Nonlinear susceptibility interference in laser SRS-CARS diagnostics of hydrogen in gas mixtures

We have studied the possibility to measure small hydrogen concentrations in dense gas mixtures by method of coherent anti-Stokes Raman scattering (CARS) in combination with biharmonic laser pumping by means of stimulated Raman scattering (SRS). It was found that the interference of nonlinear susceptibilities of a buffer gas and hydrogen can lead to a parabolic dependence of the signal intensity on the hydrogen concentration, which makes the results of analyses uncertain. The ambiguity can be eliminated by selecting an appropriate composition and pressure of the gas mixture in the cell of the SRS generator of biharmonic laser pumping. Using this approach, hydrogen in air at atmospheric pressure can be detected by the laser SRS-CARS technique on a level of 5 ppm.     

Investigation of two-photon-induced polarization spectroscopy of the a-X (1,0) transition in molecular nitrogen at elevated pressures

Two-photon-induced polarization spectroscopy of molecular nitrogen in the alpha 1IIg(nu' =) <-- X 1Sigma(g)+ (nu" =) system near 283 nm was performed, and its signal dependence investigated over the pressure range from 1.2 to 5 bars at 300 K. A significant increase of the signal intensity with pressure beyond the expected square law for a two-photon process was observed for pure nitrogen. Similar behavior was also found for a constant nitrogen partial pressure with increasing partial pressures of argon buffer gas. In both cases the spectral linewidth of the excited transitions increased dramatically with overall pressure. A possible explanation is given for the observed behavior in terms of contributions to the nonlinear susceptibility of the medium from the population of one-photon resonantly absorbing excited-state nitrogen and ground state N(2)(+) ions created in the multiphoton absorption process at the high laser intensities required.     

Simultaneous and time-resolved temperature and relative CO2-N2 and O2-CO2-N2 concentration measurements with pure rotational coherent anti-Stokes Raman scattering for pressures as great as 5 MPa

Pure rotational coherent anti-Stokes Raman-scattering (CARS) measurements have been performed in binary CO2-N2 and ternary CO2-O2-N2 mixtures in a temperature range between 300 and 773 K and pressures from 0.1 to 5 MPa to prove its potential for simultaneous single-shot thermometry and multispecies concentration measurements. In pressurized systems the CO2 component has a strong spectral influence on the pure rotational CARS spectra. Because of this dominance, pure rotational CARS proves to be a sensitive tool to measure in high-pressure combustion systems and the relative CO2-N2 concentration in the lower temperature range simultaneously with the temperature and the relative O2-N2 concentration. The evaluation of the spectra utilized a least-sum-squared differences fit of the spectral shape, weighted either constantly or inversely with respect to the normalized signal intensity. The results of the simultaneous temperature and relative CO2-N2 and O2-CO2-N2 concentration measurements provided a good accuracy and precision both in temperature and in concentrations. Because of the strong increase in the relative spectral contribution of CO2 with rising pressure, the precision of the CO2 concentration determination is in general significantly improved toward higher pressures, thus also clearly enhancing the CO2 detectability. The influence of temperature, O2 and CO2 concentration, pressure, and the evaluation techniques employed on both the accuracy and the precision is explained as well as their cross dependencies. The influence and limitations of the approximations used to model the CO2 molecule are discussed.     

Development of high-resolution n(2) coherent anti-stokes Raman scattering for measuring pressure, temperature, and density in high-speed gas flows

Mean and instantaneous measurements of pressure, temperature, and density have been acquired in an optically accessible gas cell and in the flow field of an underexpanded sonic jet by use of the high-resolution N(2) coherent anti-Stokes Raman scattering (CARS) technique. This nonintrusive method resolves the pressure- and temperature-sensitive rotational transitions of the nu = 0 ? 1 N(2) Q-branch to within Domega = 0.10 cm(-1). To extract thermodynamic information from the experimental spectra, theoretical spectra, generated by a N(2) spectral modeling program, are fit to the experimental spectra in a least-squares manner. In the gas cell, the CARS-measured pressures compare favorably with transducer-measured pressures. The precision and accuracy of the single-shot CARS pressure measurements increase at subatmospheric conditions. Along the centerline of the underexpanded jet, the agreement between the mean CARS P/T/rho measurements and similar quantities extracted from a Reynolds-averaged Navier-Stokes computational fluid dynamic simulation is generally excellent. This CARS technique is able to capture the low-pressure and low-temperature conditions of the M = 3.4 flow entering the Mach disk, as well as the subsonic conditions immediately downstream of this normal shock.     

