CW Cavity Ring-down Spectroscopy Research Articles

Kinetics of CN (v = 1) reactions with butadiene isomers at low temperature by cw-cavity ring-down in a pulsed Laval flow with theoretical modelling of rates and entrance channel branching.

We present an experimental and theoretical investigation of the reaction of vibrationally excited CN (v = 1) with isomers of butadiene at low temperature. The experiments were conducted using the newly built apparatus, UF-CRDS, which couples near-infrared cw-cavity ring-down spectroscopy with a pulsed Laval flow. The well-matched hydrodynamic time and long ring-down time decays allow measurement of the kinetics of the reactions within a single trace of a ring-down decay, termed Simultaneous Kinetics and Ring-down (SKaR). The pulsed experiments were carried out using a Laval nozzle designed for the 70 K uniform flow with nitrogen as the carrier gas. The measured bimolecular rates for the reactions of CN (v = 1) with 1,3-butadiene and 1,2-butadiene are (3.96 ± 0.28) × 10-10 and (3.06 ± 0.35) × 10-10 cm3 per molecule per s, respectively. The reaction rate measured for CN (v = 1) with the 1,3-butadiene isomer is in good agreement with the rate previously reported for the reaction with ground state CN (v = 0) under similar conditions. We report the rate of the reaction of CN (v = 1) with the 1,2-butadiene isomer here for the first time. The experimental results were interpreted with the aid of variable reaction-coordinate transition-state theory calculations to determine rates and branching of the addition channels based on a high-level multireference treatment of the potential energy surface. H-abstraction reaction rates were also theoretically determined. For the 1,2-butadiene system, theoretical estimates are then combined with literature values for the energy-dependent product yields from the initial adducts to predict overall temperature-dependent product branching. H loss giving 2-cyano-1,3-butadiene + H is the main product channel, exclusive of abstraction, at all energies, but methyl loss forming 1-cyano-prop-3-yne is 15% at low temperature growing to 35% at 500 K. Abstraction forming HCN and various radicals is important at 500 K and above. The astrochemical implications of these results are discussed.

Exploring low temperature oxidation of iso-octane under atmospheric pressure

Iso-octane is the dominant component of primary reference fuel (PRF), with high knock resistance during the combustion process. Understanding the low temperature oxidation chemistry of iso-octane aids in clarifying the auto-ignition properties of gasoline surrogates and developing their kinetic models. However, its low temperature oxidation performance has only been observed under high pressure conditions; no report in the literature includes detailed speciation, hindering a comprehensive examination of the iso-octane low temperature oxidation mechanism. In this work, low temperature oxidation behavior of iso-octane was clearly observed in jet-stirred reactors (JSRs) under an initial fuel mole fraction of 0.01, residence time of 2 s, equivalence ratio of 0.25, and pressure of 1 bar. The low temperature oxidation reaction networks of iso-octane were explored by measuring low temperature oxidation intermediates in three experimental approaches: synchrotron vacuum-ultraviolet photoionization mass spectrometry (SVUV-PIMS), gas chromatography (GC), and cw-cavity ring-down spectroscopy (cw-CRDS). More than 80 intermediates were qualitatively analyzed in combination with the photoionization efficiency (PIE) spectrum, quantum chemistry calculation of the ionization energies, and GC analysis. Hydroperoxides derived from iso-octane low temperature oxidation were reported experimentally for the first time, including keto-hydroperoxides (KHP) and/or hydroperoxyl cyclic ethers (HPCE). Approximately 40 products were quantified, including aldehydes, ketones, olefins, and carboxylic acids, etc. Kinetic models in the literature have been examined using the datasets obtained from this work and reaction pathways for the important intermediates discussed. Comparison indicated that the iso-octane models in the literature must be improved to accurately capture the performance of iso-octane low temperature oxidation.

Multicomponent gas detection based on concise CW-cavity ring-down spectroscopy with a bow-tie design.

