Further study of continuous-wave CO flame chemical laser of the CS2/O2/N2O type

This report describes an experimental examination of the output characteristics of the continuous-wave (cw) carbon monoxide flame chemical laser (FCL) of the CS₂/O₂/N₂O type in case of small CS₂/O₂ reactants ratios (tipically CS₂/O₂≦1/10). A linear burner which gives a homogeneous and stable flame was used during the experimental study. The measurements of temperature distribution in CS₂/O₂ as well as CS₂/O₂/N₂O flames show maximum temperatures of 1040 and 890 K, respectively. The addition of nitrous oxide (N₂O) leads to dramatically enhanced output laser power caused primarily by V?V transfer processes. A chemical efficiency, based on the reaction O+CS→CO^*+S, of 3% was achieved. The spectral composition of the CO FCL of the CS₂/O₂/N₂O type shows lasing in the region from 5.130 to 5.586 μm. Experimental results were obtained with a nondispersive optical cavity.     

Continuous tuning of a single laser mode over 5 cm-1 in a high-pressure N2O/CO2 transfer laser

Continuous tuning of a single laser mode in an optically pumped high-pressure N₂O/CO₂ transfer laser over 5 cm^-1 near 10.5 μm has been achieved. A spectrum of ethylene gas taken with this source indicates a resolution of 0.014 cm^-1.     

Mid-infrared laser absorption spectrometers based upon all-diode laser difference frequency generation and a room temperature quantum cascade laser for the detection of CO, N2O and NO

We describe the performance of two mid-infrared laser spectrometers for carbon monoxide, nitrous oxide and nitric oxide detection. The first spectrometer for CO and N₂O detection around 2203 cm^-1 is based upon all-diode laser difference frequency generation (DFG) in a quasi-phase matched periodically-poled lithium niobate (PPLN) crystal using two continuous-wave room-temperature distributed feedback diode lasers at 859 and 1059 nm. We also report on the performance of a mid-infrared spectrometer for NO detection at ∼ 1900 cm^-1 based upon a thermoelectrically-cooled continuous-wave distributed feedback quantum cascade laser (QCL). Both spectrometers had a single-pass optical cell and a thermoelectrically cooled HgCdZnTe photovoltaic detector. Typical minimum detection limits of 2.8 ppmv for CO, 0.6 ppmv for N₂O and 2.7 ppmv for NO have been demonstrated for a 100 averaged spectra acquired within 1.25 s and a cell base length of 21 cm at ∼ 100 Torr. Noise-equivalent absorptions of 10^-5 and 10^-4 Hz^-1/2 are typically demonstrated for the QCL and the DFG based spectrometers, respectively. PACS  42.55.Px; 42.62.Fi; 42.65.-k; 42.72.Ai; 42.68.Ca     

On-line monitoring of cyclotron target-gas output by means of tunable lead salt diode laser absorption spectroscopy

We demonstrate what is, to our knowledge, the first use of mid-infrared laser absorption spectroscopy for trace-gas measurements of cyclotron target outputs used for the generation of radioactive carbon-11 in positron emission tomography (PET). The spectrometer was based upon a liquid-nitrogen-cooled lead salt diode laser generating single-mode radiation in the wavenumber range of 2230–2240 cm^?1. The sample flowed to a multiple-pass optical cell with a total path length of 15.23 m and the laser radiation was detected by two liquid-nitrogen-cooled InSb photodetectors. We present the results of CO, N₂O and CO₂ measurements on PET trace cyclotron output and discuss future work on ¹¹CO and ¹¹CO₂ detection.     

Infrared diode laser spectroscopy of the Kr–N2O van der Waals complex: the v 1 symmetric stretch region of N2O

The rovibrational spectra of four isotopomers of the Kr–N₂O van der Waals complex, namely ⁸²Kr–N₂O, ⁸³Kr–N₂O, ⁸⁴Kr–N₂O and ⁸⁶Kr–N₂O, were measured in the v ₁ vibrational band region of the N₂O monomer (～1285?cm^?1) using a tunable diode laser spectrometer to probe a pulsed supersonic slit jet. Rotational constants for both ground and excited vibrational states of these four isotopomers were accurately determined. The band-origin of Kr–N₂O was observed to shift by +0.1065?cm^?1 from that of the monomer. The band-origin shifts of Rg–N₂O (Rg?=?Ne, Ar, Kr) in the v ₁ vibrational band region could also be well explained by the model based on a Buckingham intermolecular potential [W.A. Herrebout, H.-B. Qian, H. Yamaguchi and B.J. Howard, J. Mol. Spectrosc. 189, 235 (1998)]. But the band-origin shift of He–N₂O was found to deviate significantly from this model. The possible reason is discussed and the band-origin shift of Xe–N₂O predicted.     

