Chemical and isotopic analysis using diode laser spectroscopy: applications to volcanic gas monitoring

We report on a diode-laser-based spectrometer, operating at a wavelength of 2 μm, which enables for simultaneous measurements of CO₂ and H₂O concentrations in gaseous mixtures with high precision and accuracy. We implemented the very simple approach of a direct measurement of absorption in a multiple reflection cell with a pathlength of 50 m. Gas concentration is simply retrieved from the integrated absorbance, without using calibration cells. The spectrometer is equipped with single-mode optical fibers in order to probe remote environments. The possible application in volcanic areas will be discussed. We also investigated the possibility to measure the isotopic ratio ¹³CO₂/¹²CO₂, which is another relevant parameter for monitoring volcanic activity.     

Defect reduction and surface passivation of SiO2/Si by heat treatment with high-pressure H2O vapor

Heat treatment with high-pressure H₂O vapor was applied to improve interface properties of SiO₂/Si and passivate the silicon surface. Heat treatment at 180–420 °C with high-pressure H₂O vapor changed SiO_x films, 150 nm thick formed at room temperature by thermal evaporation in vacuum, into SiO₂ films with a Si-O-Si bonding network similar to that of thermally grown SiO₂ films. Heat treatment at 130 °C with 2.8×10⁵ Pa H₂O for 3 h reduced the recombination velocity for the electron minority carriers from 405 cm/s (as-fabricated 150-nm-thick SiO_x/Si) to 5 cm/s. Field-effect passivation was demonstrated by an additional deposition of defective SiO_x films on the SiO₂ films formed by heat treatment at 340 °C with high-pressure H₂O vapor. The SiO_x deposition reduced the recombination velocity from 100 cm/s to 48 cm/s. Received: 1 March 1999 / Accepted: 28 March 1999 / Published online: 24 June 1999     

Wavelength-agile fiber laser using group-velocity dispersion of pulsed super-continua and application to broadband absorption spectroscopy

A swept-wavelength source is created by connecting four elements in series: a femtosecond fiber laser at 1.56 μm, a non-linear fiber, a dispersive fiber and a tunable spectral bandpass filter. The 1.56-μm pulses are converted to super-continuum (1.1–2.2 μm) pulses by the non-linear fiber, and these broadband pulses are stretched and arranged into wavelength scans by the dispersive fiber. The tunable bandpass filter is used to select a portion of the super-continuum as a scan-wavelength output. A variety of scan characteristics are possible using this approach. As an example, an output with an effective linewidth of approximately 1 cm^-1 is scanned from 1350–1550 nm every 20 ns. Compared to previous scanning benchmarks of approximately 1 nm/μs, such broad, rapid scans offer new capabilities: a gas sensing application is demonstrated by monitoring absorption bands of H₂O, CO₂, C₂H₂ and C₂H₆O at a pressure of 10 bar. Received: 5 August 2002 / Revised version: 23 September 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +1-608/265-2316, E-mail: ssanders@engr.wisc.edu     

High-throughput study of the influence of H2O and CO2 on the performance of nitrogen storage and reduction (NSR) catalysts

This paper describes the application of parallel high-throughput experimentation based on FTIR spectral imaging to a tolerance study for NO_x storage and reduction (NSR) catalysts with respect to CO₂ and H₂O in the feed. It was found that both gases decrease the storage capacity of platinum/barium based NSR catalysts, with H₂O having a stronger effect than CO₂.     

Breath ammonia detection based on tunable fiber laser photoacoustic spectroscopy

A new detection method for ammonia in high concentration of CO₂ and H₂O is reported, which uses a wavelength modulated photoacoustic spectrometer based on a near-infrared tunable erbium-doped fiber laser in combination with an optical fiber amplifier. The multi-wavelength (1522.44 nm, 1522.94 nm and 1545.05 nm) photoacoustic signal measurement is established to detect multi-spectrum signal in samples. The problem of ammonia detection in high concentration of CO₂ and H₂O is resolved at atmospheric pressure. The minimum detection limit of 16 ppb (signal-to-noise ratio = 1) in simulated breath samples (5.3% CO₂ and 6.2% H₂O (100% relative humidity at 37°C)) is achieved.     