Concise open-path continuous-wave cavity ring-down spectroscopy (CW-CRDS) with a bow-tie cavity structure is demonstrated in the single- and dual-optical-path experiments for multicomponent gas detection, e.g., greenhouse gas concentration evaluation in ambient air. Owing to its features of optical feedback suppression and small free spectral range (FSR), the bow-tie configuration shows its special advantages in the realization of both a compact arrangement and two counter-propagating non-interference optical paths. The minimum of the Allan deviation reaches 1.6×10-10 cm-1 for an integration time of 100s, corresponding to the noise equivalent absorption coefficient of 1.6×10-9 cm-1 Hz-1/2. The detection sensitivity of methane is deduced to be 0.9 ppbv with its absorption cross section of 1.48×10-20 cm2/molecule in the 512 decays averaging mode. A wavelength-correction method is proposed to reduce by about 30% the uncertainty in the measurements caused by the deviation in the wavelength resonance between incident laser and ring-down cavity. The concentrations of greenhouse gases in ambient air are measured by the open-path CW-CRDS with the uncertainties of 0.02, 100, and 10 ppmv for CH4, H2O, and CO2, respectively.

Cavity ring-down spectroscopy using an EC-QCL operating at 7.5 µm for direct monitoring of methane isotopes in air

A number of atmospheric pollutants and greenhouse gases have strong fundamental vibrational transitions within the spectral range of 7.5–8 µm, which marks the region as particularly important for trace gas sensing. Here, we report the development of a mid-infrared continuous-wave (cw) cavity ring-down spectroscopy (CRDS) technique coupled with an external-cavity (EC) mode-hop-free quantum cascade laser (QCL) operating at 7.5 µm. We validated the EC-QCL based high-resolution cw-CRDS system by measuring 12CH4 and 13CH4 isotopes of methane (CH4) which served as a benchmark molecule. The direct, quantitative and selective measurements of 12C and 13C isotopes of CH4 in ambient air as well as in human breath samples in the levels of parts per billion by volume were made by probing one of the strongest fundamental vibrational transitions of CH4 arising from the asymmetric bending (ν4 band) vibrations of the bonds centred at ~1327.244 cm−1 and ~1332.946 cm−1, respectively. We achieved a noise-equivalent absorption coefficient of 1.86 × 10−9 cm−1 Hz−1/2 with 100 Hz data acquisition rate for the current cw-CRDS spectrometer. The current high-resolution cw-CRDS system could be further exploited to harness the full advantage of the spectral region covering 7.5–8 µm to monitor several other trace molecular species along with their isotopic compositions.

Cross Section of OH Radical Overtone Transition near 7028 cm(-1) and Measurement of the Rate Constant of the Reaction of OH with HO2 Radicals.

The absorption cross section of an overtone transition of OH radicals at 7028.831 cm(-1) has been measured using an improved experimental setup coupling laser photolysis to three individual time-resolved detection techniques. Time-resolved relative OH radical profiles were measured by laser-induced fluorescence (LIF), and their absolute profiles have been obtained by cw-cavity ring-down spectroscopy (cw-CRDS). HO2 radicals were quantified simultaneously at the well-characterized absorption line at 6638.21 cm(-1) by a second cw-CRDS absorption path. Initial OH concentrations and thus their absorption cross sections have been deduced from experiments of 248 nm photolysis of H2O2: OH and HO2 profiles have been fitted to a simple kinetic model using well-known rate constants. The rate constant of the reaction between OH and HO2 radicals turned out to be sensitive to the deduction of the initial OH concentration and has been revisited in this work: OH decays have been observed in the presence of varying excess HO2 concentrations. A rate constant of (1.02 ± 0.06) × 10(-10) cm(3) s(-1) has been obtained, in good agreement with previous measurements and recent recommendations. An absorption cross section of σOH = (1.54 ± 0.1) × 10(-19) cm(2) at a total pressure of 50 Torr helium has been obtained from consistent fitting of OH and HO2 profiles in a large range of concentrations.

