Two-photon predissociative fluorescence of H2O by a KrF laser for concentration and temperature measurement in flames

Two-photon laser-induced predissociative fluorescence (LIPF) of H₂O is examined as a potential measurement technique of H₂O concentration and temperature in flames. Two-photons of 248 nm light from a narrowband KrF laser excite H₂O to the highly predissociative state in a hydrogen-air flame. The subsequent bound-free emission is observed from 400–500 nm in the flame at temperatures of 1000–2000 K and is found to be free of fluorescence interference from other flame species. This LIPF signal is not affected by collisional quenching due to the short lifetime of the predissociative state (2.5 ps). Broadband laser dispersion spectra, narrowband laser dispersion spectra, laser excitation spectra and probability density functions of the H₂O fluorescence are obtained in the hydrogen flame. The H₂O LIPF signal is found to be temperature sensitive and a two-line LIPF technique is needed for concentration and temperature measurement. The accuracy of a two-line LIPF technique for H₂O concentration and temperature measurement is determined.     

Degenerate four-wave mixing in nitrogen dioxide: Application to combustion diagnostics

Single shot degenerate four wave mixing (DFWM) images of the distribution of nitrogen dioxide (NO₂) doped into a propane/air flame at concentrations of the order of 7000 ppm have been obtained. These images indicate the relative concentration of NO₂ in different parts of the flame with an estimated spatial resolution of 150 m.Initial experiments were performed using NO₂ in a glass cell with nitrogen buffer gas. DFWM signals were generated using both the frequency doubled output of a pulsed ND:YAG laser and the tunable blue output of an excimer pumped dye laser. The signal was investigated as a function of laser power, NO₂ concentration and buffer gas pressure. In addition, spectra of NO₂ in the region 450 to 480 nm were obtained.Signals were then sought in both a cold air/NO₂ gas flow and an ignited mixture of propane and air seeded with NO₂, using a DFWM imaging geometry. The resulting images from the flame demonstrate the disappearance of the NO₂ molecules in the flame interaction zone.This work was done when previously employed by AEA Technology at Harwell     

Nonlinear luminescence phenomena in fullerene crystallites

Time-resolved photoluminescence studies in powders and single crystals of C₆₀ are reported. The observed emission spectra, resulting from excitation above the HOMO-LUMO transition energy, are consistent with those reported for steady state conditions which may be associated with an intramolecular process and the observed luminescence decay times are of the order of 1 ns. No long lived decay from the triplet is observed at low intensities. A dramatic increase in the lifetime as well as a redshift and broadening of the spectrum is observed at high excitation densities. The luminescence intensity is seen to increase with the cube of the input intensity. The phenomenon is interpreted as an abrupt onset of emission from the highly populated excited state resulting from an interaction of the intramolecular states, a process which is dependent on a critical excited state density. Furthermore, the phenomenon may be photoexcited at photon energies which lie below the HOMO-LUMO transition energy, under which conditions an intensity dependence of the output luminescence on input to the sixth power is observed. Under off-resonant conditions the process is interpreted as two-photon assisted nonlinear emission.     

Ce and Pr5d-energy levels in the (pseudo) perovskites KMgF3 and NaMgF3

The location of the 5d-energy levels of Ce³⁺ and Pr³⁺ in the cubic perovskite KMgF₃ and in the distorted perovskite NaMgF₃ was determined from spectroscopic studies in the vacuum ultraviolet. It is established that Ce³⁺ and Pr³⁺ ions both occupy the same site in each host: K⁺ sites for KMgF₃ and Na⁺ sites for NaMgF₃. The small crystal field splitting and the small value of the centroid shift of the 4fⁿ⁻¹5d-configuration yield a relatively high energy for the lowest 5d state of both Ce³⁺ and Pr³⁺. The lowest 5d state of Pr³⁺ in both hosts is found at energy higher than the 4f² state, enabling the photon cascade emission to occur.     

Influence of annealing temperature on structural,electrical and optical properties of WSi2

WSi₂ polycrystalline films of different thicknesses were prepared by low pressure chemical vapor deposition on silicon wafers, and their crystallization properties were studied as a function of the annealing temperature. Structural measurements were performed by X-ray diffraction, detailing for the first time the phase transition from the amorphous to the hexagonal structure at an annealing temperature 380° C and from hexagonal to tetragonal above 700° C. The electrical sheet resistance showed the same transition temperatures.Optical characterization was performed by spectroscopic ellipsometry, and the real and imaginary part of the complex refractive index were obtained as a function of the annealing temperature in the 0.25–0.9 m wavelength range. A broad optical band was found for samples annealed up to 700° C, while for higher annealing temperatures a transparency region for wavelengths greater than 0.5 m and some significant structures appear. A corresponding behavior was observed in the infrared reflectance spectra. Furthermore, it was shown that the determination of the thickness of SiO₂ grown on WSi₂ requires a multilayer model, taking into account the transparency of tetragonal WSi₂.     