Extension of wavelength-modulation spectroscopy to large modulation depth for diode laser absorption measurements in high-pressure gases

Tunable diode laser absorption measurements at high pressures by use of wavelength-modulation spectroscopy (WMS) require large modulation depths for optimum detection of molecular absorption spectra blended by collisional broadening or dense spacing of the rovibrational transitions. Diode lasers have a large and nonlinear intensity modulation when the wavelength is modulated over a large range by injection-current tuning. In addition to this intensity modulation, other laser performance parameters are measured, including the phase shift between the frequency modulation and the intensity modulation. Following published theory, these parameters are incorporated into an improved model of the WMS signal. The influence of these nonideal laser effects is investigated by means of wavelength-scanned WMS measurements as a function of bath gas pressure on rovibrational transitions of water vapor near 1388 nm. Lock-in detection of the magnitude of the 2f signal is performed to remove the dependence on detection phase. We find good agreement between measurements and the improved model developed for the 2f component of the WMS signal. The effects of the nonideal performance parameters of commercial diode lasers are especially important away from the line center of discrete spectra, and these contributions become more pronounced for 2f signals with the large modulation depths needed for WMS at elevated pressures.     

Pressure dependence of laser-induced fluorescence from acetone

The use of laser-induced fluorescence (LIF) from acetone is becoming increasingly widespread as a diagnostic of mixing processes in both reacting and nonreacting flows. One of the major reasons for its increasing use is that the acetone LIF signal is believed to be nearly independent of pressure because of fast intersystem crossing from the first excited singlet state, from which the fluorescence signal originates, to the first excited triplet state, which does not fluoresce. To evaluate the use of acetone LIF at pressures higher than atmospheric, we have performed a study of acetone LIF in a flowing gas cell at pressures up to 8 atm. We used four different buffer gases: air, nitrogen, methane, and helium. Surprisingly, we find that the acetone fluorescence quantum efficiency increases slightly (~30%-50%) as the buffer-gas pressure increases from 0.6 to 5 atm for all four buffer gases. When the buffer gas is air, we observe a decrease in the acetone fluorescence quantum efficiency as the buffer-gas pressure is increased from 5 to 8 atm; for the other three buffer gases the quantum efficiency is constant to within experimental error in this pressure regime. The observed pressure dependence of the acetone fluorescence signal is explained by use of a four-level model. The increase in the fluorescence quantum efficiency with pressure is probably the result of incomplete vibrational relaxation coupled with an increase in the intersystem crossing rate with increasing vibrational excitation in the first excited singlet manifold.     

Viscosity Measurements and Predictions for Natural Gas

A vibrating-wire viscometer of very high precision was used to measure the viscosity of methane and of two natural gases. The experimental data were, in general, taken at temperatures of 260, 280, 300, and 320 K and at pressures up to 20 MPa, and additionally in the case of methane at temperatures of 340 and 360 K and at pressures up to 29 MPa. The estimated uncertainty is ±0.3 and ±0.5% for methane and the natural gases, respectively. The new experimental data for methane were used together with zero-density or low-density viscosity values from this study and from the literature to develop a viscosity equation for natural gas composed of two contributions. The mixing rule of Wilke [J. Chem. Phys. 18: 517 1950] was applied for the zero-density viscosity part which is based on zero-density correlations for twelve components (methane, nitrogen, carbon dioxide, ethane, propane, n- and isobutane, n- and isopentane, n-hexane, n-heptane, and n-octane) and agrees with the values derived from experiment within ±0.3%. The density dependence of the residual viscosity part was correlated with methane data only, neglecting any temperature dependence, whereas the composition dependence is characterized by a pseudo-critical viscosity value. For methane the agreement between the correlated and experimental data is within ±0.5 %. The values predicted with the correlation and the experimental data agree within ±1 % for both the high calorific, H, natural gas and the low calorific, L, natural gas.Paper presented at the Fifteenth Symposium on Thermophysical Properties, June 22–27, 2003, Boulder, Colorado, U.S.A.     