Measuring hydroperoxide chain-branching agents during n-pentane low-temperature oxidation

The reactions of chain-branching agents, such as H2O2 and hydroperoxides, have a decisive role in the occurrence of autoignition. The formation of these agents has been investigated in an atmospheric-pressure jet-stirred reactor during the low-temperature oxidation of n-pentane (initial fuel mole fraction of 0.01, residence time of 2s) using three different diagnostics: time-of-flight mass spectrometry combined with tunable synchrotron photoionization, time-of-flight mass spectrometry combined with laser photoionization, and cw-cavity ring-down spectroscopy. These three diagnostics enable a combined analysis of H2O2, C1–C2, and C5 alkylhydroperoxides, C3–C5 alkenylhydroperoxides, and C5 alkylhydroperoxides including a carbonyl function (ketohydroperoxides). Results using both types of mass spectrometry are compared for the stoichiometric mixture. Formation data are presented at equivalence ratios from 0.5 to 2 for these peroxides and of two oxygenated products, ketene and pentanediones, which are not usually analyzed during jet-stirred reactor oxidation. The formation of alkenylhydroperoxides during alkane oxidation is followed for the first time. A recently developed model of n-pentane oxidation aids discussion of the kinetics of these products and of proposed pathways for C3–C5 alkenylhydroperoxides and the pentanediones.

A study of the low-temperature oxidation of a long chain aldehyde: n-hexanal

Large aldehydes are important species among the oxygenated products formed during the oxidation of fuels (e.g. alkanes or alkenes) and biofuels (e.g. long chain alcohols), but their oxidation chemistry has been scarcely studied up to now. In this work, a study of the oxidation of hexanal has been performed in a jet-stirred reactor over the temperature range 475–1100K, at a residence time of 2s, a pressure of 106.7kPa, an inlet fuel mole fraction of 0.005 and at three equivalence ratios (0.25, 1 and 2). Reaction products were quantified using two analytical methods: gas chromatography and cw-cavity ring-down spectroscopy. In addition to species usually measured during oxidation study (such as CO, CO2, water, H2O2 and C1-C3 olefins, aldehydes and ketones), specific low temperature oxidation species were also observed: C6 species, such as δ- and γ-caprolactones and hexanoic acid, but also C5 species, such as 2-methyl-tetrahydrofuran and pentanal which are typical reaction products detected during the oxidation of n-pentane. A detailed kinetic model has been developed using an automatic generation software. This model was used to highlight the specificities of the oxidation chemistry of long chain aldehydes and to understand the formation routes of the C5 and C6 low-temperature oxidation products. Simulations indicate that the low energy of the aldehyde CH bond has a significant effect on the fuel reactivity and on the distribution of reaction products.

Overtone, 2NH (ν1 + ν3) spectroscopy of 15NH3–Ar

We report on the observation of the Π (11; 2NH)←Σ (00; ground state) band in 15NH3–Ar, with origin at 6615.943cm−1, using jet-cooled cw-cavity ring-down spectroscopy. The rotational temperature is estimated to be 7K. Nineteen rotational lines were assigned. Perturbations were evidenced from anomalous line positions and line widths, but not unraveled. Upper state rotational constants were obtained from the analysis of the nine unperturbed R/P lines. The e-symmetry upper state predissociation lifetimes appear to decrease with J′, from about 1.2ns to 250ps from J′=1 to 9.

The Water Dimer Investigated in the 2OH Spectral Range Using Cavity Ring-Down Spectroscopy.

Two setups based on CW cavity ring-down spectroscopy were used at Bruxelles and Rennes to record jet-cooled water dimer absorption between 7188 and 7285, and between 7357 and 7386 cm(-1). Some 19 absorption features are reported, significantly more than in the literature. Limited high-resolution information is available due to strong overlap between neighboring vibration-rotation-tunneling (VRT) structures and to spectral broadening induced by short upper state vibrational predissociation lifetimes, likely to range between 100 and 20 ps. Rotational band contours analyses are performed to assign the partly resolved VRT structures to the v1v2v3,vfvb = 000,11; 200,00; 000,20; and 101,00 zero-order vibrational states. Their wavenumbers are found to be 7192.34, 7225.86, 7240.57, and 7256.99 cm(-1), respectively. Both so-called acceptor-switching tunneling components are involved in the assignments whose tentative character is discussed.

