Resonant photoacoustic simultaneous detection of methane and ethylene by means. of a 1.63-μm diode laser

A compact multi-component trace-gas detector based on the resonant photoacoustic technique and a NIR external cavity diode laser has been developed. It has been characterized using a mixture of ethylene and methane diluted in ambient air. A spectroscopic investigation of combination bands and overtones between 5900 and 6250 cm^-1, obtained with an IR pulsed laser photoacoustic spectrometer, allowed us to find a wavelength region where the 2ν₃ overtone of CH₄ and the ν₅+ν₉ combination band of C₂H₄ show uncongested rotational lines. Using a single-mode scan of the diode laser in this region, around 6150 cm^-1, the sensitivity for the simultaneous detection of ethylene and methane is 8 ppm/mW and 40 ppm/mW respectively. Factors affecting the sensitivity and selectivity of the detection system and possible improvements suitable to reach the sub-ppm detection limit are discussed. Received: 1 August 2001 / Revised version: 28 November 2001 / Published online: 7 February 2002 An erratum to this article is available at .     

Relative line intensity measurement in absorption spectra using a tunable diode laser at 1.6 μm: application to the determination of 13CO2/12CO2 isotope ratio

The measurement of relative intensities in CO₂ combination bands spectrum is performed using wavelength modulation spectroscopy (WMS) and a DFB (distributed feedback) diode laser operating at 1.6 μm. The diode laser is stabilized with an external Fabry–Pérot interferometer and absorption spectroscopy is performed in a multipass gas cell. A spectrum containing spectral lines of both ¹³CO₂ and ¹²CO₂ isotopic species is recorded. The variation of laser power during frequency scanning and the line shape are taken into account to accurately extract line intensities from experimental data. The isotopic concentration ratio is deduced from the intensity ratio. Both ratios are measured with an accuracy of about 0.5% in pure CO₂. Received: 9 June 2000 / Published online: 8 November 2000     

Isotopic substitution shifts in methane and vibrational band assignment in the 5560-6200 cm region

The absorption spectra of the ¹²CH₄ and ¹³CH₄ molecules have been recorded and assigned in the 5560-6200 cm⁻¹ region. The effects of isotopic substitution for ¹²C by ¹³C on the methane vibrational energy levels have been calculated from an ab initio potential energy surface and compared with experiment. Comparison of the results obtained for two isotopic species allows us to confirm the vibrational assignment for the strongest bands of ¹²CH₄ in this region. Good agreement of ab initio calculations with observed energy levels has been demonstrated. A list of the assigned ¹³CH₄ lines valuable in atmospheric applications is reported.     

Spectral intensities in the ν3 bands of 12CH4 and 13CH4

Relative and absolute line intensities for the ν₃ bands of the ¹²C and ¹³C isotopic varieties of methane have been measured using a tunable difference-frequency laser spectrometer. From these data the integrated band strength of ¹³CH₄ is calculated to be 0.983 ± 0.007 that of ¹²CH₄, with the uncertainty representing three standard deviations. The absolute ν₃ bandstrength for ¹²CH₄ is 266.1 ± 3.0 cm^?2 atm ^?1 at 294.7 K where the errors are dominated by the pressure measurement. This band strength corresponds to an effective transition moment 〈μ₃〉 = 0.0534(3)D for ¹²CH₄ from which the ν₄ band dipole moment and the Herman-Wallis F factor can be estimated using a recent force field model for methane.     

A trace methane gas sensor using mid-infrared quantum cascaded laser at 7.5 μm

Presented is a compact instrument developed for in situ high-stable and sensitive continuous measurement of trace gases in air, with results shown for ambient methane (CH₄) concentration. This instrument takes advantage of recent technology in thermoelectrically cooled pulsed Fabry–Perot (FP) quantum cascaded (QC) laser driving in a pulse mode operating at 7.5 μm to monitor a well-isolated spectral line near the ν4 fundamental band of CH₄. A high-quality liquid nitrogen cooled mercury cadmium telluride mid-infrared detector with time discriminating electronics is used along with a total reflection coated gold ellipsoid mirror offering 20 cm single pass optical absorption in an open-path cell to achieve stability of 5.2 × 10^?3 under experimental condition of 200 ppm measured ambient CH₄. The instrument operates continuously, and integrated software for laser control using direct absorption provides quantitative trace gas measurements without calibration. One may substitute a QC laser operating at a different wavelength to measure other gases. The instrument can be applied to field measurements of gases of environmental concern.     

