Laser Cavity Ring-down Spectroscopy Research Articles

Measurement of acetylene concentration in laminar flat flames by diode laser cavity ring-down spectroscopy

Measurement of acetylene concentration in laminar flat flames by diode laser cavity ring-down spectroscopy

Quantitative measurements of singlet molecular oxygen in a low pressure ICP

We present measurements of the densities and temperatures (rotational and translational) of the metastable a1Δg (v = 0) state of O2 in a cylindrically symmetric RF driven plasma operating in inductive mode at 100 mTorr total pressure and 300 W applied power. Line-of-sight absorption across the plasma region was determined by diode laser cavity ringdown spectroscopy on the (0, 0) vibrational band of the O2(b1Σg +) ← O2(a1Δg) transition near 1.9 μm. Four rotational quantum states were studied, with a population distribution corresponding to a rotational temperature of 346 ± 38 K. The translational temperature was determined to be 359 ± 16 K from the width of the strongest absorption line, Q(12), and in equilibrium with the rotational distribution. The absolute concentration of O2(a1Δg, v = 0) was measured as (9.5 ± 1.3) × 1013 cm−3, and corresponds to an apparent (3.5 ± 0.45)% contribution to the total number density. Time-resolved CRDS measurements following plasma extinction were used to deduce a wall loss coefficient, γ, of (2.8 ± 0.3) × 10−3 on predominantly Al surfaces. Surmising reasonable concentrations for O2(b1Σg +) and an upper limit for the vibrational temperature places the total contribution of O2(a1Δg) at between 3.6% and 5.85%. The variation of the O2(a1Δg, v = 0) state concentration with RF power shows a clear transition from the E to H mode excitation near an applied power of 150 W. Allan variance analysis yields a minimum measurable concentration of O2(a1Δg, v = 0) of 1.1 × 1012 cm−3 over 100 ringdown events, an order of magnitude more sensitive than previously reported.

Impact of Residual Water Vapor on the Simultaneous Measurements of Trace CH4 and N2O in Air with Cavity Ring-Down Spectroscopy

Methane (CH4) and nitrous oxide (N2O) are among the most important atmospheric greenhouse gases. A gas sensor based on a tunable 7.6 μm continuous-wave external-cavity mode-hop-free (EC-MHF) quantum cascade laser (from 1290 to 1350 cm−1) cavity ring-down spectroscopy (CRDS) technique was developed for the simultaneous detection of CH4 and N2O in ambient air with water vapor (H2O) mostly removed via molecular sieve drying to minimize the impact of H2O on the simultaneous measurements. Still, due to the broad and strong absorption spectrum of H2O in the entire mid-infrared (mid-IR) spectral range, residual H2O in the dried ambient air due to incomplete drying and leakage, if not properly accounted for, could cause a significant influence on the measurement accuracy of the simultaneous CH4 and N2O detection. In this paper, the impact of residual H2O on the simultaneous CH4 and N2O measurements were analyzed by comparing the CH4 and N2O concentrations determined from the measured spectrum in the spectral range from 1311 to 1312.1 cm−1 via simultaneous CH4 and N2O measurements and that determined from the measured spectrum in the spectral range from 1311 to 1313 cm−1 via simultaneous CH4, N2O, and H2O measurements. The measured dependence of CH4 and N2O concentration errors on the simultaneously determined H2O concentration indicated that the residual H2O caused an under-estimation of CH4 concentration and over-estimation of N2O concentration. The H2O induced CH4 and N2O concentration errors were approximately linearly proportional to the residual H2O concentration. For the measurement of air flowing at 3 L per min, the residual H2O concentration was stabilized to approximately 14 ppmv, and the corresponding H2O induced errors were −1.3 ppbv for CH4 and 3.7 ppbv for N2O, respectively.

