Properties of laser-induced stimulated emission for diagnostic purposes

We describe an experimental and semi-quantitative theoretical investigation of the characteristics of two-photon-induced stimulated emission related to diagnostic applications. The laser power dependence, pressure dependence, and the spectral shape of the stimulated emission signal in CO are discussed and compared with those for laser-induced fluorescence. We also discuss decreases in the laser-induced fluorescence signal caused by the stimulated emission process, and propose a method for providing increased spatial resolution in measurements made using stimulated emission detection.This work was supported by the Swedish National Board for Technical Developments, the Swedish Energy Administration, and the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences     

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.     

Photochemical effects of KrF excimer excitation in laser-induced fluorescence measurements of OH in combustion environments

This paper presents experimental evidence that using the KrF excimer laser for quantitative laser-induced fluorescence (LIF) studies of the OH A-X (3,0) system is highly problematic if the effects of both photobleaching and photochemistry are not included for laser spectral irradiances greater than 20 MW/cm² cm^-1. Pump-probe and time-resolved measurements of the OH LIF signal in an atmospheric pressure, premixed CH₄-air flame at low- and high-laser-spectral-irradiance conditions show that a significant amount of OH is produced from photofragments resulting from the simultaneous 2-photon predissociation of H₂O molecules in the C-X system. A 5+2-level rate-equation model that includes the effects of both photobleaching and photochemical OH production is shown to satisfactorily predict the data using a single adjustable parameter given by the effective, spectrally integrated 2-photon cross-section of H₂O near 248 nm. The time-integrated OH LIF signal was found to depend on both the laser spectral irradiance and the local concentration of H₂O. Additionally, use of the KrF excimer laser for 2-line rotational thermometry can produce temperature errors as great as +550 K at high laser-pulse energies. Received: 21 August 2000 / Revised version: 30 October 2000 / Published online: 21 February 2001     

Time-, wavelength-, and polarization-resolved measurements of OH (A 2Σ+ ) picosecond laser-induced fluorescence in atmospheric- pressure flames

Laser-induced fluorescence of OH (A ²Σ⁺, v’=1) was measured in hydrogen/oxygen and hydrogen/air/nitrogen flames using laser pulses of 80 psec duration. A 2D signal acquisition scheme simultaneously employed wavelength, temporal, and polarization resolution. The signals emitted in different rotational branches exhibit polarization-dependent intensities, depending on the rotational branch of the absorption line used. It is possible to select experimental conditions such that rotational and vibrational relaxation as well as electronic quenching can be monitored simultaneously. Advantages and limitations of the experimental approach are discussed. Numerical simulations are presented of the LIF spectra affected by energy transfer. Received: 29 March 1999 / Revised version: 14 June 1999 / Published online: 27 October 1999     

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.     

Nitrogen atom detection in low-pressure flames by two-photon laser-excited fluorescence

Nitrogen atoms have been detected in stoichiometric flat premixed H₂/O₂/N₂ flames at 33 and 96 mbar doped with small amounts of NH₃, HCN, and (CN)₂ using two-photon laser excitation at 211 nm and fluorescence detection around 870 nm. The shape of the fluorescence intensity profiles versus height above the burner surface is markedly different for the different additives. Using measured quenching rate coefficients and calibrating with the aid of known N-atom concentrations in a discharge flow reactor, peak N-atom concentrations in these flames are estimated to be on the order of 10¹²–5×10¹³ cm^–3; the detection limit is about 1×10¹¹ cm^–3.     

Quantitative laser-induced fluorescence: Some recent developments in combustion diagnostics

This report summarizes several recent applications of quantitative laser-induced fluorescence techniques for the determination of species concentrations and temperature in combustion processes. Several lines of further development are discussed.     

Frequency and time resolved luminescence of intermediate reaction products in ir laser decomposition of silane

A systematic study of spectral and time dependence of the luminescence emitted by fragments after ir multiple-photon decomposition of silane, is presented. Data obtained at various silane pressure (1–35 Torr) and pulsed CO₂ laser fluences are discussed and compared with previous results and interpretations.     

