Sensitive detection of temperature behind reflected shock waves using wavelength modulation spectroscopy of CO2 near 2.7?μm

Tunable diode-laser absorption of CO₂ near 2.7 μm incorporating wavelength modulation spectroscopy with second-harmonic detection (WMS-2f) is used to provide a new sensor for sensitive and accurate measurement of the temperature behind reflected shock waves in a shock-tube. The temperature is inferred from the ratio of 2f signals for two selected absorption transitions, at 3633.08 and 3645.56 cm⁻¹, belonging to the ν ₁+ν ₃ combination vibrational band of CO₂ near 2.7 μm. The modulation depths of 0.078 and 0.063 cm⁻¹ are optimized for the target conditions of the shock-heated gases (P∼1–2 atm, T∼800–1600 K). The sensor is designed to achieve a high sensitivity to the temperature and a low sensitivity to cold boundary-layer effects and any changes in gas pressure or composition. The fixed-wavelength WMS-2f sensor is tested for temperature and CO₂ concentration measurements in a heated static cell (600–1200 K) and in non-reactive shock-tube experiments (900–1700 K) using CO₂–Ar mixtures. The relatively large CO₂ absorption strength near 2.7 μm and the use of a WMS-2f strategy minimizes noise and enables measurements with lower concentration, higher accuracy, better sensitivity and improved signal-to-noise ratio (SNR) relative to earlier work, using transitions in the 1.5 and 2.0 μm CO₂ combination bands. The standard deviation of the measured temperature histories behind reflected shock waves is less than 0.5%. The temperature sensor is also demonstrated in reactive shock-tube experiments of n-heptane oxidation. Seeding of relatively inert CO₂ in the initial fuel-oxidizer mixture is utilized to enable measurements of the pre-ignition temperature profiles. To our knowledge, this work represents the first application of wavelength modulation spectroscopy to this new class of diode lasers near 2.7 μm.     

Measurements of high-pressure CO2 absorption near 2.0 μm and implications on tunable diode laser sensor design

A tunable diode laser (TDL) is used to measure the absorption spectra of the R46 through R54 transitions of the 20012←00001 band of CO₂ near 2.0 μm (5000 cm⁻¹) at room temperature and pressures to 10 atm (densities to 9.2 amagat). Spectra are recorded using direct absorption spectroscopy and wavelength modulation spectroscopy with second-harmonic detection (WMS-2f) in a mixture containing 11% CO₂ in air. The direct absorption spectra are influenced by non-Lorentzian effects including finite-duration collisions which perturb far-wing absorption, and an empirical χ-function correction to the Voigt line shape is shown to greatly reduce error in the spectral model. WMS-2f spectra are shown to be at least a factor of four less-influenced by non-Lorentzian effects in this region, making this approach more resistant to errors in the far-wing line shape model and allowing a comparison between the spectral parameters of HITRAN and a new database which includes pressure-induced shift coefficients. The implications of these measurements on practical, high-pressure CO₂ sensor design are discussed.     

Wavelength-modulation spectroscopy near 2.5 μm for H2O and temperature in high-pressure and -temperature gases

The design and validation of a tunable diode laser (TDL) sensor for temperature and H₂O in high-pressure and -temperature gases are presented. High-fidelity measurements are enabled through the use of: (1) strong H₂O fundamental-band absorption near 2.5 μm, (2) calibration-free first-harmonic-normalized wavelength-modulation spectroscopy with second-harmonic detection (WMS-2f/1f), (3) an experimentally derived and validated spectroscopic database, and (4) a new approach to selecting the optimal wavelength and modulation depth of each laser. This sensor uses two TDLs near 2,474 and 2,482 nm that were fiber coupled in free space and frequency multiplexed to enable measurements along a single line-of-sight. The lasers were modulated at 35 and 45.5 kHz, respectively, to achieve a sensor bandwidth of 4.5 kHz. This sensor was validated in a shock tube at temperatures and pressures ranging from 1,000 to 2,700 K and 8 to 50 bar. There the sensor resolved transients and recovered the known steady-state temperature and H₂O mole fraction with a precision of 3.2 and 2.6 % RMS, respectively.     