Potential interferences in photolytic nitrogen dioxide converters for ambient air monitoring: Evaluation of a prototype

ABSTRACT Mixing ratios of the criteria air contaminant nitrogen dioxide (NO2) are commonly quantified by reduction to nitric oxide (NO) using a photolytic converter followed by NO-O3 chemiluminescence (CL). In this work, the performance of a photolytic NO2 converter prototype originally designed for continuous emission monitoring and emitting light at 395 nm was evaluated. Mixing ratios of NO2 and NOx (= NO + NO2) entering and exiting the converter were monitored by blue diode laser cavity ring-down spectroscopy (CRDS). The NO2 photolysis frequency was determined by measuring the rate of conversion to NO as a function of converter residence time and found to be 4.2 s−1. A maximum 96% conversion of NO2 to NO over a large dynamic range was achieved at a residence time of (1.5 ± 0.3) s, independent of relative humidity. Interferences from odd nitrogen (NOy) species such as peroxyacyl nitrates (PAN; RC(O)O2NO2), alkyl nitrates (AN; RONO2), nitrous acid (HONO), and nitric acid (HNO3) were evaluated by operating the prototype converter outside its optimum operating range (i.e., at higher pressure and longer residence time) for easier quantification of interferences. Four mechanisms that generate artifacts and interferences were identified as follows: direct photolysis, foremost of HONO at a rate constant of 6% that of NO2; thermal decomposition, primarily of PAN; surface promoted photochemistry; and secondary chemistry in the connecting tubing. These interferences are likely present to a certain degree in all photolytic converters currently in use but are rarely evaluated or reported. Recommendations for improved performance of photolytic converters include operating at lower cell pressure and higher flow rates, thermal management that ideally results in a match of photolysis cell temperature with ambient conditions, and minimization of connecting tubing length. When properly implemented, these interferences can be made negligibly small when measuring NO2 in ambient air. Implications: A new near-UV photolytic converter for measurement of the criteria pollutant nitrogen dioxide (NO2) in ambient air by CL was characterized. Four mechanisms that generate interferences were identified and investigated experimentally: direct photolysis of HONO which occurred at a rate constant 6% that of NO2, thermal decomposition of PAN and N2O5, surface promoted chemistry involving HNO3, and secondary chemistry involving NO in the tubing connecting the converter and CL analyzer. These interferences are predicted to occur in all NO2 P-CL systems but can be avoided by appropriate thermal management and operating at high flow rates.

Effect of N2 and O2 on OH radical production in an atmospheric helium microwave plasma jet

UV-pulsed laser cavity ringdown spectroscopy of the hydroxyl radical OH(A–X) (0–0) band in the wavelength range of 306–310 nm was employed to determine absolute number densities of OH in the atmospheric helium plasma jets generated by a 2.45 GHz microwave plasma source. The effect of the addition of molecular gases N2 and O2 to He plasma jets on OH generation was studied. Optical emission spectroscopy was simultaneously employed to monitor reactive plasma species. Stark broadening of the hydrogen Balmer emission line (Hβ) was used to estimate the electron density ne in the jets. For both He/N2 and He/O2 jets, ne was estimated to be on the order of 1015 cm−3. The effects of plasma power and gas flow rate were also studied. With increase in N2 and O2 flow rates, ne tended to decrease. Gas temperature in the He/O2 plasma jets was elevated compared to the temperatures in the pure He and He/N2 plasma jets. The highest OH densities in the He/N2 and He/O2 plasma jets were determined to be 1.0 × 1016 molecules/cm3 at x = 4 mm (from the jet orifice) and 1.8 × 1016 molecules/cm3 at x = 3 mm, respectively. Electron impact dissociation of water and water ion dissociative recombination were the dominant reaction pathways, respectively, for OH formation within the jet column and in the downstream and far downstream regions. The presence of strong emissions of the bands in both He/N2 and He/O2 plasma jets, as against the absence of the emissions in the Ar plasma jets, suggests that the Penning ionization process is a key reaction channel leading to the formation of in these He plasma jets.