Isotope-selective infrared multiphoton dissociation of CF3Br in a supersonic free jet

The carbon-isotope selectivity in the multiphoton dissociation of CF₃Br is studied in the collisional region of supersonic free jet. The isotopic abundance of¹²C and¹³C in C₂F₆ formed by recombination of the dissociation products is measured with a quadrupole mass spectrometer. An enrichmet factor of 9.4 is obtained for¹²C with the 9R(30)CO₂ laser line while the factor of 6.9 is obtained for¹³C with the 9P(16) line.     

Two-dimensional imaging of glyoxal (C2H2O2) in acetylene flames using laser-induced fluorescence

2 H₂O₂). Laser-induced fluorescence spectra from glyoxal vapor using the same excitation wavelength of 428 nm showed the same strongest lines as the signal from the flame. Glyoxal was visualized in two different modes; two-dimensional imaging and a spatial-spectral mode where spectra were obtained at different spatial positions in the flame simultaneously. For the premixed laminar rich flame it is shown that glyoxal is produced early in the flame, before the signals for C₂ and CH appear. For the turbulent non-premixed flames it is shown that glyoxal is produced in a layer on the fuel rich side of the flames. Here the fuel is premixed with ambient air. This layer is thin and has a high spatial resolution. The general trend was that the glyoxal signal appeared in regions with a lower temperature compared with the emission from C₂ and CH. The imaging of glyoxal in turbulent acetylene flames is a promising tool for achieving new insight into flame phenomena, as it gives very good structural information on the flame front. Tests so far do not indicate that the detected glyoxal is a result of photo-production. To our knowledge, this is the first detection of glyoxal in flames using laser-induced fluorescence. Received: 19 December 1996/Revised version: 26 May 1997     

Carbon atom fluorescence and C2 emission detected in fuel-rich flames using a UV laser

Laser-induced fluorescence from carbon atoms, excited at the two-photon resonances around 280 nm, has been detected in fuel-rich hydrocarbon flames together with Swan band emission from the C₂ radical, which was non-resonantly excited at the same wavelengths. The emission from the C atom and from the C₂ molecule exhibited several similarities, indicating a possible common photo-chemical origin.     

Quantitative measurements of OH concentration fields by two-dimensional laser-induced fluorescence

Received: 7 May 1996/Revised version: 21 October 1996     

Wide- and narrow-band saturated fluorescence measurements of hydroxyl concentration in premixed flames from 1 bar to 10 bar

We have studied the use of wide-band detection in conjunction with saturation of a rovibronic transition of OH within itsA ^{2 +}–X ²(0,0) band. For wide-band detection, in which fluorescence is detected from the entire excited rotational manifold, the fluorescence yield is sensitive to collisions in two ways. First, it is sensitive to the ratio of rate coefficients describing rotational energy transfer and electronic quenching; this ratio determines the number of neighboring rotational levels that are populated during the laser pulse. Second, the fluorescence yield can vary with the total collisional rate coefficient; only after a sufficient number of collisions, corresponding to 2.5 ns in an atmospheric flame, does the rotational manifold reach steady state. We also compare measurements employing wide-band (detecting theR ₁ andR ₂ branches) and narrow-band (detecting a single transition) saturated fluorescence of OH. Over a wide range of conditions — obtained by varying the equivalence ratio, temperature, N₂ dilution, and pressure — the wide- and narrow-band fluorescence techniques compare well. Given this good agreement, wide-band saturated fluorescence could be especially useful for analyzing atmospheric flames with XeCl-excimer lasers; one can potentially obtain 2—D images of OH which have a high signal-to-noise ratio and a reduced sensitivity to laser irradiance and quenching.     

Calibration of imaging laser-induced fluorescence measurements in highly absorbing flames

It has been described earlier that imaging measurements of laser-induced fluorescence (LIF) in flames can be calibrated to number densities with an integrated absorption measurement provided the integrated absorption is small. In this paper a method is presented that extends the technique to flames with substantial absorption, improves the number density determination and allows the experimental parameters to be chosen more freely. The method is based on an iterative computer procedure that reconstructs the 1-D spatially resolved absorption profile from laser measurements of the 1-D spatially resolved LIF and the integrated absorption of the laser beam. The technique is experimentally demonstrated by measurements of OH number densities in atmospheric flames. It is potentially a single-pulse method. Other applications of the iterative procedure are mentioned.     