Quasi-simultaneous detection of CH2O and CH by cavity ring-down absorption and laser-induced fluorescence in a methane/air low-pressure flame

The combination of two-dimensional, planar laser-induced fluorescence (PLIF) and cavity ring-down (CRD) absorption spectroscopy is applied to map quantitatively the spatial distributions of CH₂O and CH in a methane/air flame at 25 Torr. Both species are detected in the same spectral region using the overlapping CH₂O A ¹ A ₂ -X ¹ A ₁ 4₁ ⁰ and CH B-X(1,0 )bands. The combination of diagnostic techniques exploits the spatial resolution of LIF and the quantitative CRD absorption measure of column density. The spatially resolved PLIF provides the distribution of absorbers and line-of-sight CRD absorption the absolute number density needed for quantitative concentration images. The peak CH₂O concentration is (3.5±1.4 )×10¹⁴ cm^-3, or 1450±550 ppm at 1000 K. The lack of precise absorption cross-section data produces these large error limits. Although a flame model predicts lower amounts, these large uncertainties limit this measurement’susefulness as a test of the flame chemistry. Received: 24 April 2001 / Revised version: 10 July 2001 / Published online: 10 October 2001     

A quantum cascade laser-based optical feedback cavity-enhanced absorption spectrometer for the simultaneous measurement of CH4 and N2O in air

Optical feedback cavity-enhanced absorption spectroscopy (OF CEAS) has been demonstrated with a thermoelectrically cooled continuous wave distributed feedback quantum cascade laser (QCL) operating at wavelengths around 7.84 μm. The QCL is coupled to an optical cavity which creates an absorption pathlength greater than 1000 m. The experimental design allows optical feedback of infra-red light, resonant within the cavity, to the QCL, which initiates self-locking at each TEM₀₀ cavity mode frequency excited. The QCL linewidth is narrowed to below the mode linewidth, greatly increasing the efficiency of injection of light into the cavity. At the frequency of each longitudinal cavity mode, the absorption coefficient of an intracavity sample is obtained from the transmission at the mode maximum, measured with a thermoelectrically cooled detector: spectral line profiles of CH₄ and N₂O in ambient air were recorded simultaneously and with a resolution of 0.01386 cm^?1. A minimum detectable absorption coefficient of 5.5×10^?8 cm^?1 was demonstrated after an averaging time of 1 s for this completely thermoelectrically cooled system. The bandwidth-normalised limit for a single cavity mode is 5.6×10^?9 cm^?1?Hz^?1/2 (1σ).     

Detection of methane in air using diode-laser pumped difference-frequency generation near 3.2 μm

Spectroscopic detection of the methane in natural air using an 800 nm diode laser and a diode-pumped 1064 nm Nd:YAG laser to produce tunable light near 3.2 µm is reported. The lasers were pump sources for ring-cavity-enhanced tunable difference-frequency mixing in AgGaS₂. IR frequency tuning between 3076 and 3183 cm^–1 was performed by crystal rotation and tuning of the extended-cavity diode laser. Feedback stabilization of the IR power reduced intensity noise below the detector noise level. Direct absorption and wavelength-modulation (2f) spectroscopy of the methane in natural air at 10.7 kPa (80 torr) were performed in a 1 m single-pass cell with 1 µW probe power. Methane has also been detected using a 3.2 µm confocal build-up cavity in conjunction with an intracavity absorption cell. The best methane detection limit observed was 12 ppb m (Hz.)^–1/2.     