A broadband cavity-enhanced spectrometer for atmospheric trace gas measurements and Rayleigh scattering cross sections in the cyan region (470–540 nm)

Abstract. An incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) instrument for quantification of atmospheric trace gases that absorb in the cyan region of the electromagnetic spectrum (470 to 540 nm), including NO2 and I2, is described. The instrument uses a light-emitting diode coupled to a 1 m optical cavity consisting of a pair of mirrors in stable resonator configuration. Transmitted light is monitored using a grating spectrometer and charge-coupled device array detector. The average mirror reflectivity was determined from the N2∕He and Ar∕He ratios of scattering coefficients and was ∼99.98 % at its maximum, yielding an effective optical path length of 6.3 km. Cross sections of N2, O2, air, Ar, CO2, and CH4 scattering and of O4 absorption were measured and agree with literature values within the measurement uncertainty. Trace gas mixing ratios were retrieved using the spectral fitting software DOASIS (DOAS intelligent system) from 480 to 535 nm. Under laboratory conditions, the 60 s, 1σ measurement precisions were ±124 and ±44 pptv for NO2 and I2, respectively. The IBBCEAS instrument sampled ambient air in Ucluelet, BC, Canada, in July 2015. IBBCEAS retrievals agreed with independent measurements of NO2 by blue diode laser cavity ring-down spectroscopy (r2=0.975), but ambient I2 concentrations were below the detection limit.

Nuclear spin symmetry conservation and relaxation of water (H216O) seeded in supersonic jets of argon and oxygen: measurements by cavity ring-down laser spectroscopy

ABSTRACTWe have investigated nuclear spin symmetry conservation and relaxation of water seeded in argon or molecular oxygen using supersonic jet expansions probed by high-resolution continuous-wave laser cavity ring-down spectroscopy. The probing of the R-branch of the 2 band (above 7500 cm1) was used to complement our previous investigation. We were able to further cool down the water samples (down to 17 K., i.e. 7 K colder than before) and double the data set (regarding argon as carrier gas). We confirm our first observations: at the lowest rotational temperatures and low partial pressures of HO, nuclear spin symmetry is conserved, in agreement with theoretical expectation for inelastic collisions. For high concentrations of water in the gas mixture, we obtained higher rotational temperatures and were able to observe nuclear spin symmetry relaxation. This can be related to the formation of water clusters at the early stage of the supersonic jet expansion. We also present the analogous series of measurements performed using molecular oxygen (O) as carrier gas: we observed similar behaviour for low and high concentrations of water in the gas mixtures when cooling the samples to 16 K. The conservation of nuclear spin symmetry at low temperature and low concentration using oxygen as carrier gas indicates that the paramagnetic collisional partner does not play a significant role regarding the possible nuclear spin symmetry conversion of water. We discuss possible mechanisms related to our observations of apparent nuclear spin symmetry relaxation.

Determination of dissolved methane in natural waters using headspace analysis with cavity ring-down spectroscopy

Methane (CH4) is the third most abundant greenhouse gas (GHG) but is vastly understudied in comparison to carbon dioxide. Sources and sinks to the atmosphere vary considerably in estimation, including sources such as fresh and marine water systems. A new method to determine dissolved methane concentrations in discrete water samples has been evaluated. By analyzing an equilibrated headspace using laser cavity ring-down spectroscopy (CRDS), low nanomolar dissolved methane concentrations can be determined with high reproducibility (i.e., 0.13nM detection limit and typical 4% RSD). While CRDS instruments cost roughly twice that of gas chromatographs (GC) usually used for methane determination, the process presented herein is substantially simpler, faster, and requires fewer materials than GC methods. Typically, 70-mL water samples are equilibrated with an equivalent amount of zero air in plastic syringes. The equilibrated headspace is transferred to a clean, dry syringe and then drawn into a Picarro G2301 CRDS analyzer via the instrument’s pump. We demonstrate that this instrument holds a linear calibration into the sub-ppmv methane concentration range and holds a stable calibration for at least two years. Application of the method to shipboard dissolved methane determination in the northern Gulf of Mexico as well as river water is shown. Concentrations spanning nearly six orders of magnitude have been determined with this method.

