quantification of methane in a wide pressure range. To our knowledge, we report here the first realization and application of a pulsed DFG-based CRDS experiment used for sensitive trace gas detection.

Cavity ring-down spectroscopy for atmospheric pressure plasma jet analysis

Pulsed cavity ring-down spectroscopy (CRDS) is a direct absorption, highly sensitive, versatile technique suitable for the analysis of a wide range of plasmas. CRDS belongs to a wide class of cavity-enhanced spectroscopies. This work briefly describes the basic principles of CRDS and summarizes the main literature specifically related to applications for atmospheric pressure plasma jets (APPJ). Emphasis is given to the temporal resolution and pulsed character of the plasma sources. As an example we present in more detail the determination of metastable helium density in the state produced in a single-electrode atmospheric pressure plasma jet driven by a pulsed high-voltage waveform. Measured He() number densities ranged from near the capillary orifice to at a distance 3.5 mm downstream.

Sensitive and ultra-fast species detection using pulsed cavity ringdown spectroscopy.

Pulsed cavity ringdown spectroscopy (CRDS) is used to develop a novel, ultra-fast, high-sensitivity diagnostic for measuring species concentrations in shock tube experiments. The diagnostic is demonstrated by monitoring trace concentrations of ethylene in the mid-IR region near 949.47 cm⁻¹. Each ringdown measurement is completed in less than 1 µs and the time period between successive pulses is 10 µs. The high sensitivity diagnostic has a noise-equivalent detection limit of 1.08 x 10⁻⁵ cm⁻¹ which enables detection of 15 ppm ethylene at fuel pyrolysis conditions (1845 K and 2 bar) and 294 ppb ethylene under ambient conditions (297 K and 1 bar). To our knowledge, this is the first successful application of the cavity ringdown method to the measurement of species time-histories in a shock tube.

Eddy covariance flux measurements of gaseous elemental mercury using cavity ring-down spectroscopy.

A newly developed pulsed cavity ring-down spectroscopy (CRDS) system for measuring atmospheric gaseous elemental mercury (GEM) concentrations at high temporal resolution (25 Hz) was used to successfully conduct the first eddy covariance (EC) flux measurements of GEM. GEM is the main gaseous atmospheric form, and quantification of bidirectional exchange between the Earth's surface and the atmosphere is important because gas exchange is important on a global scale. For example, surface GEM emissions from natural sources, legacy emissions, and re-emission of previously deposited anthropogenic pollution may exceed direct primary anthropogenic emissions. Using the EC technique for flux measurements requires subsecond measurements, which so far has not been feasible because of the slow time response of available instrumentation. The CRDS system measured GEM fluxes, which were compared to fluxes measured with the modified Bowen ratio (MBR) and a dynamic flux chamber (DFC). Measurements took place near Reno, NV, in September and October 2012 encompassing natural, low-mercury (Hg) background soils and Hg-enriched soils. During nine days of measurements with deployment of Hg-enriched soil in boxes within 60 m upwind of the EC tower, the covariance of GEM concentration and vertical wind speed was measured, showing that EC fluxes over an Hg-enriched area were detectable. During three separate days of flux measurements over background soils (without Hg-enriched soils), no covariance was detected, indicating fluxes below the detection limit. When fluxes were measurable, they strongly correlated with wind direction; the highest fluxes occurred when winds originated from the Hg-enriched area. Comparisons among the three methods showed good agreement in direction (e.g., emission or deposition) and magnitude, especially when measured fluxes originated within the Hg-enriched soil area. EC fluxes averaged 849 ng m(-2) h(-1), compared to DFC fluxes of 1105 ng m(-2) h(-1) and MBR fluxes of 1309 ng m(-2) h(-1). This study demonstrated that a CRDS system can be used to measure GEM fluxes over Hg-enriched areas, with a conservative detection limit estimate of 32 ng m(-2) h(-1).

Comparative Study on Microwave Plasma-Assisted Combustion of Premixed and Nonpremixed Methane/Air Mixtures

Employing a novel microwave plasma-assisted combustion (PAC) facility, we conducted a comparative study on PAC of premixed and nonpremixed methane/air mixtures. Results showed that three different reaction zones (plasma zone, hybrid plasma-flame zone, and flame zone) exist, regardless of the fuel injection patterns of the premixed or nonpremixed. Different fuel injection patterns had different optical emission spectra for the hybrid zones and flame zones, with a major difference being the presence or absence of emissions from CH and C2. A U-shaped ignition curve of plasma power versus fuel equivalence ratio was observed in the premixed PAC, whereas an approximately linear increasing line was in the nonpremixed PAC. Pulsed cavity ringdown spectroscopy was also utilized to measure absolute number densities of the ground state OH(X) radicals in the flame zones for both fuel patterns. Results showed different number density profiles along the flame propagation direction between the premixed and nonpremixed PAC.