Bestimmung der 15N Häufigkeit bei nichtstatistischer 15N-verteilung in N2 sowie bei N2O in Bodenluftproben mittels GC-R-IRMS-Kopplung in einem Probenlauf

Abstract

An analysis technique based on GC-R-IRMS coupling (Gas-Chromatography-Reduction-Isotope Ratio Mass Spectrometry) is demonstrated. The ¹⁵N abundance of N² and N²O in atmospheric air or soil atmosphere from nitrification or denitrification processes with nonrandom distribution is determined in one run. The 12 ml sample is separated from CO² and transported by a helium gas stream through a cooling trap. The N²O is trapped in the cooling trap while the N² passes through it and enters the GC. After GC separation and O² removal in a reduction column, part of the N² enters an isotope mass spectrometer to determine the masses m/z 28,29 and 30. The interferences on mass 30 by the formation of NO in the ion source of the mass spectrometer are eliminated by a calibration and a correction procedure. Upon removing the cooling trap, the N²O is injected into the GC, where it is separated and then quantitatively reduced to N² in a reduction column. The measurement of one sample takes 16 minutes. The detection limit of the ³⁰R_t in alteration N₂ is Δ³⁰R_t = 5 · 10^?7. The detection limit of the N₂O is 3.6 nl.     

New N2O laser band in the 10 μm wavelength region

Fifty new laser lines have been observed in continuous emission from an N₂O laser by placing a hot N₂O absorber cell inside the laser cavity to suppress the regular laser lines. The new lines are identified as rotation-vibration transitions of the (00⁰2–10⁰1) band of N₂O.     

Single-QCL-based absorption sensor for simultaneous trace-gas detection of CH4 and N2O

A quantum cascade laser (QCL)-based absorption sensor for the simultaneous dual-species monitoring of CH₄ and N₂O was developed using a novel compact multipass gas cell (MGC). This sensor uses a thermoelectrically cooled, continuous wave, distributed feedback QCL operating at ~7.8 µm. The QCL wavelength was scanned over two neighboring CH₄ (1275.04 cm^?1) and N₂O (1274.61 cm^?1) lines at a 1 Hz repetition rate. Wavelength modulation spectroscopy (f = 10 kHz) with second harmonic (2f) detection was performed to enhance the signal-to-noise ratio. An ultra-compact MGC (16.9 cm long and a 225 ml sampling volume) was utilized to achieve an effective optical path length of 57.6 m. With such a sensor configuration, a detection limit of 5.9 ppb for CH₄ and 2.6 ppb for N₂O was achieved, respectively, at 1-s averaging time.     

Pulsed reactive crossed beam laser ablation of La0.6Ca0.4CoO3 using O: Where does the oxygen come from?

The composition of thin perovskite films, especially the oxygen content, is a crucial parameter which influences many physical properties, such as conductivity and catalytic activity. Films produced by pulsed laser deposition are normally annealed in an oxygen atmosphere after deposition to achieve a desired oxygen content. In pulsed reactive crossed beam laser ablation, no annealing step is necessary, but a fundamental question regarding this deposition technique is still open: where does the oxygen in the films come from?There are three possibilities, i.e. from the target, from the gas background, or from the gas pulse. To answer this question two experiments were performed: ¹⁸O₂ was used during the deposition process as background gas with ¹⁶O anions in the target and ¹⁶O₂ gas pulse, and a ¹⁸O₂ gas pulse with ¹⁶O from the target and background. These experiments revealed that the quantification of the oxygen origin is only possible, when no oxygen exchange occurs at the deposition temperature. The films are characterized after deposition by elastic recoil detection analysis (ERDA) to determine the ¹⁶O/¹⁸O ratio. Experiments with different oxidizing species in the gas pulse (N₂O and O₂) confirm that the oxidizing potential (N₂O > O₂) as well as the number of molecules are important.     