Wavelength-modulation-spectroscopy for real-time, in situ NO detection in combustion gases with a 5.2 μm quantum-cascade laser

A mid-infrared absorption strategy with calibration-free wavelength-modulation-spectroscopy (WMS) has been developed and demonstrated for real-time, in situ detection of nitric oxide in particulate-laden combustion-exhaust gases up to temperatures of 700 K. An external-cavity quantum-cascade laser (ECQCL) near 5.2 μm accessed the fundamental absorption band of NO, and a wavelength-scanned, 1f-normalized WMS with second-harmonic detection (WMS-2f/1f) strategy was developed. Due to the external-cavity laser architecture, large nonlinear intensity modulation (IM) was observed when the wavelength was modulated by injection-current modulation, and the IM indices were also found to be strongly wavelength-dependent as the center wavelength was scanned with piezoelectric tuning of the cavity. A quantitative model of the 1f-normalized WMS-2f signal was developed and validated under laboratory conditions. A sensor was subsequently designed, built and demonstrated for real-time, in situ measurements of NO across a 3 m path in the particulate-laden exhaust of a pulverized-coal-fired power plant boiler. The 1f-normalized WMS-2f method proved to have better noise immunity for non-absorption transmission, than wavelength-scanned direct absorption. A 0.3 ppm-m detection limit was estimated using the R15.5 transition near 1927 cm⁻¹ with 1 s averaging. Mid-infrared QCL-based NO absorption with 1f-normalized WMS-2f detection shows excellent promise for practical sensing in the combustion exhaust.     

Performance evaluation of a near infrared QEPAS based ethylene sensor

A sensor based on quartz-enhanced photoacoustic spectroscopy (QEPAS) was evaluated for the detection of trace levels of ethylene at atmospheric pressure using a fiber coupled DFB diode laser emitting in the 1.62 μm spectral range. A noise-equivalent QEPAS signal of ∼4 ppm C₂H₄ was achieved for a 0.7 s data acquisition time using wavelength-modulation with a second-harmonic detection scheme on the strongest C₂H₄ absorption peak at 6177.14 cm⁻¹ with an average optical power of ∼15 mW. Improved detection sensitivity of 0.5 and 0.3 ppm C₂H₄ (1σ) was demonstrated using longer averaging time of 70 and 700 s, respectively. Important characteristics for the QEPAS based sensor operation in real-world conditions are presented, particularly the influence of external temperature variations. Furthermore, the response time of the ethylene sensor was measured in different configurations and it is shown that the QEPAS technique can provide a response time in a few seconds range even without active gas flow.     

Application of wavelength-scanned wavelength-modulation spectroscopy H2O absorption measurements in an engineering-scale high-pressure coal gasifier

A real-time, in situ water vapor (H₂O) sensor using a tunable diode laser near 1,352 nm was developed to continuously monitor water vapor in the synthesis gas of an engineering-scale high-pressure coal gasifier. Wavelength-scanned wavelength-modulation spectroscopy with second harmonic detection (WMS-2f) was used to determine the absorption magnitude. The 1f-normalized, WMS-2f signal (WMS-2f/1f) was insensitive to non-absorption transmission losses including beam steering and light scattering by the particulate in the synthesis gas. A fitting strategy was used to simultaneously determine the water vapor mole fraction and the collisional-broadening width of the transition from the scanned 1f-normalized WMS-2f waveform at pressures up to 15 atm, which can be used for large absorbance values. This strategy is analogous to the fitting strategy for wavelength-scanned direct absorption measurements. In a test campaign at the US National Carbon Capture Center, the sensor demonstrated a water vapor detection limit of ~800 ppm (25 Hz bandwidth) at conditions with more than 99.99 % non-absorption transmission losses. Successful unattended monitoring was demonstrated over a 435 h period. Strong correlations between the sensor measurements and transient gasifier operation conditions were observed, demonstrating the capability of laser absorption to monitor the gasification process.     

N2O molecular tagging velocimetry

A new seeded velocity measurement technique, N₂O molecular tagging velocimetry (MTV), is developed to measure velocity in wind tunnels by photochemically creating an NO tag line. Nitrous oxide “laughing gas” is seeded into the air flow. A 193 nm ArF excimer laser dissociates the N₂O to O(¹D) that subsequently reacts with N₂O to form NO. O₂ fluorescence induced by the ArF laser “writes” the original position of the NO line. After a time delay, the shifted NO line is “read” by a 226-nm laser sheet and the velocity is determined by time-of-flight. At standard atmospheric conditions with 4% N₂O in air, ∼1000 ppm of NO is photochemically created in an air jet based on experiment and simulation. Chemical kinetic simulations predict 800–1200 ppm of NO for 190–750 K at 1 atm and 850–1000 ppm of NO for 0.25–1 atm at 190 K. Decreasing the gas pressure (or increasing the temperature) increases the NO ppm level. The presence of humid air has no significant effect on NO formation. The very short NO formation time (<10 ns) makes the N₂O MTV method amenable to low- and high-speed air flow measurements. The N₂O MTV technique is demonstrated in air jet to measure its velocity profile. The N₂O MTV method should work in other gas flows as well (e.g., helium) since the NO tag line is created by chemical reaction of N₂O with O(¹D) from N₂O photodissociation and thus does not depend on the bulk gas composition.     