Simultaneous vibrational and pure rotational coherent anti-stokes Raman spectroscopy for temperature and multispecies concentration measurements demonstrated in sooting flames

The potential of measuring temperature and multiple species concentrations (N2, O2, CO) by use of combined vibrational coherent anti-Stokes Raman spectroscopy (CARS) and pure rotational CARS has been investigated. This was achieved with only one Nd:YAG laser and one dye laser together with a single spectrograph and CCD camera. From measurements in premixed sooting C2H4-air flames it was possible to evaluate temperatures from both vibrational CARS and rotational CARS spectra, O2 concentration from the rotational CARS spectra, and CO concentration from the vibrational CARS spectra. Quantitative results from premixed sooting C2H4-air flames are presented, and the uncertainties in the results as well as the possibility of extending the combined CARS technique for probing of additional species are discussed.     

Quantitative temperature measurements in high-pressure flames with multiline NO-LIF thermometry

An accurate temperature measurement technique for steady, high-pressure flames is investigated using excitation wavelength-scanned laser-induced fluorescence (LIF) within the nitric oxide (NO) A-X(0, 0) band, and demonstration experiments are performed in premixed methane/air flames at pressures between 1 and 60 bars with a fuel/air ratio of 0.9. Excitation spectra are simulated with a computational spectral simulation program (LIFSim) and fit to the experimental data to extract gas temperature. The LIF scan range was chosen to provide sensitivity over a wide temperature range and to minimize LIF interference from oxygen. The fitting method is robust against elastic scattering and broadband LIF interference from other species, and yields absolute, calibration-free temperature measurements. Because of loss of structure in the excitation spectra at high pressures, background signal intensities were determined using a NO addition method that simultaneously yields nascent NO concentrations in the postflame gases. In addition, fluorescence emission spectra were also analyzed to quantify the contribution of background signal and to investigate interference in the detection band-width. The NO-LIF temperatures are in good agreement with intrusive single-color pyrometry. The proposed thermometry method could provide a useful tool for studing high-pressure flame chemistry as well as provide a standard to evaluate and validate fast-imaging thermometry techniques for practical diagnostics of high-pressure combustion systems.     

Spontaneous anti-stokes Raman probe for gas temperature measurements in industrial furnaces.

A compact, pulsed Nd:YAG laser-based instrument has been built to measure in situ absolute gas temperatures in large industrial furnaces by use of spontaneous anti-Stokes Raman scattering. The backscattering configuration was used to simplify the optics alignment and increase signal-to-noise ratios. Gated signal detection significantly reduced the background emission that is found in combustion environments. The anti-Stokes instead of the Stokes component was used to eliminate contributions to spectra from cold atmospheric nitrogen. The system was evaluated in a methane/air flame and in a bench-top oven, and the technique was found to be a reliable tool for nonintrusive absolute temperature measurements with relatively clean gas streams. A water-cooled insertion probe was integrated with the Raman system for measurement of the temperature profiles inside an industrial furnace. Gas temperatures near 1500-1800 K at atmospheric pressure in an industrial furnace were inferred by fitting calculated profiles to experimental spectra with a standard deviation of less than 1% for averaging times of ~200 s. The temperatures inferred from Raman spectra are in good agreement with data recorded with a thermocouple probe.     

Strategies for laser-induced fluorescence detection of nitric oxide in high-pressure flames. II. A-X(0,1) excitation

A-X(0,1) excitation is a promising new approach for NO laser-induced fluorescence (LIF) diagnostics at elevated pressures and temperatures. We present what to our knowledge are the first detailed spectroscopic investigations within this excitation band using wavelength-resolved LIF measurements in premixed methane/air flames at pressures between 1 and 60 bar and a range of fuel/air ratios. Interference from O2 LIF is a significant problem in lean flames for NO LIF measurements, and pressure broadening and quenching lead to increased interference with increased pressure. Three different excitation schemes are identified that maximize NO/O2 LIF signal ratios, thereby minimizing the O2 interference. The NO LIF signal strength, interference by hot molecular oxygen, and temperature dependence of the three schemes are investigated.     

Effect of laser pumping power on the SRS-CARS signal in compressed hydrogen

In the context of developing the diagnostics of hydrogen in gas mixtures by means of coherent anti-Stokes Raman scattering (CARS) in combination with biharmonic laser pumping by means of stimulated Raman scattering (SRS), the effect of laser pumping power on the SRS-CARS signal in compressed hydrogen has been studied. It is established that an increase in the pumping power at the input of the SRS generator leads to a shift of the CARS signal intensity maximum measured as a function of the gas pressure in the cell. This behavior is probably explained by changes in the positions of energy levels, which are caused by a significant modification of populations in the SRS process at high laser pumping powers.     