Release of nitrous oxide and dinitrogen from a transition bog under drained and rewetted conditions due to denitrification: results from a [15N]nitrate–bromide double-tracer study

Denitrification is well known being the most important nitrate-consuming process in water-logged peat soils, whereby the intermediate compound nitrous oxide (N₂O) and the end product dinitrogen (N₂) are ultimately released. The present study was aimed at evaluating the release of these gases (due to denitrification) from a nutrient-poor transition bog ecosystem under drained and three differently rewetted conditions at the field scale using a ¹⁵N-tracer approach ([¹⁵N]nitrate application, 30?kg N ha^?1) and a common closed-chamber technique. The drained site is characterized by a constant water table (WT) of –30?cm (here referred to as D30), while rewetted sites represent a constant WT of –15?cm, a constant WT of 0?cm (i.e. waterlogged), and an initial WT of 0?cm (which decreased slightly during the experiment), respectively, (here referred to as R15, R0, and R0_d, respectively). The highest N₂O emissions were observed at D30 (291?µg N₂O–N m^?2 h^?1) as well as at R0_d (665?µg N₂O–N m^?2 h^?1). At the rewetted peat sites with a constant WT (i.e. R15 and R0), considerably lower N₂O emissions were observed (maximal 37?µg N₂O–N m^?2 h^?1). Concerning N₂ only at the initially water-logged peat site R0_d considerable release rates (up to 3110?µg N₂–N m^?2 h^?1) were observed, while under drained conditions (D30) no N₂ emission and under rewetted conditions with a constant WT (R15 and R0) significantly lower N₂ release rates (maximal 668?µg N₂–N m^–2 h^?1) could be detected. In addition, it has been found that natural WT fluctuations at rewetted peat sites, in particular a rapid drop down of the WT, can induce high emission rates for both N₂O and N₂.     

Development of a field-deployable method for simultaneous,real-time measurements of the four most abundant N2O isotopocules*

Understanding and quantifying the biogeochemical cycle of N₂O is essential to develop effective N₂O emission mitigation strategies. This study presents a novel, fully automated measurement technique that allows simultaneous, high-precision quantification of the four main N₂O isotopocules (¹⁴N¹⁴N¹⁶O, ¹⁴N¹⁵N¹⁶O, ¹⁵N¹⁴N¹⁶O and ¹⁴N¹⁴N¹⁸O) in ambient air. The instrumentation consists of a trace gas extractor (TREX) coupled to a quantum cascade laser absorption spectrometer, designed for autonomous operation at remote measurement sites. The main advantages this system has over its predecessors are a compact spectrometer design with improved temperature control and a more compact and powerful TREX device. The adopted TREX device enhances the flexibility of the preconcentration technique for higher adsorption volumes to target rare isotope species and lower adsorption temperatures for highly volatile substances. All system components have been integrated into a standardized instrument rack to improve portability and accessibility for maintenance. With an average sampling frequency of approximately 1 h^–1, this instrumentation achieves a repeatability of 0.09, 0.13, 0.17 and 0.12?‰ for δ¹⁵N^α, δ¹⁵N^β, δ¹⁸O and site preference of N₂O, respectively, for pressurized ambient air. The repeatability for N₂O mole fraction measurements is better than 1 ppb (parts per billion, 10^–9 moles per mole of dry air).     

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     

Isotope-selective ionization utilizing molecular alignment and non-resonant multiphoton ionization

We demonstrate laser nitrogen isotope separation, which is based on field-free alignment and angular-dependent ionization of ¹⁴N₂ and ¹⁵N₂ isotopologues. A linearly polarized short laser pulse (???~?795?nm, ?????~?60?fs) creates rotational wave packets in the isotopologues, which periodically revive with different revival times as a result of different moments of inertia. Another linearly polarized short laser pulse (???~?795?nm, ?????~?60?fs) ionizes one of the isotopologues selectively as a result of their different angular distributions. In the present experiments, the ion yield ratio R [=I(¹⁵N₂ ⁺)/I(¹⁴N₂ ⁺)] can be changed in the range from 0.85 to 1.22, depending on the time delay between the two laser pulses.     

Optimization of optical characteristics of In0.29Ga0.71As0.99N0.01/GaAs straddled nano-heterostructure

Designing of a nanoscale Quantum Well (QW) heterostructure with a well thickness of ～60?Å is critical for many applications and remains a challenge. This paper has a detailed study directed towards designing of In_0.29Ga_0.71As_0.99N_0.01/GaAs straddled nanoscale-heterostructure having a single QW of thickness ～60?Å and optimization of optical and lasing characteristics such as optical and mode gain, differential gain, gain compression, anti-guiding factor, transparency wavelength, relaxation oscillation frequency (ROF), optical power and their mutual variation behavior. The outcomes of the simulation study imply that for the carrier concentration of ～2?×?10¹⁸cm^?3 the optical gain of the nano-heterostructure is of 2100?cm^?1 at the wavelength is of 1.30?μm. Though the obtained gain is almost half of the gain of InGaAlAs/InP heterostructure, but from the wavelength point of view the InGaAsN/GaAs nano-heterostructure is also more desirable because the 1.30?μm wavelength is attractive due to negligible dispersion in the silica based optical fiber. Hence, the InGaAsN/GaAs nano-heterostructure can be very valuable in optical fiber based communication systems.     