Hydroxyl ion conduction in ytterbium-doped strontium cerate at T<580 K in H2O/air atmosphere

Open circuit voltage (OCV) measurements in H₂O/air concentration cells at T<580 K using Yb-doped SrCeO₃ electrolyte indicate that under these conditions, protons are transported through the electrolyte as -ve ions, possibly as hydroxyl (OH⁻) ions. The H⁺ ionic transport, which is generally reported, becomes the dominant mode for H₂O/air concentration cells at temperatures greater than 750 K or when H₂O/air electrodes are replaced by H₂/Ar, and the anomalous OCV sign disappears. The combination of low temperature and the presence of hydrogen and oxygen as provided by the H₂O/air system appears to be necessary for the postulated hydroxyl ion electrode reactions to take place. In addition to OCV measurements, results from impedance spectroscopy are used to provide evidence in support of the suggested hydroxyl ion mode of protonic transport under the specified conditions. These findings are directly relevant in the development of novel humidity sensors in the temperature range 450–580K and is reported in a separate paper in this conference. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Electrical conductivity of BaCe0.9Nd0.1O3−α in H2+H2O+Ar gas mixture

Electroconductivity of BaCe_0.9Nd_0.1O_3−α was studied as a function of the composition of the H₂+H₂O+Ar mixture and temperature in the interval from 873 to 1173 K. It was shown that the electroconductivity was independent of P_H2 (0.97 to 0.10 atm) and P_O2 (10⁻²¹ to 10⁻²⁶ atm), but depended on P_H2O (0.08 to 0.005 atm). A mathematical processing of the P_H2O dependencies of the electroconductivity, which was performed in terms of a classical model of defect formation in high-temperature proton-conducting solid electrolytes, yielded equilibrium constants of the reaction of water dissolution in BaCe_0.9Nd_0.1O_3−α and mobilities of protons and oxygen ions. The temperature dependencies of these quantities were used to determine the mobility activation energies of protons (E_a=34±7 kJ/mole) and oxygen ions (E_a=72±8 kJ/mole), and also the enthalpy (ΔH=−150±25 kJ/mole) and the entropy (ΔS=153±26 kJ/mole·K) of the reaction of water dissolution in BaCe_0.9Nd_0.1O_3−α.     

Absorption measurements of H2O at high temperatures using a CO laser

Absorption of CO laser radiation (v = 8→7, J = 14→15 transition at 1901.762 cm^-1) by H₂O has been studied in shock-heated H₂/O₂/Ar mixtures over the temperature range 1300–2300 K. This laser transition is nearly coincident with the v₂-band 12_3,10←11_2,9 transition of H₂O at 1901.760 cm^-1, thereby providing a convenient and sensitive absorption-based H₂O diagnostic useful for studies of combustion. The collision-broadening parameter for this H₂O line, due to broadening by Ar, was determined to be 2γ (cm^-1atm^-1) = 0.027 (T/1300)^-0.9 in the temperature range 1300–2300 K. Calculations of the H₂O absorption coefficient (at 1901.762 cm^-1) based on this expression for 2γ are presented for the temperature range 300–2500 K and pressure range 0.3–1 atm.     

Epitaxial growth of CeO2 on (100) InP using reactive r.f. magnetron sputtering

The epitaxial growth of CeO₂ thin films has been realized on (100) InP substrates using reactive r.f. magnetron sputtering. Oxide films were nucleated in the presence of molecular hydrogen (4% H₂/Ar sputtering gas) in order to reduce the native oxide formation on the InP surface, which interferes with CeO₂ epitaxy. A metal cerium target was used as the cation source, with water vapor serving as the oxidizing species. Epitaxial films were sputter-deposited at a substrate temperature of 550 °C in a H₂O vapor pressure of approximately 10^-3 Torr. Crystallinity of the oxide films was examined using θ–2θ X-ray diffraction, ω-rocking curves, and in-plane φ-scans. The best results were obtained when the initial nucleation layer was deposited with P(H₂O)<10^-5 Torr, followed by deposition at P(H₂O)=10^-3 Torr. The epitaxial growth of CeO₂ on InP could prove enabling in efforts to integrate functional oxides with InP-based optoelectronic and microwave technologies. Received: 20 February 20002 / Accepted: 21 February 2002 / Published online: 19 July 2002     