Simultaneous detection of methane, oxygen and water vapour utilising near-infrared diode lasers in conjunction with difference-frequency generation

An all-diode-laser-based spectrometer is used for the simultaneous detection of methane, oxygen and water vapour. This is accomplished using a 760-nm diode laser and a 980-nm diode laser in conjunction with difference-frequency generation to 3.4 μm in a periodically poled lithium niobate crystal. Each of the output wavelengths is resonant with one of the molecular species. Simultaneous recordings over a 15-m open path of laboratory air are demonstrated. The recording scheme shows the wide applicability of a diode-laser-based difference-frequency spectrometer for the detection of molecular species in different wavelength ranges. By increasing the frequency of the 760-nm diode laser and decreasing the frequency of the 980-nm diode laser, a maximum continuous tuning range in the mid infrared of 3.6 cm^-1 is achieved. This enables the recording of several methane lines at atmospheric pressure. Pressure-dependence studies of methane lineshapes are also performed in an absorption cell. An indoor-air methane background level of 3 ppm is measured. The signal-to-noise ratio in the recorded methane spectra indicates that sub-ppm detection of methane at atmospheric pressure is feasible. Received: 6 March 2000 / Revised version: 19 June 2000 / Published online: 11 October 2000     

Sensitive detection of methane and nitrous oxide isotopomers using a continuous wave quantum cascade laser

A continuous wave quantum cascade laser (QCL), operating near 8.1 μm, was used for wavelength modulation spectroscopy of methane (CH₄) and nitrous oxide (N₂O) stable isotopes. Several rotational transitions of ¹⁴N₂ ¹⁶O, ¹⁵N¹⁴N¹⁶O, ¹⁴N₂ ¹⁸O, ¹⁴N₂ ¹⁷O, ¹³CH₄ and ¹²CH₄ fundamental bands were detected. The noise-equivalent absorbance was measured to be less than 10^-5 in a 1-Hz bandwidth. A characterization of the laser source was also performed. The use of a QCL spectrometer for high-precision isotope ratio measurements is discussed. Received 14 March 2002     

Mid-infrared laser-induced grating experiments of C2H4 and NH3 from 0.1–2 MPa and 300–800 K

Laser-induced thermal gratings (LITG) were generated in mixtures of ethylene and ammonia in nitrogen using mid-infrared laser radiation from a grating-tuned, low-pressure, pulsed (5 ms pulse width) CO₂ laser, and read out with a continuous wave Nd:YLF laser. The LITG signal intensity was measured as a function of pressure (0.1–2 MPa) and temperature (300–800 K, at 0.1 and 1 MPa) by tuning the laser to the accidental coincidences of the 10P(14) and 10R(6) emission lines with molecular absorption transitions of C₂H₄ and NH₃, respectively. Comparisons are made with theoretical predictions for the grating efficiency from a simple thermalization model. A theoretical comparison of the temporal LITG signal response for three excitation pulse shapes – a delta function, a realistic pulse, and a square wave is presented. Furthermore, it is shown that for NH₃, most of the decrease of the LITG signal intensity with increasing temperature is due to the corresponding decrease in fractional molecular absorption of the pump beam radiation. The diagnostic capabilities of the mid-infrared LITG experiment is demonstrated for spatially resolved ethylene measurements with long laser pulses in a premixed stoichiometric CH₄–air flame at atmospheric pressure. Received: 17 March 2000 / Revised version: 23 March 2000 / Published online: 13 September 2000     

Sensitive absorption measurements in the near-infrared region using off-axis integrated-cavity-output spectroscopy

A novel instrument that employs a high-finesse optical cavity as an absorption cell has been developed for sensitive measurements of gas mixing ratios using near-infrared diode lasers and absorption-spectroscopy techniques. The instrument employs an off-axis trajectory of the laser beam through the cell to yield an effective optical path length of several kilometers without significant unwanted effects due to cavity resonances. As a result, a minimum detectable absorption of approximately 1.4×10^-5 over an effective optical path of 4.2 km was obtained in a 1.1-Hz detection bandwidth to yield a detection sensitivity of approximately 3.1×10^-11 cm^-1 Hz^-1/2. The instrument has been used for sensitive measurements of CO, CH₄, C₂H₂ and NH₃. Received: 6 May 2002 / Revised version: 31 May 2002 / Published online: 2 September 2002 RID="*" ID="*"Corresponding author. Fax: +1-650/965-7074, E-mail: d.baer@lgrinc.com     