Diode laser spectroscopy of H2O and CO2 in the 1.877-μm region for the in situ monitoring of the Martian atmosphere

A diode laser spectrometer was used in the laboratory to study H₂O and CO₂ line intensities and self-broadening coefficients around 1.877 μm. The spectral region ranging from 5327 cm^-1 to 5329 cm^-1, which is suitable for the in situ sensing of water vapor and carbon dioxide in the Martian atmosphere, was studied using a distributed feedback GaInSb diode laser from Nanoplus GmbH. We have studied one line from the (011)←(000)band of H₂O and two lines from the (01¹2)_I←(000) band of CO₂. The results of intensity and self-broadening measurements are compared to available databases, ab initio calculations and previous experimental determinations. Finally, we discuss the current development of the tunable diode laser absorption spectrometer instrument, a laser diode sensor devoted to the in situ measurement of H₂O and CO₂ in the Martian atmosphere. PACS 07.57.Ty; 07.87.+v     

Excitation of inner-shell states of CO2, COS,CS2, N2O and C2H2 by low energy electron impact

The design of an electron impact spectrometer for use in the study of inner-shell electric-dipole-forbidden transitions is described. The use of the spectrometer to investigate spin-forbidden transitions to inner-shell excited states of CO₂, COS, CS₂, N₂O and C₂H₂ is reported. For all these molecules, excitation of inner-shell triplet states has been observed.     

Near infrared diode laser spectroscopy of C2H2, H2O, CO2 and their isotopologues and the application to TDLAS, a tunable diode laser spectrometer for the martian PHOBOS-GRUNT space mission

A near-infrared tunable diode laser spectrometer called TDLAS has been developed that combines telecommunication-type as well as new-generation antimonide laser diodes to measure C₂H₂, H₂O, CO₂ and their isotopologues in the near infrared. This sensor is devoted to the in situ analysis of the soil of the Martian satellite PHOBOS, within the framework of the Russian space mission PHOBOS-GRUNT. In the first part of the paper, we report accurate spectroscopic measurements of C₂H₂ and ¹³C¹²CH₂ near 1.533 μm, of H₂O and CO₂ at 2.682 μm and of the isotopologues ¹³C¹⁶O₂ and ¹⁶O¹²C¹⁸O near 2.041 μm and H₂ ¹⁷O, H₂ ¹⁸O and HDO near 2.642 μm. The achieved line strengths are thoroughly compared to data from molecular databases or from former experimental determinations. In the second part of the paper, we describe the TDLAS spectrometer for the PHOBOS-GRUNT mission.     

Isotope-analytical results of a study of gas generation in L/ILW

The paper presents the results of a long-term measurement series using hermetic containers to make more precise quantitative estimation of the generation rates and radioactivity of the gas in a drum of low and intermediate level radioactive waste (L/ILW) packages. Development of special preparation lines and isotope-analytical measurements of the headspace gas samples were performed in the ATOMKI. Stable isotope measurements were executed from the CO₂ and CH₄ fractions by stable isotope ratio mass spectrometer. Noble gas (He) measurements were done by noble gas mass spectrometer. The tritium content of the vapour, H₂ and CH₄ fractions was measured in H₂O chemical form by a low background liquid scintillation counter. The ¹⁴C content of the CO₂ and CH₄ fractions of the headspace gas samples was measured by a low background gas proportional counter system.     

Pressure broadening coefficients of induced by for Venus atmosphere

Carbon dioxide (CO₂) induced pressure broadening coefficients of water vapor (H₂O) lines have been determined using a terahertz time-domain spectrometer (TDS). Thirty-two rotational transitions of H₂O were observed in the spectral range of 18– (550–3050 GHz) for the first time. Using TDS allows one to measure absorption spectra with one order of magnitude better precision than Fourier transform spectrometer in this frequency region. The precision of our broadening coefficient measurements was 2.4% in average. The measured CO₂ induced pressure broadening coefficients are compared to those calculated by the complex Robert–Bonamy formalism. The difference between the measurement and the theoretical estimation was in the range of -10.7% to +19.0% confirming the credibility of the theoretical approach. The impact on retrieval of water vapor abundance was examined by performing inversion analysis on H₂O spectra of Venus atmosphere obtained with the Submillimeter Wave Astronomy Satellite. In this example case, the retrieved water vapor mixing ratio reduces by half at the altitude region of 70–85 km when applying the newly measured broadening coefficient compared to the air-broadening coefficient, and changes by 5% compared to that estimated by the complex Robert–Bonamy formalism.     

Synthesis of basic magnesium carbonate microrods with a “house of cards” surface structure using rod-like particle template

Basic magnesium carbonate (Mg₅(CO₃)₄(OH)₂·4H₂O) microrods with a surface structure of “house of cards” have been synthesized without any alkaline reagent, using rod-like particles, magnesium carbonate trihydrate, as templates. The product was characterized by X-ray diffraction (XRD) and field-emission scanning electron microscopy (FE-SEM). The transformation process from rod-like MgCO₃·3H₂O particles to Mg₅(CO₃)₄(OH)₂·4H₂O microrods with a surface structure of “house of cards” was recorded. Preliminary discussions on possible growth mechanisms of Mg₅(CO₃)₄(OH)₂·4H₂O microrods are also proposed in this paper.     