Absolute frequency measurements of the 0002-0000, 2001-0000, and 1201-0000 bands of N2O by heterodyne spectroscopy

The absolute frequencies of 39 lines in the 00⁰2-00⁰0, 20⁰1-00⁰0, and 12⁰1-00⁰0 bands of N₂O in the range 4300–4800 cm^?1 have been measured by heterodyne frequency techniques. The lines were each measured in Doppler-limited absorption, with a color-center laser as a tunable probe of the N₂O and two stabilized CO₂ lasers as reference frequencies. New rovibrational constants have been fitted to these measurements. Tables of calculated transition frequencies are given, with estimated absolute uncertainties as small as 10^?4 cm^?1. The pressure shifts of four lines have been measured, and the values fall within the range of 0 to ?2 MHz/kPa (0 to ?0.2 MHz/Torr).     

Development of 385-μm D2O laser for plasma diagnostics

An optically-pumped 385-μm D₂O laser has been constructed for Thomson scattering ion temperature measurements in tokamak plasma. Stable single mode and stable tunable (over ±1 GHz) operation of the pump 9R (22) TEA CO₂ laser is performed by using an intracavity ZnSe etalon which is temperature-controlled within ±0.01°C. The saturation broadening with D₂O absorption line is observed for the first time using the tunable 9R (22) CO₂ laser. The pressure broadening coefficient of the N₂O absorption line is measured to be 7.6 MHz/torr using the ±1 GHz tunable 385-μm D₂O Raman laser. At 385 μm, an output quantum efficiency as high as 21% is obtained.     

Quartz-enhanced photoacoustic spectroscopy-based sensor system for sulfur dioxide detection using a CW DFB-QCL

Sulfur dioxide (SO₂) trace gas detection based on quartz-enhanced photoacoustic spectroscopy (QEPAS) using a continuous wave, distributed feedback quantum cascade laser operating at 7.24 μm was performed. Influence of water vapor addition on monitored QEPAS SO₂ signal was also investigated. A normalized noise equivalent absorption coefficient of NNEA (1σ) = 1.21 × 10^?8 cm^?1 W Hz^?1/2 was obtained for the ν ₃ SO₂ line centered at 1,380.93 cm^?1 when the gas sample was moisturized with 2.3 % H₂O. This corresponds to a minimum detection limit (1σ) of 63 parts per billion by volume for a 1 s lock-in time constant.     

Absolute frequency measurement of a water-stabilized diode laser at 1.384?��m by means of a fiber frequency comb

We report on the absolute frequency measurement of an extended-cavity diode laser stabilized against a Doppler-free vibration?Crotation transition of the H₂ ¹⁸O isotopologue near 1.384???m. Absolute determination is performed by comparing the water-stabilized laser frequency with respect to a GPS-disciplined Rb microwave standard by means of a self-referenced fiber-based optical frequency comb. The line center frequency of the 2_2,1??2_2,0 transition of the H₂ ¹⁸O ??₁+??₃ combination band is found to be 216?519?045?955(13)?kHz.     

N2O emissions and source processes in snow-covered soils in the Swiss Alps

Nitrous oxide (N₂O) emissions from snow-covered soils represent a significant fraction of the annual flux from alpine, subalpine or cold-temperate regions. In winter 2010–2011, we investigated the temporal variability of N₂O emissions and source processes from a subalpine valley in the Swiss Alps. The study included regular measurements of N₂O snow profiles at a fixed location and an intensive sampling campaign along a transversal cut through the valley with grassland at the bottom and coniferous forest at the slopes. During the intensive campaign, recently developed laser spectroscopy was employed for high-precision N₂O isotopomer analysis. Maximum N₂O fluxes (0.77±0.64 nmol m^?2 h^?1) were found for periods with elevated air temperature and, in contrast to our expectations, were higher from forest than from grassland in mid-February. At maximum snow height (63 cm) the main N₂O source processes were heterotrophic denitrification and nitrifier denitrification. The reduction of N₂O by heterotrophic denitrifiers was much more pronounced for the grassland compared with the forest soil, as indicated by the ¹⁵N site preferences of 16.4±11.5 ‰ (grassland) and?1.6±2.1 ‰ (forest). This illustrates the potential of laser spectroscopic N₂O isotopomer analysis for the identification of source processes even at low emission rates in nutrient poor ecosystems.