CRDS of water vapor at 0.1 Torr between 6886 and 7406 cm−1

The absorption spectrum of water vapor in “natural” isotopic abundance has been recorded by high sensitivity CW-Cavity Ring Down Spectroscopy (CW-CRDS) between 6885.79 and 7405.91cm−1. This strong absorbing region includes the first hexad of interacting vibrational bands which was previously studied by Fourier Transform Spectroscopy. The achieved sensitivity of the recordings varies from αmin∼2×10–11 to 2×10−10cm−1 allowing us to use a sample pressure of 0.1Torr, making pressure broadening of the line profile mostly negligible. Weak lines in the vicinity of much stronger lines could then be accurately measured. The weakest lines have intensity on the order of 5×10–28cm/molecule at 296K. A set of 4471 lines were assigned to 4916 transitions of five water isotopologues (H216O, H218O, H217O, HD16O and HD18O). A small number of new energy levels was determined mostly for the H217O isotopologue. The previous investigations and existing databases are critically evaluated. In particular, a number of corrections and new assignments are proposed for the water list provided by the HITRAN database in the considered region. As a result, a complete list of 12,700 transitions for water in “natural” isotopic abundance is provided as Supplementary Material for the 6885–7408cm−1 region.

Refinements of the WKMC empirical line lists (5852–7919 cm−1) for methane between 80 K and 296 K

During the last 4 years, empirical line lists for methane at room temperature and at 80K were constructed from spectra recorded by (i) differential absorption spectroscopy (DAS) in the high energy part of the tetradecad (5852−6195cm−1) and in the icosad (6717–7589cm−1) and (ii) high sensitivity CW-Cavity Ring Down Spectroscopy (CRDS) in the 1.58μm and 1.28μm transparency windows (6165–6750cm−1 and 7541–7919cm−1, respectively). We have recently constructed the global line lists for methane in “natural” isotopic abundance, covering the spectral region from 5854 to 7919cm−1 (Campargue A, Wang L, Kassi S, Mondelain D, Bézard B, Lellouch E, et al., An empirical line list for methane in the 1.26–1.71μm region for planetary investigations (T=80–300K). Application to Titan, Icarus 219 (2012) 110–128). These WKMC (Wang, Kassi, Mondelain, Campargue) empirical lists include about 43,000 and 46,420 lines at 80±3K and 296±3K, respectively. The “two temperature method” provided lower state energy values, Eemp, for about 24,000 transitions allowing us to account satisfactorily for the temperature dependence of the methane absorption over the considered region. The obtained lists have been already successfully applied in a large range of temperature conditions existing on Titan, Uranus, Pluto, Saturn and Jupiter.In the present contribution, we provide some improvements to our lists by using literature data to extend the set of lower state energy values and by correcting the distortion of the high Eemp values (J>10) due to the temperature gradient existing in the cryogenic cell used for the recordings. The proposed refinements are found to have an overall limited impact but they may be significant in some spectral intervals below 6500cm−1.The new version of our lists at 80K and 296K is provided as Supplementary Material: the WKMC@80K+ and WKMC@296K lists are adapted for planetary and atmospheric applications, respectively. The WKMC@80K+ list is made applicable over a wider range of temperatures and shows satisfactory extrapolation capabilities up to room temperature. It was obtained by transferring to the 80K list the 27,580 single lines present only in the 296K list, with corresponding lower state energy values chosen to make them below the detectivity limit at 80K.In the discussion, the different line lists and databases available for methane in the near infrared are compared and some suggestions are given.