Diode laser cavity ring-down spectroscopy for atmospheric NO3 radical measurement

This article presents an external modulation diode laser (with wavelength 661.85 nm and line width 0.3 nm) cavity ring-down spectrometer for measurement of NO3 radical in the atmosphere. The output spectrum of the diode laser is optimized by changing the external modulation signal; the effective cross-section of the instrument is a convolution of the measured cross-section and the laser spectrum. Interference of other gases (O3, NO2, and water vapor)in NO3 radical detection has also been investigated. Considering the loss from the PFA tube wall collision and the membrane filtration, NO3 radical inlet transmission efficiencies of the system is about 70% through the preliminary quantitative analysis. When the time resolution is 7 s, the laboratory detection limit of the system is 2.0 pptv. This instrument was deployed in the night atmosphere to measure the NO3 radical. NO3 concentration varied from 17.9 to 51.7 pptv with an average level of 36.3 pptv. A typical detection sensitivity in the night atmosphere is 3.5 pptv. Because of the uncertainty in the inlet transmission efficiencies and other factors, the total uncertainty of the measured NO3 radicals is about 8%(1 ). Experimental results show that the diode laser cavity ring-down spectroscopy can achieve high sensitivity in insitu detection of NO3 radicals in the atmosphere.

Detection of chemical warfare agents based on quantum cascade laser cavity ring-down spectroscopy

Chemical warfare agents (CWAs) are recognized as serious threats of terrorist acts against the civilian population. Minimizing the impact of these threats requires early detection of the presence of CWAs. Cavity ring-down spectroscopy (CRDS) is an exquisitely sensitive technique for the detection of trace gaseous species. In this letter, the CRDS technique is employed using a pulsed quantum cascade laser for the detection of dimethyl methylphosphonate (DMMP). A limit of DMMP detection of approximately 77 ppb is achieved. The best achievable sensitivity that corresponds to noise-equivalent absorption is approximately 2 x 10(-7) cm(-1).

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.

Challenges in measuring the δ13C of the soil surface CO2 efflux

The δ13C of the soil surface efflux of carbon dioxide (δ13CRS) has emerged as a powerful tool enabling investigation of a wide range of soil processes from characterising entire ecosystem respiration to detailed compound-specific isotope studies. δ13CRS can be used to trace assimilated carbon transfer below ground and to partition the overall surface efflux into heterotrophic and autotrophic components. Despite this wide range of applications no consensus currently exists on the most appropriate method of sampling this surface efflux of CO2 in order to measure δ13CRS. Here we consider and compare the methods which have been used, and examine the pitfalls. We also consider a number of analysis options, isotope ratio mass spectrometry (IRMS), tuneable diode laser spectroscopy (TDLS) and cavity ring-down laser spectroscopy (CRDS). δ13CRS is typically measured using chamber systems, which fall into three types: closed, open and dynamic. All are imperfect. Closed chambers often rely on Keeling plots to estimate δ13CRS, which may not be appropriate without free turbulent air mixing. Open chambers have the advantage of being able to maintain steady-state conditions but analytical errors may become limiting with low efflux rates. Dynamic chambers like open chambers are complex, and controlling pressure fluctuations caused by air movement is a key concern. Both open and dynamic chambers in conjunction with field portable TDLS and CRDS analysis systems have opened up the possibility of measuring δ13CRS in real time permitting new research opportunities and are on balance the most suited to this type of measurement.

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.