Spectroscopic properties of Tm -doped Ba3Gd2(BO3)4 crystal

crystal with the size up to Φ 13 mm×44 mm was grown successfully by the Czochralski technique and its optical properties were presented. The absorption cross-section and emission cross-section were presented. Also, the potential laser gain near 1.9 μm was investigated. In the framework of the Judd-Ofelt (J-O) theory, the intensity parameters were calculated to be: Ω₂=11.375×10⁻²⁰ cm², Ω₄=5.077×10⁻²⁰ cm² and Ω₆=6.524×10⁻²⁰ cm². The spectroscopic parameters of this crystal such as the oscillator strengths, radiative transition probabilities, radiative lifetime as well as the branching ratios were calculated, too. This crystal is promising as a tunable infrared laser crystal.     

Suppression of stimulated hyer-Raman scattering in lithium vapor

The stimulated hyper-Raman scattering (SHRS) in lithium vapor has been observed in both the forward and the backward directions as the laser was tuned near the 2S–3S and 2S–4S two-photon resonances. It is found that in the forward direction the SHRS associated with the 2P excitation near the 2S–3S or the 2S–4S two-photon resonance is completely suppressed, but the SHRS associated with the 3P excitation near the 2S–4S two-photon resonance can be observed. A condition under which the forward SHRS takes place, given by Malakyan [3], has been examined. A reasonable agreement between the theory and the experiment is obtained. Near the 2S–3S two-photon resonance, instead of the expected emission _2P–2S corresponding to the 2P–2S transition, an emission at the 2_P–_3S–2P frequency, which is close to the _2P–2S, has been observed in the backward direction. The process generating this emission could be considered as an inverse SHRS process induced by the 3S–2P spontaneous emission. For the SHRS emission a shift of the observed frequency from the calculated frequency has been found. In order to explain the shift, the Stark effect has been calculated. The results show that the shift can not be attributed to the Stark effect.     

Two-photon degenerate four-wave mixing (DFWM) for the detection of ammonia: Applications to flames

Two-photon Degenerate four-wave mixing (DFWM) has been used for the detection of ammonia in a cell and in an atmospheric pressure flame. The NH₃ molecules were excited from the ground state X to either the C' or the B state and the DFWM signal was recorded simultaneously with the laser-induced fluorescence signal, originating from the C-A and B-A transitions, respectively. During the flame experiments sequential measurements of NH₃ and OH in an NH₃/O₂ flame were also performed.     

Low-frequency stimulated emission and induced absorption in a three-level atom

Investigations of the motion of conventional negative ions (electron bubbles) in He II under pressures above 11 bar have provided the only means of measuring the Landau critical velocity for roton creation,v _L, and for studying supercritical dissipation at higher velocities. Earlier work on roton creation is reviewed and it is pointed out that there is still no generally agreed explanation of the fact that the rotons seem to be emitted from the moving ionin pairs; nor is it known why the matrix element characterising the pair emission process should decrease rapidly with pressure. The possibility of studying these phenomena through use either of the fast ion (whose nature remains unknown), or of selected ions from the large variety of species that can be injected into He II by the recently developed technique of laser ablation, is discussed.     

Preparation effects on the spectroscopic properties of V2O5-GeO2 glass

The spectroscopic studies of V₂O₅-GeO₂ (45:55) glasses fused and equilibrated at various temperatures (T) in air were measured. Their spectroscopic properties are shown to be sensitive to microstructure. This observation is consistent with previously reported results that the structure of glasses depend upon the temperature at which the glass was fused and equilibrated.     

Simultaneous measurement of localized heat release with OH/CH2O-LIF imaging and spatially integrated OH∗ chemiluminescence in turbulent swirl flames

The in-situ and localized observation of heat release in turbulent flames is important for the validation of computational modeling of turbulent flows with combustion. In the present work we obtain localized information on heat release rate (HRR) by the commonly accepted technique of the simultaneous and single-shot planar imaging of OH and CH₂O concentrations by laser-induced fluorescence (LIF). Additionally, we combine this with the simultaneous line-of-sight and temporally resolved chemiluminescence detection of OH^?, spatially integrated within the flame volume, interrogated by the laser sheets used for the HRR imaging technique. The combined diagnostic methods are demonstrated for a swirl-stabilized, premixed turbulent methane/air flame of 30-kW thermal power, and they show the existence of correlations between both HRR-sensitive diagnostic techniques.     