The absorption spectrum of D2: Ultrasensitive cavity ring down spectroscopy of the (2–0) band near 1.7 μm and accurate ab initio line list up to 24 000 cm−1

Eleven very weak electric quadrupole transitions Q(2), Q(1), S(0)-S(8) of the first overtone band of D(2) have been measured by very high sensitivity CW-cavity ring down spectroscopy (CRDS) between 5850 and 6720 cm(-1). The noise equivalent absorption of the recordings is on the order of α(min) ≈ 3 × 10(-11) cm(-1). By averaging a high number of spectra, the noise level was lowered to α(min) ≈ 4 × 10(-12) cm(-1) in order to detect the S(8) transition which is among the weakest transitions ever detected in laboratory experiments (line intensity on the order of 1.8 × 10(-31) cm/molecule at 296 K). A Galatry profile was used to reproduce the measured line shape and derive the line strengths. The pressure shift and position at zero pressure limit were determined from recordings with pressures ranging between 10 and 750 Torr. A highly accurate theoretical line list was constructed for pure D(2) at 296 K. The intensity threshold was fixed to a value of 1 × 10(-34) cm/molecule at 296 K. The obtained line list is provided as supplementary material. It extends up to 24,000 cm(-1) and includes 201 transitions belonging to ten v-0 cold bands (v = 0-9) and three v-1 hot bands (v = 1-3). The energy levels include the relativistic and quantum electrodynamic corrections as well as the effects of the finite nuclear mass. The quadrupole transition moments are calculated using highly accurate adiabatic wave functions. The CRDS line positions and intensities of the first overtone band are compared to the corresponding calculated values and to previous measurements of the S(0)-S(3) lines. The agreement between the CRDS and theoretical results is found within the claimed experimental uncertainties (on the order of 1 × 10(-3) cm(-1) and 2% for the positions and intensities, respectively) while the previous S(0)-S(3) measurements showed important deviations for the line intensities.

An empirical line list for methane in the 1.26–1.71 μm region for planetary investigations (T = 80–300 K). Application to Titan

Insufficient knowledge of the near infrared spectrum of methane is an important limitation for the analysis of the spectra of Titan and of the outer planetary atmospheres in general. The work reported here is the result of a long-term project aiming to provide astronomers with a line by line list for precise calculations of the methane absorption in the near infrared region. We thus present here our best to date empirical line list between 5854 and 7919cm−1 (1.71–1.26μm) and apply it to Titan, demonstrating its capability to significantly improve planetary spectral analysis.In recent contributions, we have obtained empirical line lists at room temperature and at 80K (suitable for Titan conditions) from spectra recorded by (i) Differential Absorption Spectroscopy (DAS) in the high energy part of the tetradecad (5854–6195cm−1) and in the icosad (6717–7589cm−1) (ii) high sensitivity CW-Cavity Ring Down Spectroscopy (CRDS) in the 1.58μm and 1.28μm transparency windows (6165–6750cm−1 and 7541–7919cm−1, respectively). In this work, we construct the global line lists for methane in “natural” isotopic abundance, covering the entire spectral region from 5854 to 7919cm−1. These WKMC (for Wang, Kassi, Mondelain, Campargue) empirical lists include 42,988 and 46,320 lines at 80±3K and 296±3K, respectively and are assembled here with some important improvements:(i)New spectroscopic parameters for the 5854–6148cm−1 region at 80K which increase significantly the number of observations and lower state energy determinations.(ii)Transitions of 13CH4 and CH3D were systematically identified by comparison with DAS spectra of highly enriched 13CH4 and CH3D, recorded at the same temperatures.(iii)In the 1.58μm transparency window where CH3D lines contribute importantly to the methane spectrum at 80K, the set of CH3D lower state energy values was completed by using recent DAS results for pure CH3D.The “two temperature method” provided lower state energy values for about 24,000 transitions from the ratios of their line intensities at 80K and 296K. The clear propensity of the derived low J values of 12CH4 and 13CH4 to be integer illustrates the quality of the lower state energy values. The obtained data sets allow us to account for most of the temperature dependence of the absorption over the considered region.To illustrate the interest of the WKMC line lists for planetary applications, we perform simulations of Titan spectra at different resolutions taken from the ground with instruments such as the FTS at the CFHT and CRIRES at the VLT or by the DISR instrument on board the Huygens probe. The agreement between the simulations and the observations clearly demonstrates an important improvement with respect to previous works.