Laser diagnostics of NO reburning in fuel-rich propene flames

Absolute NO concentrations were measured by laser-induced fluorescence (LIF) in three different fuel-rich non-sooting propene flames (φ=1.5, 1.8 and 2.3). The experiments were performed in low-pressure premixed propene flames with 0.2%-1% NO added. Laser diagnostics was applied as a tool for investigating reburn chemistry. The Q₂(25.5) line in the A-X(0,0) band was excited because of the small temperature dependence of its ground state population. The NO fluorescence lifetimes were measured directly and compared to theoretical values. The initial NO levels are strongly reduced in all three flames. According to modeling results, the HCN mole fraction increases strongly with stoichiometry. As guidelines for laser diagnostics applications in such systems, the modeling results were analyzed with respect to the main reaction channels and reaction partners in fuel-rich flames. Received: 1 March 2000 / Revised version: 20 April 2000 / Published online: 20 September 2000     

Photolytic interference affecting two-photon laser-induced fluorescence detection of atomic oxygen in hydrocarbon flames

This study reports on photochemical interferences affecting atomic oxygen detection using two-photon laser-induced fluorescence at 226 nm. In contrast to previous studies in which molecular oxygen was proven to be the relevant photochemical precursor molecule in a hydrogen-fueled flame, the present investigations were carried out in a laminar diffusion flame of methane and air. The most significant interferences were found at the fuel side of the flame in the absence of molecular oxygen, and vibrationally excited carbon dioxide was identified as the most probable precursor molecule for the photochemical production of oxygen atoms. Received: 11 December 2002 / Revised version: 10 March 2003 / Published online: 16 April 2003 RID="*" ID="*"Corresponding author. Fax: +1-925/294-2595, E-mail: tbsette@sandia.gov     

Simultaneous CH planar laser-induced fluorescence and particle imaging velocimetry in turbulent nonpremixed flames

jet =18600). Here, PLIF images reveal a CH layer of thickness typically <1 mm from flame base to tip. Furthermore, in these permanently blue flames, we observe instantaneous flamefront strain rates – derived from the PIV data – in excess of ±10⁴ s^-1 without flame extinction. Received: 16 October 1997/Revised version: 30 October 1997     

Imaging of flames and cold flows in air by diffraction from a laser-induced grating

Nonresonant laser-induced gratings are created in gases employing the second-harmonic output of a Nd: YAG laser in a degenerate four-wave mixing beam geometry. The diffraction efficiency of the gratings has been investigated as a function of laser intensity and gas pressure. Single-shot images of a helium flow in ambient air illustrate that diffraction of light from a laser-induced grating has the potential for remote, two-dimensional diagnostics of gas mixing processes. In addition, this coherent technique is used to image a sooty flame.     

Single-pulse collision-insensitive picosecond planar laser-induced fluorescence of OHA 2 Σ + (v′ = 2) in atmospheric-pressure flames

Single-pulse two-dimensional picosecond Laser-Induced Fluorescence (LIF) imaging of the OH density in a single quantum state was performed for the first time, using a premixed methane-oxygen flame at atmospheric pressure. A picosecond, excimer-Raman-laser system (268 nm, 470 ps FWHM) was used for excitation of OH. The fluorescence from the laser sheet was imaged onto a fast gated intensified camera with a 400 ps gate width. The short laser pulse minimizes the collisional redistribution of population in the ground state during excitation, while the short camera gate avoids significant quenching of the excited-state fluorescence. The fluorescence signal obtained in this way is a direct measure of the population in a selected quantum state. In contrast to common nanosecond LIF signals no corrections on variations of the collisional environment are necessary. This collision-insensitive approach to two-dimensional LIF yields an OH detection limit of 10 ppm in a cube of 330 µm per side with a single 1 mJ laser pulse. A rate-equation model is used to estimate the effects on the observed signal of fluctuations in pulse energy and duration, laser-camera timing jitter, and spatial variations in the collisional environment.