Laser diode spectroscopy of H2O at 2.63 μm for atmospheric applications

The 2.7 μm spectral range is highly suitable for the in situ monitoring of atmospheric H₂O using compact balloonborne laser diode spectrometers. Water vapour spectroscopic parameters of the 2₀₂   1₀₁ and the 4₁₃   4₁₄ transitions of the ν₃ band are revisited in this spectral region using a new distributed-feedback InGaAsSb laser diode emitting at 2.63 μm. Accurate line strengths are provided which are well adapted for the in situ probing of the middle atmosphere. Our measurements are thoroughly compared to an existing molecular database, laboratory measurements and ab-initio calculations. A laser hygrometer was developed for operation from small stratospheric balloons using this new laser diode technology, with emission at 2.6 μm. The realized sensor is described and results from a recent test-flight are reported. PACS 07.57.Ty; 92.60 Jq     

Laser-induced fluorescence detection of singlet CH2 in low-pressure methane/oxygen flames

Methylene, CH₂, is a chemically important intermediate in hydrocarbon combustion but has previously eluded optical detection in a combustion environment. The CH₂ signal as a function of height above the burner surface in a premixed, laminar, methane/oxygen flame (5.6 Torr and fuel equivalence ratio 1.05) is measured by laser-induced fluorescence (LIF) in the B ₁ – ã¹ A ₁ electronic system. The ã state which lies 3165 cm^–1 above the ground state is populated at the high temperatures of the flame (800–1800 K). Although less than one photon for each laser pulse is detected, we can unambiguously attribute the LIF features in the region 450 to 650 nm to CH₂ by both scanning the excitation laser and dispersing fluorescence. LIF temperatures and CH and OH LIF concentration profiles are also obtained for the flame. The CH₂ radical concentration maximum occurs closer to the burner than that of either OH or CH, as expected from models of methane combustion chemistry.     

Spectroscopic trace-gas sensor with rapidly scanned wavelengths of a pulsed quantum cascade laser for in situ NO monitoring of industrial exhaust systems

Development of a pulsed quantum cascade laser (QCL)-based spectroscopic trace-gas sensor for sub-part-per-million detection of nitric oxide (NO) and capable of monitoring other molecular species such as CO₂, H₂O, and NH₃ in industrial combustion exhaust systems is reported. Rapid frequency modulation is applied to the QCL to minimize the influence of fluctuating non-selective absorption. A novel method utilizes only a few laser pulses within a single wavelength scan to probe an absorption spectrum at precisely selected optical frequencies. A high-temperature gas cell was used for laboratory evaluation of the NO sensor performance. A noise-equivalent sensitivity (1) of 100 ppb × m/ at room temperature and 200 ppb × m/ at 630 K was achieved by measuring the NO R(6.5) absorption doublet at 1900.075 cm^–1.     

Characterization of a collinear double pulse laser-induced plasma at several ambient gas pressures by spectrally- and time-resolved imaging

A collinear double pulse laser-induced plasma was characterised by means of a spectrally and time resolved imaging technique. The beams of two Q-switched Nd:YAG lasers were focused on a brass target in a vacuum chamber to form the plasma. The plume emission intensity and spatial distribution were recorded with temporal resolution using an intensified CCD. Using a set of interference filters, we collected images of the emission from the major target components as well as from oxygen. Both the laser inter-pulse separation (in the range between 0 and 10 s) and the ambient air pressure value (in the range between 10⁵ and 10 Pa) were varied during the experiment. At atmospheric pressure, an enhancement of the line emission from the target elements was observed for delayed laser pulses compared to coincident pulses. However, this enhancement effect tends to fall at low pressure values, and a decrease of the signal is observed for pressures under about 10⁴ Pa. Moreover, it was observed that the evaluation of the enhancement factor strongly depends on the detector field of view. The propagation of the emitting plume was also studied at several pressures and inter-pulse delays.     