Roles of the state-resolved OH(A) and OH(X) radicals in microwave plasma assisted combustion of premixed methane/air: An exploratory study

We report a novel microwave plasma-assisted combustion (PAC) system that is developed as a new test platform to study roles of plasma in PAC. The system included two major components, an atmospheric pressure microwave plasma cavity and a cross-shape quartz combustor. This new PAC system allows one to study PAC using complicated yet well-controlled combinations of operating parameters, such as fuel equivalence ratio (ϕ), fuel mixture flow rate, plasma gas flow rate, plasma gases, symmetric or asymmetric fuel-oxidant injection patterns, with and without plasma. In this work, ignitions at the fuel (lean and rich) flammability limits at different plasma powers and fuel flow rates were investigated. The ignition curves of plasma power versus ϕFL at the different flow rates revealed a stretched U-shape, showing clear evidences of the plasma enhancement effects on ignition and flame stabilization, i.e. the fuel lean flammability limit (ϕLFL) was extended to ϕ=0.2, as compared to ϕ=0.6 at the same combustion parameters except with no plasma. Optical emission spectroscopy (OES) showed that the combustor had three distinct reaction zones: plasma zone, hybrid plasma-flame zone, and flame zone; and each of the reaction zones was well defined by its OES features. Furthermore, a detailed survey of OES of OH (A–X) conducted along the plasma jet axis (x direction) with a spatial resolution of 0.5mm revealed that OH(A) had a double-peak feature in its relative emission intensity curve (I∼x) in the hybrid zone where plasma-assisted ignition (PAI) started, as evidenced by a significant surge of OH(A) and by a large increase in OH rotational temperature, i.e. from 1450K to 2400K. Moving from the hybrid zone to the flame zone, OH(A) decreased by more than four orders of magnitude. However, the electronic ground state OH(X) measured simultaneously using pulsed cavity ringdown spectroscopy around 308nm showed that absolute number density of the OH(X) decreased by smaller than a factor of ten from the downstream of the hybrid zone to the flame zone. The different changing rates of the OH(A) and OH(X) radicals from the hybrid zone to the flame zone allow us to propose a hypothesis that if both the electronically excited state OH(A) and the electronic ground state OH(X) assisted the ignition and flame stabilization processes, the role of OH(X) radicals was more dominant in the flame stabilization but the role of OH(A) radicals was more dominant in the ignition enhancement.

High-Sensitivity In-Situ Diagnosis of NO2 Production and Removal in DBD Using Cavity Ring-Down Spectroscopy

A highly-sensitive in-situ diagnosis approach for nitrogen dioxide (NO2) has been developed in dielectric barrier discharge (DBD) based on pulsed cavity ring-down spectroscopy (CRDS). Absorption bands of NO2 in a spectral region from 508 nm to 509 nm were used, and a detection limit of 17.5 ppb was achieved. At this level of sensitivity, the quantitative and real-time monitoring of the production and removal of NO2 are accomplished for the first time in the discharge region. By measuring the removal amount and rate at different NO2 initial number densities from 1.54 × 1013 cm−3 to 2.79 × 1014 cm−3, we determined the relationship between them and NO2 initial number densities. The removal amount linearly increases with the initial number density, while the removal rate increases logarithmically. At a lower initial number density, the removal rate is limited. By considering the chemical kinetic mechanism in plasma, a qualitative explanation for the above phenomena is proposed: the additional NO2 produced by discharge limits the removal rate, since the NO2 concentration is dominated by the competition between the forward reactions (production) and the reverse reactions (removal).

Simultaneous measurements of OH(A) and OH(X) radicals in microwave plasma jet-assisted combustion of methane/air mixtures around the lean-burn limit using optical emission spectroscopy and cavity ringdown spectroscopy

We report a new plasma-assisted combustion system, in which a continuous atmospheric argon microwave plasma jet is employed to enhance combustion of methane/air mixtures in different fuel equivalence ratios (φ) ranging from 0.35 to 1.5. The combustor has three distinct reaction zones along the jet axis (the combustion flame direction): the pure plasma zone, the hybrid plasma-flame zone and the combustion flame zone. Each of the three zones is clearly defined by its emission spectral fingerprints. The plasma zone was featured by strong emissions from OH and NH electronic bands and atomic lines of Ar, Hα and Hβ. In the hybrid zone where the plasma jet met fuel mixtures, emission spectra were dominated by OH, NH and CN transitions and by weak or no atomic transitions. In the combustion flame zone, only weak OH emissions were observed. Simulations of optical emission spectroscopy (OES) yielded gas kinetic temperatures to be 1175 ± 50 K, 1450 ± 50 K and 1865 ± 50 K in each of the three zones, respectively. The plasma-enhancement effect was investigated by comparing the lean-burn limits of the combustion with and without plasma. At the same fuel mixture flow rate of 1.0 standard litre per minute and plasma power of 100 W, the lean-burn limit in terms of the fuel equivalence ratio φ was extended from 0.72 without assistance of the plasma to 0.35 with assistance of the plasma. In addition to OES that was employed to characterize the excited state species including OH(A) in the three different zones, pulsed cavity ringdown spectroscopy was utilized to measure absolute number densities of the ground state OH(X) using the OH A–X (0–0) R2 (1) line in different locations in the flame zone at φ = 0.51, 0.87, 1.10 and 1.45. For rich and lean combustions, significantly different OH(X) number densities and density profiles in the flame zone were observed. At φ = 0.51, the OH(X, V″ = 0, J″ = 0.5) number density increased from 2.29 × 1015 molecule cm−3 at the combustor nozzle to the maximum, 3.13 × 1015 molecule cm−3 at 2 mm downstream, and to the lowest detectable level of 0.12 × 1015 molecule cm−3 in the far downstream where optical emissions were too weak to be detected. Results from the simultaneous measurements of the electronically excited state OH(A) and the ground state OH(X) allow us to discuss the roles of OH(A) and OH(X) in the plasma-assisted ignition and the flame stabilization, respectively.