QCLAS and CRDS-Based CO Quantification as Aimed at in Breath Measurements

Laser-spectrometric methods to derive absolute and traceable carbon monoxide (CO) amount fractions in exhaled human breath could be of advantage for early disease detection as well as for treatment monitoring. As proof-of-principle laboratory experiment, we employed intra-pulse and continuous wave (cw) quantum cascade laser spectroscopy (QCLAS), both at 4.6 μm. Additional experiments were carried out applying cw cavity ring-down spectroscopy (CRDS) with a CO sideband laser and a QCL. We emphasize metrological data quality objectives, thatis, traceability and uncertainty, which could serve as essential benefits to exhaled breath measurements. The results were evaluated and compared on a 100 μmol/mol CO level using the two QCLAS spectrometers, and the cw CO sideband laser CRDS setup. The relative standard uncertainties of the pulsed and the cw QCLAS CO amount fraction results were ±4.8 and ±2.8%, respectively, that from the CO sideband laser CRDS was ±2.7%. Sensitivities down to a 3 nmol/mol CO level were finally demonstrated and quantified by means of cw CRDS equipped with a QCL yielding standard uncertainties of about ±2.5 that are exclusively limited by the available line strength figure quality. With this study we demonstrate the achieved comparability of CO quantifications, adhering metrological principles.

The absorption spectrum of H2: CRDS measurements of the (2-0) band, review of the literature data and accurate ab initio line list up to 35 000 cm−1

Five very weak transitions-O(2), O(3), O(4), O(5) and Q(5)-of the first overtone band of H(2) are measured by very high sensitivity CW-Cavity Ring Down Spectroscopy (CRDS) between 6900 and 7920 cm(-1). The noise equivalent absorption of the recordings is on the order of α(min)≈ 5 × 10(-11) cm(-1) allowing for the detection of the O(5) transition with an intensity of 1.1 × 10(-30) cm per molecule, the smallest intensity value measured so far for an H(2) absorption line. A Galatry profile was used to reproduce the measured line shape and derive the line strengths. The pressure shift of the O(2) and O(3) lines was accurately determined from a series of recordings with pressure ranging between 10 and 700 Torr. From an exhaustive review of the literature data, the list of H(2) absorption lines detected so far has been constructed. It includes a total of 39 transitions ranging from the S(0) pure rotational line near 354 cm(-1) up to the S(1) transition of the (5-0) band near 18,908 cm(-1). These experimental values are compared to a highly accurate theoretical line list constructed for pure H(2) at 296 K (0-35,000 cm(-1), intensity cut off of 1 × 10(-34) cm per molecule). The energy levels and transition moments were computed from high level quantum mechanics calculations. The overall agreement between the theoretical and experimental values is found to be very good for the line positions. Some deviations for the intensities of the high overtone bands (V > 2) are discussed in relation with possible pressure effects affecting the retrieved intensity values. We conclude that the hydrogen molecule is probably a unique case in rovibrational spectroscopy for which first principles theory can provide accurate spectroscopic parameters at the level of the performances of the state of the art experimental techniques.

The absorption spectrum of water in the 1.25 μm transparency window (7408–7920 cm −1)

The absorption spectrum of water vapor in “natural” isotopic abundance has been recorded between 7408 and 7920 cm −1 by high sensitivity cw-cavity ring down spectroscopy (CW-CRDS). This region covers the low energy part of the 1.25 μm transparency window and corresponds to weak water absorption of the first hexad of interacting vibrational bands. The achieved sensitivity – on the order of α min ∼ 2 × 10 −10 cm −1 – has allowed one to newly measure 2028 weak transitions with intensities down to 2 × 10 −29 cm/molecule at 296 K i.e. more than two orders of magnitude lower than previous observations. Three hundred and fourty-one new and corrected energy levels belonging to 26 vibrational states of H 2 16O, H 2 18O, H 2 17O, and HD 16O could be determined from the vibration–rotation analysis based on variational calculations by Schwenke and Partridge. The previous investigations in the studied region by Fourier Transform Spectroscopy and existing databases have been critically evaluated. The most complete list for water in the region is provided as Supplementary Material.