Inelastic neutron scattering from H2 on vapour deposited CO2

Inelastic neutron scattering spectra at 17 meV and 68 meV incident energies of molecular hydrogen adsorbed on vapour deposited substrates of pure CO₂, 90:10 and 50:50 CO₂:Kr are reported. The ortho-para transition is shifted from 14.7 meV in the free H₂-molecule to 9.4 meV in a presumably commensurate ortho-H₂ monolayer on the CO₂ surface. The quadrupole-quadrupole interaction of ortho-H₂ molecules with the CO₂ substrate results in a strongly anisotropic potential. In addition to rotations the dynamics of this layer comprise a local Einstein mode and phonons in resonance with the substrate, giving rise to intense multiphonon transitions. Quasielastic scattering on warmer samples is assigned to a liquidlike adsorption layer, in which the H₂ rotations are strongly perturbed. Received: 15 October 1996 / Revised: 25 August 1997 / Accepted: 24 October 1997     

CO2 laser CVD of a-Si:H: in situ gas analysis and model calculations

4 and disilane Si₂H₆ induced by continuous wave CO₂ laser irradiation has been investigated under the conditions of chemical vapor deposition (CVD) of amorphous hydrogenated silicon a-Si:H. At the very position of depositing the thin film the stationary chemical composition of the processing gas is probed in situ by an effusive molecular beam which passes through a differential pumping stage into a quadrupole mass spectrometer (QMS). With SiH₄ as educt and SF₆ as a sensitizer, SiH₄ and Si₂H₆ are found in the processing gas while Si₃H₈ or higher silanes are lacking. Si₂H₆ and SF₆ lead to SiH₄, Si₂H₆, and Si₃H₈, but higher silanes are missing. The experimentally determined composition of the processing gas is semi-quantitatively reproduced by model calculations based on the assumption of stationary local equilibrium conditions and applying thermodynamic and spectroscopic data (molecular statistics). The mass balance of the processing gas entering and leaving the CVD chamber states an atomic ratio Si:H of 1:2 for the gas phase species forming the solid deposit on the reactor walls. This finding together with theoretical considerations indicates the intermediate Si₂H₄ to be the dominating gas phase species forming the a-Si:H thin films. Received: 17 July 1998/Accepted: 20 July 1998     

Development of a cw-laser-based cavity-ringdown sensor aboard a spacecraft for trace air constituents

The progress in the development of a sensor for the detection of trace air constituents to monitor spacecraft air quality is reported. A continuous-wave (cw), external-cavity tunable diode laser centered at 1.55 μm is used to pump an optical cavity absorption cell in cw-cavity ringdown spectroscopy (cw-CRDS). Preliminary results are presented that demonstrate the sensitivity, selectivity and reproducibility of this method. Detection limits of 2.0 ppm for CO, 2.5 ppm for CO₂, 1.8 ppm for H₂O, 19.4 ppb for NH₃, 7.9 ppb for HCN and 4.0 ppb for C₂H₂ are calculated. Received: 3 April 2002 / Revised version: 3 June 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +1-202/994-5873, E-mail: Houston@gwu.edu     

A diode-laser-based spectrometer for in-situ measurements of volcanic gases

We report the first results of the field operation of a novel, portable diode-laser spectrometer for gas-concentration measurements in volcanic areas. Remote detection of direct absorption line shapes was possible thanks to a telecom single-mode optical fiber that delivered radiation from a room-temperature distributed-feedback diode laser, emitting at 1.997 m, to an open-path multiple-reflection cell, placed on gas effluxes. The system was deployed on two different active volcanoes in Italy, where simultaneous and continuous monitoring of CO₂ and H₂O concentrations has been demonstrated. PACS 42.62.Fi; 07.60.Vg; 93.85.+q     

High-resolution investigation of the weak ν1+3ν21-ν21+ν3 band of CO2 around 2 μm

Fundamental spectroscopical parameters of the weak ν₁+3ν₂ ¹-ν₂ ¹+ν₃ band of CO₂ are reported using a high-resolution, direct-absorption spectrometer, based on a distributed feed-back diode laser emitting at 2 μm. Line intensities and self-broadening coefficients have been measured for the first time with high accuracy, for nine lines of the R branch, from R(44) up to R(59). Comparison with available data has been made, and a generally good agreement has been found. Received: 30 August 1999 / Published online: 24 March 2000     