Microstructures and photoluminescence of hydrogenated amorphous carbon films produced by using organic hydrocarbon source

Thin layers of hydrogenated amorphous carbon were prepared by using organic hydrocarbon source, xylene (C₈H₁₀), in plasma-enhanced chemical vapor deposition (PECVD) system. The microstructures were characterized by using Fourier-transform infrared and Raman scattering spectra. The appearance of a sharp vibration signal in 1600 cm^-1 strongly suggests the existence of sp² carbon clusters with aromatic rings. With increasing the deposition rf power, the content of these aromatic structures is increased in the films. In contrast to a broad single PL peak in methane (CH₄)-baseda-C:H films, the PL band of xylene-based a-C:H films contains multiple peaks in blue-green light region, which is influenced by rf power. We tentatively attributed it to the radiative recombination of electron-hole pairs through some luminescent centers associated with aromatic structures. Received: 26 April 2000 / Accepted: 9 May 2000 / Published online: 13 September 2000     

Mid-infrared trace gas detection using continuous-wave difference frequency generation in periodically poled RbTiOAsO4

A tunable mid-infrared continuous-wave (cw) spectroscopic source in the 3.4–4.5 μm region is reported, based on difference frequency generation (DFG) in a quasi-phase-matched periodically poled RbTiOAsO₄ (PPRTA) crystal, DFG power levels of 10 μW were generated at approximately 4 μm in a 20-mm long PPRTA crystal by mixing two cw single-frequency Ti:Al₂O₃ lasers operating near 713 nm and 871 nm, respectively, using a laser pump power of 300 mW. A quasi-phase-matched infrared wavelength-tuning bandwidth (FWHM) of ∼12 cm^-1 and a temperature tuning rate of 1.02 cm^-1/°C were achieved. Experimental details regarding the feasibility of trace gas detection based on absorption spectroscopy of CO₂ in ambient air using this DFG radiation source are also described. Received: 23 October 2000 / Revised version: 22 January 2001 / Published online: 27 April 2001     

Application of Brønsted's principle of congruence to n-alkane mixtures

The second virial coefficient of normal methane, ¹²CH₄, has been compared with that of the corresponding deutero compound ¹²CD₄, as well as with the other isotopic molecule, ¹³CH₄. These measurements were performed by means of a differential apparatus over a range of temperature extending from 110°k up to 300°k.     

High-power diode-end-pumped Nd:YVO4 laser: thermally induced fracture versus pump-wavelength sensitivity

We report two kinds of compact and efficient diode-end-pumped TEM₀₀ lasers with output power >25 W at ≈50 W of incident pump power. One laser consists of a single 0.3 at. % Nd:YVO₄ crystal in a V-type cavity, the other laser includes two 0.5 at. % Nd:YVO₄ crystals in a linear cavity. Experimental results show that lowering Nd³⁺ concentration can be beneficial in extending the fracture-limited pump power but it also increases the sensitivity of the pump wavelength due to the overlapping efficiency. Received: 19 February 2000 / Revised version: 30 May 2000 / Published online: 20 September 2000     

Estimation of microbial methane generation and oxidation rates in the municipal solid waste landfill of Kaluga city,Russia