A study of OH radicals in an atmospheric AC discharge plasma using near infrared diode laser cavity ringdown spectroscopy combined with optical emission spectroscopy

Simultaneous measurements of absolute concentrations of H2O and OH radicals in an atmospheric AC discharge using continuous wave cavity ringdown spectroscopy (cw-CRDS) are reported. Formation of OH radicals and plasma temperatures are characterized by optical emission spectroscopy. The concentration of OH radical at the edge of the discharge plume at 380 K is measured by the cw-CRDS technique to be 1.1 ×1015 molecule cm-3. Ringdown measurements of the H2O (120-000) band and the OH first overtone around 1515 nm enable us to determine an OH generation yield, \(\Phi =\frac{N_{\rm OH} }{N_{\rm H_2 O}}\), to be 4.8 ×10-3, where NOH and \( N_{H_2 O} \) are the number densities of OH and H2O, respectively. The minimum detectable absorption coefficient of the cw-CRDS system is 8.9 ×10-9 cm-1, which corresponds to a 1σ detection limit of OH number density of 1.2 ×1013 molecule cm-3 in the discharge. This experimental approach is demonstrated for the first time ever in an AC discharge, and can be applied in general to a variety of atmospheric plasmas to help study OH formation mechanisms and OH-related plasma applications.

High-Resolution Continuous-Wave-Diode Laser Cavity Ring-Down Spectroscopy of the Hydrogen Fluoride Dimer in a Pulsed Slit Jet Expansion: Two Components of the N = 2 Triad near 1.3 μm

The near-infrared overtone spectra of the prototypical hydrogen-bond hydrogen fluoride dimer (HF)2 contain rich information on hydrogen bond dynamics. We report a study of the N=2 triad involving excitations with two quanta of HF stretching in (HF)2 around 1.3 microm (7500-7800 cm(-1)) by means of continuous-wave-diode laser cavity ring-down spectroscopy in a pulsed supersonic slit jet expansion. The analysis of the rotationally resolved overtone spectra allows the study of vibrational mode-selective kinetics, such as hydrogen bond predissociation with lifetimes tauPD and tunneling rearrangement (switching) processes with periods tausw obtained from the tunneling splitting DeltaT in highly excited vibrational states. The Ka=1<--0 transition of the Nj=22 band of (HF)2 has been reinvestigated by us in a supersonic jet expansion; the much improved data obtained here are in excellent agreement with several previous experimental results. Our analysis provides subband-level positions and properties 0(Ka=1(A+))=7711.37956(66) cm(-1), DeltaT=0.0936(10) cm(-1), and tauPD=1.3-1.9x10(-9) s, depending on the level symmetry A+ and B+. We have also analyzed spectra of the Nj=21 band, which we have observed for the first time in a supersonic jet with rotational resolution. For the Ka=0<--0 transition of this band, we find the band center at 0(A+)=7550.3555(26) cm(-1) and a tunneling splitting of DeltaT=0.0150(37) cm(-1). This level involves mostly excitation of the H-bonded HF stretching with two quanta. The mode-selective tunneling switching is in agreement with a simple picture of inhibited tunneling. These experimental values are close to those calculated on the "SO(-3)" potential energy hypersurface of Klopper, Quack, and Suhm. The N=2 triad also exhibits a strongly mode-selective predissociation dynamics, with a predissociation lifetime tauPD=4.99(84)x10(-11) s in the Nj=21 level, which is more than 20 times shorter than that for the Nj=22 level.

Measurements of the 3ν 3 band of 14N15N16O and 15N14N16O using continuous-wave cavity ring-down spectroscopy

The absorption spectra of the 3ν3 band of nitrous oxide isotopologues, 14N15N16O and 15N14N16O, have been measured using diode laser cavity ring-down spectroscopy in 6400–6463 and 6465–6532 cm-1, respectively. Spectroscopic parameters and the rotational line intensities of the bands have been determined. We have applied this spectroscopic technique to the measurements of the absolute isotope ratio of those isotopologues using the absolute line intensities.