Dilution effects analysis of opposed-jet H2/CO syngas diffusion flames

This paper reported the analysis of dilution effects on the opposed-jet H₂/CO syngas diffusion flames. A computational model, OPPDIF coupled with narrowband radiation calculation, was used to study one-dimensional counterflow syngas diffusion flames with fuel side dilution from CO₂, H₂O and N₂. To distinguish the contributing effects from inert, thermal/diffusion, chemical, and radiation effects, five artificial and chemically inert species XH₂, XCO, XCO₂, XH₂O and XN₂ with the same physical properties as their counterparts were assumed. By comparing the realistic and hypothetical flames, the individual dilution effects on the syngas flames were revealed. Results show, for equal-molar syngas (H₂/CO = 1) at strain rate of 10 s^?1, the maximum flame temperature decreases the most by CO₂ dilution, followed by H₂O and N₂. The inert effect, which reduces the chemical reaction rates by behaving as the inert part of mixtures, drops flame temperature the most. The thermal/diffusion effect of N₂ and the chemical effect of H₂O actually contribute the increase of flame temperature. However, the chemical effect of CO₂ and the radiation effect always decreases flame temperature. For flame extinction by adding diluents, CO₂ dilution favours flame extinction from all contributing effects, while thermal/diffusion effects of H₂O and N₂ extend the flammability. Therefore, extinction dilution percentage is the least for CO₂. The dilution effects on chemical kinetics are also examined. Due to the inert effect, the reaction rate of R84 (OH+H₂ = H+H₂O) is decreasing greatly with increasing dilution percentage while R99 (CO+OH→CO₂+H) is less affected. When the diluents participate chemically, reaction R99 is promoted and R84 is inhibited with H₂O addition, but the trend reverses with CO₂ dilution. Besides, the main chain-branching reaction of R38 (H+O₂→O+OH) is enhanced by the chemical effect of H₂O dilution, but suppressed by CO₂ dilution. Relatively, the influences of thermal/diffusion and radiation effects on the reaction kinetics are then small.     

Simultaneous online detection of isoprene and isoprene-d2 using infrared photoacoustic spectroscopy

A photoacoustic spectrometer for the simultaneous detection of isoprene and the deuterated species [4,4-²H]-2-methyl-1,3-butadiene (isoprene-d₂) is presented. Using a sealed-off ¹³CO₂ laser a single-component detection limit of 400 ppt (isoprene) and 600 ppt (isoprene-d₂) was achieved. Simultaneous monitoring of both compounds allowed the detection of labelling levels down to 6% (isoprene-d₂ in total isoprene) with a time resolution of 3 min. In emission studies with Eucalyptus globulus, the deuterated precursor [4,4-²H]-1-deoxy-D-xylulose was fed to a leaf through the transpiration stream. Emission of isoprene-d₂ started as early as 10 min after application of the precursor. Received: 3 May 2002 / Revised version: 31 May 2002 / Published online: 21 August 2002 RID="*" ID="*"Corresponding author. Fax: +49-228/733474, E-mail: frank.kuehnemann@iap.uni-bonn.de     

Use of High-Resolution Mass Spectrometry to Identify Products from Microwave Discharges in COAL-D2O Mixtures

Reactions of carbonaceous materials and H O in microwave discharges are known to produce H₂, HCN, CO, CO₂, and light hydrocarbon gases (primarily C₁ and C₂) in varying amounts. To determine if the solid or the H₂O is the source of hydrogen in formation of the above products, Fu and Blaustein reacted coal and graphite with D₂O.¹ Low-resolution mass spectra of the gaseous products from the D₂O experiments indicated the possibility of non-deuterated and corresponding deuterated compounds in the reaction mixture. Conventional separation and analytical techniques are not applicable to mixtures of this type. This communication describes the use of a high-resolution mass spectrometer, operated at a resolution 35 percent less than theoretically required for separation of the H₂-D doublet, to electrically measure precise masses for mixtures containing micromole amounts of deuterated and non-deuterated light gases.     

Laser photoacoustic spectroscopy of biosystems gas exchange with the atmosphere

2 evolution by some plants exposed to elevated concentration of pollutants and pressure decrease is studied using a photoacoustic spectrometer with a CO₂ laser. The measurements show a considerable CO₂ evolution by all kinds of the test plants. The quantity of CO₂ emitted by pea seedlings at 8 kPa, for example, exceeds the control one by about 20 times (24 h after the exposure start). The exposure of pea seedlings to C₂H₄ and O₃ at various concentrations also increases CO₂ evolution: the 48-h exposure of test plants to C₂H₄ (at 0.01 ppm) increases CO₂ evolution by approximately 100% with respect to the control plants. Received: 09 April 1998