Impact of humidity on quartz-enhanced photoacoustic spectroscopy based detection of HCN

The architecture and operation of a trace hydrogen cyanide (HCN) gas sensor based on quartz-enhanced photoacoustic spectroscopy and using a λ=1.53 μm telecommunication diode laser are described. The influence of humidity content in the analyzed gas on the sensor performance is investigated. A kinetic model describing the vibrational to translational (V–T) energy transfer following the laser excitation of a HCN molecule is developed. Based on this model and the experimental data, the V–T relaxation time of HCN was found to be (1.91±0.07)10^-3 s Torr in collisions with N₂ molecules and (2.1±0.2)10^-6 s Torr in collisions with H₂O molecules. The noise-equivalent concentration of HCN in air at normal indoor conditions was determined to be at the 155-ppbv level with a 1-s sensor time constant. PACS 82.80.Kq; 42.62.Fi     

Detection of trace gases by rapidly-swept continuous-wave cavity ringdown spectroscopy: pushing the limits of sensitivity

Cavity ringdown (CRD) absorption spectroscopy enables spectroscopic sensing of gases with a high sensitivity and accuracy. Instrumental improvements result in a new high-performance continuous-wave (cw) CRD spectrometer using a rapidly-swept cavity of simple design. It employs efficient data-acquisition procedures, high-reflectivity mirrors, a low-adsorption flow cell, and various compact fibre-optical components in a single-ended transmitter-receiver configuration suitable for remote sensing. Baseline noise levels in our latest cw-CRD experiments yield a competitive noise-equivalent absorption limit of ∼5×10^-10 cm^-1Hz^-1/2, independent of whatever molecules are to be detected. Measurements in the near-infrared wavelength range of 1.51–1.56 μm yield sub-ppmv (i.e., ppbv or better) sensitivity in the gas phase for several representative molecules (notably CO₂, CO, H₂O, NH₃, C₂H₂, and other hydrocarbons). By measuring spectroscopic features in the 1.525 μm band of C₂H₂ gas, we realise detection limits of 19 nTorr (2.5×10^-11 atm) of neat C₂H₂ (Doppler-limited at low pressure) and 0.37 ppbv of C₂H₂ in air (pressure-broadened at 1 atm). Our cw-CRD spectrometer is a high-performance sensor in a relatively simple, low-cost, compact instrument that is amenable to chemical analysis of trace gases in medicine, agriculture, industry, and the environment. PACS 07.07.Df; 07.57.Ty; 42.62.Fi     

Multi-species laser absorption sensors for in situ monitoring of syngas composition

Tunable diode laser absorption spectroscopy sensors for detection of CO, CO₂, CH₄ and H₂O at elevated pressures in mixtures of synthesis gas (syngas: products of coal and/or biomass gasification) were developed and tested. Wavelength modulation spectroscopy (WMS) with 1f-normalized 2f detection was employed. Fiber-coupled DFB diode lasers operating at 2325, 2017, 2290 and 1352 nm were used for simultaneously measuring CO, CO₂, CH₄ and H₂O, respectively. Criteria for the selection of transitions were developed, and transitions were selected to optimize the signal and minimize interference from other species. For quantitative WMS measurements, the collision-broadening coefficients of the selected transitions were determined for collisions with possible syngas components, namely CO, CO₂, CH₄, H₂O, N₂ and H₂. Sample measurements were performed for each species in gas cells at a temperature of 25 °C up to pressures of 20 atm. To validate the sensor performance, the composition of synthetic syngas was determined by the absorption sensor and compared with the known values. A method of estimating the lower heating value and Wobbe index of the syngas mixture from these measurements was also demonstrated.     