Silicon nitride films prepared by reactive plasma sputtering

Silicon nitride films were prepared by reactive plasma sputtering in nitrogen at a pressure of 2×10^-4 Torr. The residual gas in the reactor during film sputtering was analysed. The chemical composition of the films was determined from infrared absorption spectra in the wavelength region 2.0–15.0 μm and by the elastic scattering of ³He particles.The best quality silicon nitride films were obtained in pure nitrogen at the minimum residual gas pressures. An absorption minimum at 11.0 μm in the infrared spectra, corresponding to the Si-N chemical bond in the Si₃N₄ molecule, was observed in our films, indicating that their composition was close to stoichiometric.With a residual hydrogen pressure above 10% or a residual oxygen pressure above 2% the generation of new chemical bonds Si-H, N-H and Si-O respectively was observed in the silicon nitride films.     

Hysteresis,structural and composition characteristics of deposited thin films by reactively sputtered titanium target in Ar/CH4/N2 gas mixture

Abstract

Hysteresis, crystal structure and chemical composition of thin films deposited through reactive sputtering of titanium metal target in Ar/CH₄/N₂ gas mixture have been investigated. The transition from metallic to compound sputtering mode was clearly seen as the reactive gases (CH₄ and N₂) flowrate concentration first increased and subsequently decreased. Abrupt cathode current drop from 273 mA to reach a minimum value of 195 mA was observed upon addition of nitrogen gas from 0 to 10% flowrate concentration to the Ar/CH₄ gas mixture. This was also accompanied by an abrupt change in reactive gas partial pressure. Exploration of the deposition rate and film thickness showed that it decreased from 4·5 to 1·5 nm min^?1 and from 140 to 40 nm as the N₂ flowrate concentration increased from 1·5 to 7·5% at 5·5%CH₄ flowrate concentration respectively. X-ray diffraction and X-ray photoelectron spectroscopy analyses of the deposited films confirmed the formation of titanium carbide and carbonitride phases as the methane and nitrogen gas concentrations in the sputtering gas were increased.     

Helium detection in gas mixtures by laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) has been evaluated as a tool for monitoring trace levels of helium in gas mixtures consisting mostly of hydrogen. Calibration data for helium in hydrogen was investigated at different helium concentration levels. At high concentrations of helium (>7.25%), the LIBS signal is quenched due to Penning ionization. The hydrogen alpha line (656.28 nm) was observed to broaden as the concentration of helium impurities in the hydrogen gas mixture increased. The helium line at 587.56 nm was selected as the analyte line for helium impurity detection. The effects of laser energy, the delay time between the laser pulse and data acquisition, and the gas pressure on the LIBS signal of helium were investigated to determine the optimum conditions for helium detection. The LIBS signal from the helium line at 587.56 nm shows good linear correlation with helium concentration for He concentrations below 1%. Thus, LIBS can be reliably used to detect the low levels of helium. The limit of detection for helium was found to be 78 ppm.     

Broadband coherent anti-Stokes Raman spectroscopy characterization of polymer thin films

Broadband coherent anti-Stokes Raman spectroscopy (CARS) is demonstrated as an effective probe of polymer thin film materials. A simple modification to a 1 kHz broad bandwidth sum frequency generation (SFG) spectrometer permits acquisition of CARS spectra for polymer thin films less than 100 nm thick, a dimension relevant to organic electronic device applications. CARS spectra are compared to the conventional Raman spectra of polystyrene and the resonance-enhanced Raman spectra of poly(3-hexylthiophene). The CARS spectra obtained under these conditions consistently demonstrate enhanced signal-to-noise ratio compared to the spontaneous Raman scattering. The sensitivity of the CARS measurement is limited by the damage threshold of the samples. The dielectic properties of the substrate have a dramatic effect on the detected signal intensity. For ultrathin films, the strongest signals are obtained from fused silica surfaces. Similar to surface-enhanced Raman scattering (SERS), Au also gives a large signal, but contrary to SERS, no surface roughening is necessary.     

