Laser fluorescence flames

Using laser fluorescence measurements on fuel-rich H2/02/N2 flames seeded with H2S, Muller et al. [43] determined the concentrations of SH, S2, SO, S02, and OH in the post-flame gases. From their results and an evaluation of rate constants, they postulated that the flame chemistry of sulfur under rich conditions could be described by the eight fast bimolecular reactions and the two three-body recombination reactions given in Table 8.4. 

Fluorescence excitation and emission spectra of the two sodium D lines in an air-acetylene flame, (a) In the excitation spectrum, the laser was scanned, (to) In the emission spectrum, the monochromator was scanned. The monochromator slit width was the same for both spectra. [From s. J. Weeks, H. Haraguchl, and J. D. Wlnefordner, Improvement of Detection Limits in Laser-Excited Atomic Fluorescence Flame Spectrometry," Anal. Chem. 1976t 50,360.]... 

The measurements of temperature and species concentrations profiles in premixed, laminar flames play a key role in the development of detailed models of hydrocarbon combustion. Systematic comparisons are given here between a recent laminar methane-air flame model and laser measurements of temperature and species concentrations. These results are obtained by both laser Raman spectroscopy and laser fluorescence. These laser probes provide nonintrusive measurements of combustion species for combustion processes that require high spatial resolution. The measurements reported here demonstrate that the comparison between a model and the measured concentrations of CH, O2,... 

Figure 2. Optical system for laser-fluorescence measurements in flames...

Low power laser fluorescence measurements of OH, S2, SH, SO and S02 have been made in a series of sulfur bearing H2/O2/N2 flames. A simple generally applicable method for taking account of quench effects has been employed to convert relative fluorescence data into absolute concentrations. The technique has been employed to develop a kinetic model for the coupled chemistry of sulfur in rich H2/02/N2 flames (6). 

Nitric Oxide Detection in Flames by Laser Fluorescence... 

Laser-Fluorescence techniques for NO are of interest for studying the mechanisms of NO formation and its influence on chemical processes and pollutant formation in flames. In general, the optical fluorescence techniques provide very high detection sensitivities and good spatial resolution. 

In summary, the results in this paper demonstrate that laser fluorescence can be used to detect NO in atmospheric-pressure flames. Detection sensitivities in the ppm range were observed with laser pulse energies in the range of about 3 pJ. This sensitivity can be increased significantly by using a higher intensity laser. 

The concept of saturated laser fluorescence appears attractive in that the fluorescence intensity is directly related to the particular species concentration and becomes roughly independent of the laser intensity at saturation. Such a mode has been invoked already to monitor absolutely flame concentrations of Na a-4), OH (5), C2 (6,7), CH (7,8), CN (8), and MgO (4). However, during a recent study of the behavior of Na and Li in flames (9-11), we have observed evidence for laser induced chemical reactions under saturated conditions which has significant implications for the quantitative exactness of such measurements. 

Summary. In saturated laser fluorescence studies of sodium and lithium in a series of fuel rich H2/02/N2 flames there is evidence for the involvement of laser induced chemical reactions with H20 and H2. Although their reactive probabilities have been shown to be small relative to the corresponding physical quenching interactions they are still sufficient to establish significant... 

Two techniques, which appear well suited to the diagnostic probing of practical flames with good spatial and temporal resolution, are coherent anti-Stokes Raman spectroscopy (CARS) and saturated laser fluorescence. The two techniques are complementary in regard to their measurement capabilities. CARS appears most appropriate for thermometry and major species concentration measurements, saturated laser fluorescence to trace radical concentrations. With electronic resonant enhancement (6), CARS may be potentially useful for the latter as well. Fluorescence thermometry is also possible (7, 8) but generally, is more tedious to use than CARS. In this paper, recent research investi-... 

CARS appears ideally suited to thermometry and major species concentration measurements in both practical and clean flame environments. It should see widespread application in both practical combustors and fundamental flame investigations, particularly where soot levels are high. Saturated laser fluorescence has great potential for the measurement of selected species in low concentration (ppm) in both practical and clean flames. 

F9. Fraser, L. M., and Winefordner, J. D., Laser-excited atomic fluorescence flame spectrometry. Anal. Chem. 43, 1693-1696 (1971). 

Laser Fluorescence Noise Sources. Finally, let us examine a technique with very complex noise characteristics, laser excited flame atomic fluorescence spectrometry (LEAFS). In this technique, not only are we dealing with a radiation source as well as an atomic vapor cell, as In atomic absorption, but the source Is pulsed with pulse widths of nanoseconds to microseconds, so that we must deal with very large Incident source photon fluxes which may result in optical saturation, and very small average signals from the atomic vapor cell at the detection limit [22]. Detection schemes involve gated amplifiers, which are synchronized to the laser pulse incident on the flame and which average the analyte fluorescence pulses [23]. 

The excitation source can be a continuum or a line-like radiation source. Research on atomic fluorescence spectrometry has been connected with the examination of intense radiation sources such as electrodeless discharge lamps and lasers. Various flames, plasmas, and furnaces have been employed as atomizing devices. 

Confirmation of the formation of the radicals during combustion reactions has been made by inuoducing a sample of dre flames into a mass spectrometer. The sample is withdrawn from a turbulent flame which is formed into a thin column, by admitting a sample of the flame to the spectrometer drrough a piidrole orifice, usually of diameter a few tenths of a millimetre. An alternative procedure which has been successful in identifying the presence of radicals, such as CHO, has been the use of laser-induced fluorescence. 

The LIF technique is extremely versatile. The determination of absolute intermediate species concentrations, however, needs either an independent calibration or knowledge of the fluorescence quantum yield, i.e., the ratio of radiative events (detectable fluorescence light) over the sum of all decay processes from the excited quantum state—including predissociation, col-lisional quenching, and energy transfer. This fraction may be quite small (some tenths of a percent, e.g., for the detection of the OH radical in a flame at ambient pressure) and will depend on the local flame composition, pressure, and temperature as well as on the excited electronic state and ro-vibronic level. Short-pulse techniques with picosecond lasers enable direct determination of the quantum yield [14] and permit study of the relevant energy transfer processes [17-20]. 

Daily, J.W., Laser induced fluorescence spectroscopy in flames. Prog. Energy Combust. Sci., 23,133,1997. 

Dreyer, C.B., Spuler, S.M., and Linne, M., Calibration of laser induced fluorescence of the OH radical by cavity ringdown spectroscopy in premixed atmospheric pressure flames. Combust. Sci. Tech., 171,163, 2001. 

In AFS, the analyte is introduced into an atomiser (flame, plasma, glow discharge, furnace) and excited by monochromatic radiation emitted by a primary source. The latter can be a continuous source (xenon lamp) or a line source (HCL, EDL, or tuned laser). Subsequently, the fluorescence radiation is measured. In the past, AFS has been used for elemental analysis. It has better sensitivity than many atomic absorption techniques, and offers a substantially longer linear range. However, despite these advantages, it has not gained the widespread usage of atomic absorption or emission techniques. The problem in AFS has been to obtain a... 

---