Excimer ArF laser with an output energy of 1.3 J at 2.0% efficiency on the He:Ar:F2 mixture

In this paper it is shown that to achieve a maximum efficiency and high output energy of an ArF (193 nm) excimer laser, one should use optimal pump intensity. It has been shown experimentally that the optimal pump intensity for an ArF excimer laser with the mixture of He:Ar:F₂ has a value of 4.5–5.0 MW/cm³. The results of an experimental study of the pump and active medium parameters effect on the efficiency and output energy of the ArF excimer laser on the mixture of He:Ar:F₂ are presented. To provide high pump intensity of an active medium, the excitation scheme of the LC-inverter type has been used where the current return conductor inductance had been increased from 30 to 80 nH. This allows the pump to achieve levels of intensity above 5.0 MW/cm³. By using the pump intensity of 5.0 MW/cm³ in an active medium of He:Ar:F₂–79.7:20:0.3 at total pressure of 2.4 atm, we are the first to obtain the output energy of 1.3 J at the total efficiency of 2.0%. The pulse duration (FWHM) was 15±1 ns and the peak pulse power was 85 MW. PACS 42.55.Lt; 42.60.Lh     

Apodized 2f/1f wavelength modulation spectroscopy method for calibration-free trace detection of carbon monoxide in the near-infrared region: theory and experiment

We introduce the basics of an apodized 2f/1f wavelength modulation method for the spectroscopy of the R(9) transition line in the first overtone band of carbon monoxide (¹²C¹⁶O) in near-infrared (NIR) region around 2.33 μm. Performance of the method is investigated for high gas concentrations beyond the optically thin limit to generalize common 2f/1f wavelength modulation spectroscopy (WMS) reported by Rieker et al. (Appl Opt 48:5546, [28]). Numerical simulations are performed based on real experimental parameters associated with a NIR spectrometer designed in our laboratory. The results primarily show a more linear response and less error than occurred in the common WMS-2f/1f method for an optically thick sample. It is also theoretically shown that the apodized method enables sharpening the spectrum without peak displacement compared to the common WMS-2f/1f method. The validity of the method is verified experimentally by the trace detection of an air-broadened R(9) CO absorption line centered at 4,294.637 cm^?1 at atmospheric pressure and room temperature. The effect of a so-called scaling k-factor on the sharpening of WMS-2f/1f signal is investigated through trace simulation and detection of CO and methane (CH₄) lines in the scanning range of a distributed feedback laser. The obtained results show very good agreement between simulation and experiment.     

Linear and nonlinear optical properties of yttrium formate dihydrate,Y(HCOO)<Subscript>3</Subscript>·2H<Subscript>2</Subscript>O

Yttrium formate dihydrate, Y(HCOO)₃·2H₂O (point group 222), a promising new material for Raman laser frequency converters, has been reinvestigated with respect to the linear and nonlinear optical (second-harmonic generation, SHG) properties. High-precision data of refractive indices and their dispersion in the wavelength range 365–1083 nm are given, together with the three independent components of the tensor of nonlinear optical susceptibility (SHG) [d_ijk ^SHG]. A calculation of the macroscopic nonlinear optical susceptibility from the hyperpolarizabilities of the formate groups results in good agreement with the experimental values. This signals that the nonlinear optical interaction in Y(HCOO)₃·2H₂O can mainly be attributed to the formate groups. PACS 42.65.Ky; 42.70.Mp; 78.20.Ci     

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     

Initiation of nuclear reactions under laser irradiation of Au nanoparticles in the aqueous solution of Uranium salt

Laser exposure of a suspension of either gold or palladium nanoparticles in aqueous solutions of UO₂Cl₂ of natural isotope abundance was experimentally studied. Picosecond Nd:YAG lasers at peak power of 10¹¹–10¹³ W/cm² at the wavelength of 1.06–0.355 μm were used as well as a visible-range Cu vapor laser at a peak power of 10¹⁰ W/cm². The composition of colloidal solutions before and after laser exposure was analyzed using atomic absorption and gamma spectroscopy in the 0.06–1 MeV range of photon energy. Real-time gamma spectroscopy was used to characterize the kinetics of nuclear reactions during laser exposure. It was found that laser exposure initiated nuclear reactions involving both ²³⁸U and ²³⁵U nuclei via different channels in H₂O and D₂O. The influence of saturation of both the liquid and nanoparticles by gaseous H₂ and D₂ on the kinetics of nuclear transformations was found. Possible mechanisms of observed processes are discussed.     

Diode-laser measurements and calculations of CO 1-0 P(4) line broadening in the 294- to 765-K temperature range

We present first diode-laser measurements of C¹²O¹⁶ 1-0 P(4) line broadening by Ar, N₂ and H₂O at high temperatures, together with the line-intensity value at 300 K. A T^-β dependence of line-widths on temperature is deduced from measurements in the 294- to 765-K and 0.2- to 1.2- atm ranges (β = 0.69±0.02, β = 0.69±0.02, β = 0.59±0.05 for Ar, N₂ and H₂O broadening, respectively). Calculations for CO-Ar and CO-N₂ with the model proposed by Robert and Bonamy give very satisfactory results.