Simultaneous, time-resolved measurements of OH and HO2 radicals by coupling of high repetition rate LIF and cw-CRDS techniques to a laser photolysis reactor and its application to the photolysis of H2O2

A new experimental set-up for the simultaneous, time-resolved measurement of OH and HO2 radicals by coupling high repetition rate Laser Induced Fluorescence (LIF) and cw-Cavity Ring-Down Spectroscopy (cw-CRDS) detection techniques to a laser photolysis reactor has been installed. The first experiments concern the well-known photolysis of H2O2: the absorption cross sections of H2O2 lines in the near-infrared at 6636.75 and 6639.90 cm−1 have been determined by combined results of spectroscopic and kinetic measurements of OH and HO2 radicals produced by the photolysis of H2O2 at 248 nm. These experiments will enable the measurement of absolute H2O2 concentrations in future experiments with the same set-up.

High sensitivity CRDS of the a1Δ g− X3Σg− band of oxygen near 1.27 μm: Extended observations, quadrupole transitions, hot bands and minor isotopologues

The CW-Cavity Ring Down Spectroscopy (CW-CRDS) technique has been used to record the high sensitivity absorption spectrum of the a 1Δ g– X 3 Σ g − band of oxygen near 1.27 μm. The spectra were obtained between 7640 and 7917 cm −1 with “natural” oxygen and with a sample highly enriched in 18O. The absolute intensities of 376 and 643 oxygen transitions were measured in the two spectra. They include the a 1Δ g− X 3 Σ g − (0–0) bands of 16O 2, 16O 18O, 16O 17O, 17O 18O and 18O 2. The (0–0) bands of 16O 2 and 18O 2 show weak quadrupole transitions with line intensities ranging from 1×10 −30 to 1.9×10 −28 cm/molecule. They are accompanied by the a 1Δ g− X 3 Σ g − (1–1) hot bands, which are reported for the first time. The line profiles of the transitions of the 16O 17O and 17O 18O isotopologues were observed to be broadened due to an unresolved magnetic hyperfine structure. Accurate spectroscopic parameters of the different energy levels involved in the observed bands were derived from a global fit of the observed line positions, combined with microwave and Raman measurements available in the literature.

Detection and identification of TNT, 2,4-DNT and 2,6-DNT by near-infrared cavity ringdown spectroscopy

A dedicated instrument based on cw cavity ringdown (CRD) spectroscopy has been used to study the feasibility of solid explosive detection in the near infrared. A flash heater was used to evaporate solid samples in a CRD absorption cell. The vapor phase spectra of TNT, 2,4-DNT and 2,6-DNT have been recorded for the first time in the spectral range between 1560 and 1680 nm. Although the three explosives are chemically very similar the recorded spectra are distinct and permit an unambiguous identification of the three pure substances. The fast sample preparation and short detection times render the technique useful for trace detection of solid particles. The detection sensitivity is estimated in the 10–100 ng range and is adequate for trace detection and identification of solid explosives.

Development of a Custom CRDS-based Trace Gas Analyzer for Argon and Xenon

Noble liquid-based dark matter detectors are sensitive to parts-per-billion (ppb) concentrations of impurities of O 2, N 2, and H 2O, which reduce the scintillation light yield and disrupt the ionization signal. In order to achieve an optimal light yield and collection of the ionization signal, DarkSide is designing a cryogenic distillation unit to achieve large quantities of depleted argon with a purity at sub-ppb levels. Critical to the success of the depleted argon dark matter detector will be the development of a system capable of analyzing these impurities at the sub-ppb level. A trace gas analyzer based on cw-Cavity Ring-Down Spectroscopy (CRDS) technology is being built at Black Hills State University that exceeds the current limits of commercially available systems. CRDS involves measuring the decay rate of the intensity of monochromatic light transmitted at the mirrors enclosing a cavity filled with a sample of gas. By comparing the decay rates for light at a resonance frequency of the contaminant of interest with that of light slightly off-resonance, a direct measurement of the amount of absorbing material (the contaminant) is obtained. Designed to measure ultra-low levels of elemental impurities, the BHSU system will be critical to such experiments as DarkSide and other dark matter searches based on liquid argon and xenon.