Simultaneous temperature and relative O2-N2 concentration measurements by single-shot pure rotational coherent anti-stokes Raman scattering for pressures as great as 5 MPa

Dual-broadband pure rotational coherent anti-Stokes Raman scattering is a valuable nonintrusive tool for gas diagnosis that provides simultaneous and time-resolved information about temperature and relative species concentration. A systematic investigation of single-shot precision and accuracy of simultaneous measurement of temperature and O(2)/N(2) concentration is presented. Various O(2) concentrations (1.0-15.6%) in binary mixtures with N(2) have been investigated in a temperature range from 300 to 773 K and for pressures of 1-50 bars (0.1-5 MPa). A comparison of two least-sum-squared differences fit evaluation procedures for the spectral shape, weighted constantly or inversely with respect to the relative signal intensity, is given. The results yielded good accuracy and precision for measuring temperature as well as concentration. The influence of temperature, O(2) concentration, pressure, and evaluation techniques on both accuracy and precision is discussed.     

Validation of a rotational coherent anti-Stokes Raman spectroscopy model for carbon dioxide using high-resolution detection in the temperature range 294-1143 K

Experiments were performed in the temperature range of 294-1143 K in pure CO(2) using high-resolution rotational coherent anti-Stokes Raman spectroscopy (CARS), in the dual-broadband approach. Experimental single-shot spectra were recorded with high spectral resolution using a single-mode Nd:YAG laser and a relay imaging lens system on the exit of a 1 m spectrometer. A theoretical rotational CARS model for CO(2) was developed for evaluation of the experimental spectra. The evaluated mean temperatures of the recorded single-shot dual-broadband rotational coherent anti-Stokes Raman spectroscopy (DB-RCARS) spectra using this model showed good agreement with thermocouple temperatures, and the relative standard deviation of evaluated single-shot temperatures was generally 2-3%. Simultaneous thermometry and relative CO(2)/N(2)-concentration measurements were demonstrated in the product gas of premixed laminar CO/air flames at atmospheric pressure. Although the model proved to be accurate for thermometry up to 1143 K, limitations were observed at flame temperatures where temperatures were overestimated and relative CO(2)/N(2) concentrations were underestimated. Potential sources for these discrepancies are discussed.     

Optical emission spectroscopy study of positive direct current bias enhanced diamond nucleation

High nucleation density and crystalline diamond films were deposited on a mirror-polished Si(100) substrate by horizontal microwave plasma chemical vapor deposition using a two step process consisting of positive direct current (dc) bias enhanced nucleation and growth. Optical emission spectroscopy was employed to investigate in situ the plasma emission characterization during positive biasing process. Emission lines from the Balmer series of atomic hydrogen, molecular hydrogen, CH, C₂, and Ar were observed in the visible and ultraviolet ranges when CH₄, H₂, and Ar were used as the reactant gases. The dependence of plasma emission spectra on the deposition parameters, such as biasing voltage, methane concentration and working pressure was investigated. The relative concentrations of neutral atomic hydrogen were estimated by using the Ar emission at 750.4 nm as an actinometer. A significant variation in the emission intensity of the radicals was measured with a change in the biasing voltage. The correlation between the spectra of some species and the quality of diamond films was studied. The results show that CH and C₂ both were important precursor in the diamond deposition, while C₂ was associated with the presence of amorphous phase in the films during positive dc biasing process.     

Radiation of X-Rays Using Polarized LiNbO3 Single Crystal in Low-Pressure Ambient Gas

The dependence of X-ray intensity on the pressure and type of ambient gas was investigated for LiNbO₃ single crystals polarized in the c-axis direction at pressures of approximately 1 to 30 Pa. Ionization of surrounding gas molecules by the electric field generated by the crystal led to the production of both positive ions and free electrons. The electrons were accelerated toward a Cu target, radiating both white X-rays and X-rays specific to the crystal or target material by bremsstrahlung. The integrated X-ray intensity per cycle in the energy range 1 to 20 keV showed a local maximum value at a pressure P_max. The logarithm of P_max was proportional to the Boltzmann factor using the first ionization energy of each ambient gas molecule. The value of P_max was found to be independent of the electrical surface area of the crystal. The integrated X-ray intensity was approximated qualitatively by a quadratic function with pressure, which was upwardly convex. It was found that one of the causes of the reduction in X-ray intensity at pressures P > P_max is the adsorption of positive ions generated by the ionization of gas molecules on the negative electric surface. It was also discovered that the lifetime of the X-ray radiation device could be improved when the X-ray radiation case was covered with another hermetically sealed decompression case. The gas with the smallest first ionization energy, with a partial pressure of P_max, was enclosed inside the X-ray radiation case (inner case) and the gas with the largest first ionization energy was enclosed at a suitable pressure between the inner and outer cases.