Cavity ring-down spectroscopy sensor development for high-time-resolution measurements of gaseous elemental mercury in ambient air

Abstract. We describe further development of a previous laboratory prototype pulsed cavity ring-down spectroscopy (CRDS) sensor into a field-deployable system for high-time-resolution, continuous, and automated measurement of gaseous elemental mercury (GEM) concentrations in ambient air. We employed an external, isotopically enriched Hg cell for automated locking and stabilization of the laser wavelength on the GEM peak absorption during measurements. Further, we describe implementation of differential absorption measurements via a piezoelectric tuning element for pulse-by-pulse tuning of the laser wavelength onto and off of the GEM absorption line. This allowed us to continuously correct (at 25 Hz) for system baseline extinction losses unrelated to GEM absorption. Extensive measurement and calibration data obtained with the system were based on spike addition in both GEM-free air and ambient air. Challenges and interferences that occurred during measurements (particularly in ambient air) are discussed including temperature and ozone (O3) concentration fluctuations, and steps taken to reduce these. CRDS data were highly linear (r2 ≥ 0.98) with data from a commercial Tekran 2537 Hg analyzer across a wide range of GEM concentrations (0 to 127 ng m−3) in Hg-free and ambient air. Measurements during periods of stable background GEM concentrations provided a conservative instrument sensitivity estimate of 0.35 ng m−3 for the CRDS system when time averaged for 5 min. This sensitivity, along with concentration patterns observed in ambient air (with the CRDS system and verified with the Tekran analyzer), showed that the sensor was capable of characterizing GEM fluctuations in ambient air. The value of fast-response GEM measurements was shown by a series of GEM spike additions – highlighting that high-temporal-resolution measurement allowed for detailed characterization of fast concentration fluctuations not possible with traditional analyzers.

Real-time measurement of nitrogen dioxide in vehicle exhaust gas by mid-infrared cavity ring-down spectroscopy

The application of pulsed cavity ring-down spectroscopy (CRDS) was demonstrated for the measurement of nitrogen dioxide (NO2) in automotive exhaust gas. The transition of the ν 3 vibrational band assigned to the antisymmetric stretching mode of NO2 was probed with a thermoelectrically cooled, pulsed, mid-infrared, distributed feedback, quantum cascade laser (QCL) at 6.13 μm. The measurement of NO2 in the exhaust gas from two diesel vehicles equipped with different aftertreatment devices was demonstrated using a CRDS-based NO2 sensor, which employs a HEPA filter and a membrane gas dryer to remove interference from water as well as particulates in the exhaust gas. Stable and sensitive measurement of NO2 in the exhaust gas was achieved for more than 30 minutes with a time resolution of 1 s.

The effect of collisions with nitrogen on absorption by oxygen in the A-band using cavity ring-down spectroscopy.

This paper reports on the effect of collisions between nitrogen and oxygen on absorption in the A-band near 760 nm under atmospheric conditions relevant for satellite retrieval studies. We use pulsed laser cavity ring-down spectroscopy with a narrow bandwidth laser and use pressure scans to increase the accuracy of the measured oxygen extinction coefficients. We use the so-called Adjustable Branch Coupling model to describe line mixing in the magnetic allowed A-band dipole absorption and we retrieve the collision induced absorption spectrum due to N2–O2 collisions.

Line mixing and collision induced absorption in the oxygen A-band using cavity ring-down spectroscopy

This paper reports on the absorption of molecular oxygen in the region of the A-band near 760 nm under atmospheric conditions relevant for satellite retrieval studies. We use pulsed laser cavity ring-down spectroscopy with a narrow bandwidth laser and use pressure scans to increase the accuracy of the measured oxygen extinction coefficients. Absolute binary absorption coefficients in minima between absorption lines of the A-band spectrum have been measured and tabulated. We use the so-called adjustable branch coupling model including line mixing to calculate the magnetic dipole absorption in order to determine the contribution of collision induced absorption. The line mixing model has been optimized such that the collision induced absorption spectrum is smooth.