Detailed study of the effect of a short prepulse on soft X-ray spectra generated by a high-intensity KrF* laser pulse

The effect of a short prepulse (0.5 ps) on soft X-ray spectra from a plasma generated by a high intensity KrF* laser pulse (main pulse: 0.5 ps, intensity I _main=5.3×10¹⁵ W/cm²) on flat targets of Al and Cu has been studied in detail. The spectra have been measured as a function of the pulse separation t between the two pulses and the prepulse intensity I _pre. It was found that both the overall emission and the line emission increased with t (at constant I _pre) and with I _pre (at constant t). In particular, lines in the shorter wavelength region had higher intensity. The influence of the prepulse on the line emission of specific transitions in the Al spectra was investigated systematicly. An explanation for the observed effects is given.     

Spectroscopic properties of the photoproducts of pyridoxal-5′-P irradiation: Catalytic site recognition of ribonuclease A

Photoproducts of pyridoxal-5-P, i.e., 4-pyridoxic-5-P and bis-pyridoxal-5-P, have been studied by spectroscopic methods. The spectroscopic properties of bis-pyridoxal-5-P (bis-PLP) resemble those of pyridoxal-5-P (PLP) under similar experimental conditions. The coupling of methylen hydrogens to the phosphorus atom has been shown by NMR spectroscopy. The singlet in the³¹P-NMR spectra and the triplet in¹H-undecoupled experiments confirm the presence of the phosphate group in the 5 position of the structure of the vitamin. The effect of pH and solvent composition on the relative distribution of species of bis-pyridoxine-5-P (bis-PNP) has been investigated by absorption and fluorescence spectroscopy. The acid-base dissociation of the phosphate group is easily detected by emission spectroscopy. Bis-PNP and bis-PLP bind to the enzyme RNase A and they behave as competitive inhibitors with respect to the substrate cytidine-2-3-cyclic phosphate. The natural forms of vitamin B₆, pyridoxine, and pyridoxine-5-P have no effect on the catalytic activity of the protein. Experimental evidence derived from fluorescence and inhibition experiments is consistent with the hypothesis that bis-PNP recognizes the catalytic site of RNase A.     

Linewise Raman-Stokes/anti-Stokes temperature measurements in flames using an unintensified charge-coupled device

One-dimensional, spatially resolved (linewise), temporally averaged Stokes/anti-Stokes temperature measurements with 200 m spatial resolution and 6% precision uncertainty are demonstrated in an opposed-jet-diffusion flame of air vs N₂-diluted H₂ using the Raman-Stokes/anti-Stokes technique. The Stokes/anti-Stokes temperature profile is compared to temperature measurements obtained using all major species'Q-branches and differences between the two sets of results are discussed. In addition, single-pulse, linewise measurements with 710 m spatial resolution and 19% precision uncertainty are also demonstrated. To the authors' knowledge this is the first report of linewise Stokes/anti-Stokes temperature measurements in flames. Simultaneous measurements of the N₂ Stokes and anti-Stokes vibrationalQ-branches are obtained by using an unintensified Charge-Coupled Device (CCD) that is gated by a ferro-electric liquid crystal light valve and/or a low duty cycle optical chopper to suppress detected flame luminosity. This detection system provides increased detection efficiency, dynamic range, and imaged spectral range over an intensified CCD, allowing spatially resolved Stokes and anti-Stokes signals to be imaged onto one detector.     

Scaling-up the Production of Nanosized SiO2-particles in a Double Diffusion Flame Aerosol Reactor

Synthesis of nanophase silica (SiO₂) from hexamethyldisiloxane (HMDS) oxidation in a co-flow diffusion flame reactor at atmospheric pressure is investigated focusing on high production rates of powder. A new experimental set-up is introduced, including a diffusion burner which is operated with a ring-shape double diffusion flame. Significantly high HMDS concentrations are used resulting in SiO₂ production rates of up to 130g/h. Deposition of silica powder on the burner face is eliminated by the design of a special diffusion burner and higher collection rates are achieved using a baghouse filter. The specific surface area and the product powder composition are analyzed. Carbon black coated silica particles were produced at high production rates (130g/h) at low oxygen flow rates or using a mixture of nitrogen and oxygen as oxidant. The size of the product particles was controlled in the range of 15–170nm.     

Interconnected β-Ni(OH)2 sheets and their morphology-retained transformation into mesostructured Ni

A facile route was described for the synthesis of interconnected β-Ni(OH)₂ sheets, which was realized by thermal treatment of solution at 95 °C. X-ray diffraction (XRD), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), infrared absorption spectrum (IR) and Raman spectrum were used to characterize the product. The complexation of ammonia and temperature played important roles for the formation of the assembled structure. Thus-prepared β-Ni(OH)₂ were morphology-retained transformed into mesostructured Ni by further reduction in H₂ current for 1.5 h.