Using a theoretical model and mass isotopic balance, biogas (methane and CO₂) released from buried products at their microbial degradation was analysed in the landfill of municipal and non-toxic industrial solid organic waste near Kaluga city, Russia. The landfill contains about 1.34×10⁶ tons of waste buried using a ‘sandwich technique’ (successive application of sand–clay and waste layers). The δ¹³C values of biogenic methane with respect to CO₂ were?56.8 (±2.5) ‰, whereas the δ¹³C of CO₂ peaked at+9.12‰ (+1.4±2.3‰ on average), reflecting a virtual fractionation of carbon isotopes in the course of bacterial CO₂ reduction at the landfill body. After passing through the aerated soil layers, methane was partially oxidised and characterised by δ¹³C in the range of?50.6 to?38.2‰, evidencing enrichment in ¹³C, while the released carbon dioxide had δ¹³C of?23.3 to?4.04‰, respectively. On the mass isotopic balance for the δ¹³C values, the methane production in the landfill anaerobic zone and the methane emitted through the aerated landfill surface to the atmosphere, the portion of methane oxidised by methanotrophic bacteria was calculated to be from 10 to 40% (averaged about 25%). According to the theoretical estimation and field measurements, the annual rate of methane production in the landfill reached about 2.9(±1.4)×10⁹ g C CH₄ yr^?1 or 5.3(±2.6)×10⁶ m³ CH₄ yr^?1. The average rates of methane production in the landfill and methane emission from landfill to the atmosphere are estimated as about 53 (±26) g C CH₄ m^?2 d^?1 (or 4 (±2) mol CH₄ m^?2 d^?1) and 33 (±12) g C CH₄ m^?2 d^?1 (or 2.7 (±1) mol CH₄ m^?2 d^?1), respectively. The calculated part of methane consumed by methanotrophic bacteria in the aerated part of the landfill was 13(±7) g C CH₄ m^?2 d^?1 (or 1.1(±0.6) mol CH₄ m^?2 d^?1) on average.     

PreCon: A Fully Automated Interface for the Pre-Gc Concentration of Trace Gases on Air for Isotopic Analysis

Abstract

For the high precision isotope analysis of atmospheric trace gases a computer controlled concentration interface has been developed. From small air samples it collects either N₂O or CO₂ derived from CH₄ at their respective concentrations (0.3 ppm for N₂O, 1.7 ppm for CH₄) into a small diameter cold trap (?196°C) and interfaces via GC and open split to an isotope ratio mass spectrometer (Finnigan MAT 252) for on-line isotope evaluation. External reproducibilities for repeated measurements of 100 ml air samples from the same source of < 0.2° (δ-notation) have been achieved for ¹³C/¹²C from CH₄ and for ¹⁵N/¹⁴N and ¹⁸O/¹⁶O from N₂O. The precision is adequate to monitor the isotopic changes in these gases during a day's course.     

Automatically tunable continuous-wave optical parametric oscillator for high-resolution spectroscopy and sensitive trace-gas detection

We present a high-power (2.75 W), broadly tunable (2.75–3.83 μm) continuous-wave optical parametric oscillator based on MgO-doped periodically poled lithium niobate. Automated tuning of the pump laser, etalon and crystal temperature results in a continuous wavelength coverage up to 450 cm^-1 per poling period at <5×10^-4 cm^-1 resolution. The versatility of the optical parametric oscillator as a coherent light source in trace-gas detection is demonstrated with photoacoustic and cavity ring-down spectroscopy. A 17-cm^-1-wide CO₂ spectrum at 2.8 μm and multi-component gas mixtures of methane, ethane and water in human breath were measured using photoacoustics. Methane (at 3.2 μm) and ethane (at 3.3 μm) were detected using cavity ring-down spectroscopy with detection limits of 0.16 and 0.07 parts per billion by volume, respectively. A recording of ¹²CH₄ and ¹³CH₄ isotopes of methane shows the ability to detect both species simultaneously at similar sensitivities. PACS 42.65.Yj; 42.72.Ai; 42.62.Fi     

Far infrared laser lines produced by methanol and its isotopic species: A review

Methanol (CH₃OH) is considered today one of the most important active media for the generation of laser radiation in the far-infrared (FIR) spectral region. Together with ten of its other isotopic species, it is responsible for the major part of the laser lines generated by the optical pumping technique. Due to the extreme importance of those molecules as laser generators, we understood that there was a necessity of a comprehensive work which would include as much pratical information as possible about each line.Chang et al⁽¹⁾ early recognized methanol as a source of strong FIR laser lines. Since then, more than 100 papers were published containing information about new laser emission. Recently, Moruzzi et al⁽¹¹⁴⁾ presented a review of 575 lines produced by¹²CH₃OH. In the present paper, we have extended the review to the various isotopic modifications of this molecule (namely¹³CH₃OH, CD₃OH,¹³CD₃OH, CD₃OD,¹³CD₃OD, CH₃OD, CH ₃ ¹⁸ OH, CH₂DOH, CH₂DOD and CHD₂OH), a total of nearly 2000 lines with wavelengths ranging from 19µm to 3030µm.