Intracavity laser absorption spectroscopy and cavity ring-down spectroscopy in low-pressure flames

Intracavity laser absorption spectroscopy (ICLAS) and cavity ring-down spectroscopy (CRDS) have been used for measurement of the NH2-radical spectrum near 643 nm. NH2 was obtained in low-pressure methane/air flat flames doped with minor amounts of ammonia (as low as 0.023%). The NH2 concentration was measured both by CRDS and ICLAS in the same conditions. This enables us to compare the practical sensitivity of the two methods. Both methods were also used for measurements in a sooting acetylene/air flame (ϕ = 2.6). The comparative advantages of the methods and their complementarities are discussed.

Absorption cross-sections and pressure broadening of rotational lines in the ν5+ ν9band of ethene measured by diode laser cavity ring down spectroscopy

Absorption cross sections are reported for several rotationally resolved lines in the QQ-branch of the ν5 + ν9 combination vibrational band of ethene (ethylene) at wavelengths around 1.625 μm. The measurements were made using cavity ring-down spectroscopy with a near infra-red diode laser. In the strongest region of the band, a line-integrated absorption cross section for the QQ4(4) feature of 5.08 ± 0.38 × 10−22 cm2 molecule−1 cm−1 is obtained, with a peak cross section at line centre of 3.23 ± 0.03 × 10−20 cm2 molecule−1. The effects of addition of pressures of up to 200 Torr of air on spectral linewidths are analysed to obtain an air broadening coefficient (HWHM) of γ = 0.1060 ± 0.0011 cm−1 atm−1. In 1 atm of air, the line centre absorption cross sections in the QQ4 branch are reduced by a factor of ∼8 because of line broadening. The consequences of these data for direct monitoring of ethene in the atmosphere using sensors based on diode laser cavity ring-down spectroscopy are discussed.

Absorption cross-sections and pressure broadening of rotational lines in the 3 ν3 band of N 2O determined by diode laser cavity ring-down spectroscopy

Diode laser cavity ring-down spectroscopy (CRDS) has been used to measure rotational line intensities and air pressure broadening coefficients for the 3 ν 3 (N–N stretching) vibrational overtone band of N 2O. The line intensity measurements are in quantitative agreement with previously reported Fourier transform spectroscopy results [J. Mol. Spectrosc. 197 (1999) 158]. The air pressure broadening coefficients increase from 0.086 ± 0.015 cm −1 atm −1 for the P(34) line to 0.139 ± 0.010 cm −1 atm −1 for the P(3) line, and are greater than values reported for other ro-vibrational bands of N 2O observed at lower frequencies. The effects of pressure broadening on line-centre absorption cross-sections, and thus for the sensitivity of CRDS detection of N 2O using narrow bandwidth near-IR lasers are quantified.

Number density and temperature of acetylene in hot-filament and arc-jet activated CH 4/H 2 gas mixtures measured using diode laser cavity ring-down absorption spectroscopy

Continuous-wave cavity ring-down spectroscopy (CW-CRDS) with an external cavity diode laser has been used to make diagnostic measurements of acetylene absorption in hot-filament and DC-arc plasma jet reactors operating with 1% CH 4 in H 2 and ≤0.8% CH 4/H 2 in excess Ar, respectively, and used for diamond chemical vapour deposition. The acetylene was widely distributed throughout the reactor, extending well beyond the regions of highest thermal activation as a result of diffusion processes. The average acetylene concentration was determined as (2.9±0.6)×10 13 molecules cm −3 in the hot filament reactor and (1.2±0.2)×10 14 molecules cm −3 in the DC arc jet reactor, accounting for ∼1.7% and 40% of the total C budget in the respective cases. The average acetylene gas temperature along the viewing column was 550±150 K in both environments. Variations in the concentration and temperature of acetylene with the gas feed composition and input power processing conditions for the two reactors are presented. The experiments demonstrate the effectiveness of near-IR diode laser CRDS as a probe of highly luminous environments.