Saturated induced fluorescence

Altkorn R and Zare R N 1984 Effects of saturation on laser-induced fluorescence measurements of population and polarization Annual Review of Physical Chemistry ed B S Rabinovitch, J M Schurr and H L Strauss (Palo Alto, CA Annual Reviews)... 

In the gas-liquid two-phase flows illuminated by a laser sheet, for example, the intensity of light reflected from the gas-liquid interface (mostly the gas bubble s surface) not only saturate the CCD camera, but also overwhelm the intensity of light from the seeded tracer particles in its vicinity. Fluorescent particles are often used to realize the laser-induced fluorescence (LIF) technique together with PIV (e.g., Broder and Sommerfeld, 2002 Fujiwara et al., 2004a, b Kitagawa et al., 2005 Liu et al., 2005 Tokuhiro et al., 1998,1999), so that both images of gas-liquid interface (e.g., bubble s geometry) and velocity distribution in the liquid phase around the gas bubbles can be obtained. Issues on PIV measurement of gas-liquid two-phase flows will be further illustrated in the latter sections. 

As I have indicated, this presentation will be divided into two parts. In the first part we will discuss the development of Coherent Anti-Stokes Raman Spectroscopy, the problems inherent in applications to combustion sources, recent developments which address operational problems, and the state-of-the-art today. This will be followed by a similar discussion involving the use of saturated laser-induced fluorescence spectroscopy as a combustion diagnostic. 

Saturated Laser Induced Fluorescence Spectroscopy. The development of saturated laser induced fluorescence spectroscopy is more recent than CARS and is less published. Even though this is the case, this introductory review will not be comprehensive. I will likely miss some work and I apologize in advance to those authors. I will not attempt to discuss laser absorption experiments or laser induced fluorescence experiments in the low laser power, i.e., non-saturated, limit. There is much work in the latter area of merit and several important papers on LIF in this conference. 

As we will see the use of saturated laser induced fluorescence spectroscopy will allow us to ignore some of these effects. we can infer the importance of others and for the time being the remainder have to be evaluated on a case by case basis for each molecular system and for the operating parameters of the experiments. 

Experimental Setup. The instrumentation (both optics and electronics) for studying saturated laser induced fluorescence spectroscopy is much less conplicated than for CARS. The experimental setup shown in Figure 18, as used in our laboratory, is typical for these studies. In some experiments it is advantageous to use a monochromator rather than band pass filters to isolate the laser induced fluorescence signal. The lasers used are either flash lamp pumped systems or NdsYAG pumped dye lasers. 

The recent availability of tunable dye lasers has markedly enhanced our ability to inquire into the chemistry and physics of combustion systems. The high sensitivity, spectral and spatial resolution, and non-perturbing nature of laser induced fluorescence makes this technique well suited to the study of trace chemistry in complex combustion media. A barrier to the quantitative application of fluorescence to species analysis in flames has been the need to take into account or bypass the effects of quenching. The use of saturated fluorescence eliminates quenching as a problem and has the further advantage that fluorescence intensity is insensitive to variations in laser power (1, 2 ). However, the generation of high concentrations of excited states under saturated excitation in an active flame environment opens up the possibilities for laser induced chemistry effects that also must be taken into account or avoided (3,4,5). 

One of the main problems met in Laser Induced Fluorescence measurements is the excited population dependence on the quenching due to collisional deexcitation. The saturation mode proposed to avoid this dependence is very difficult to achieve U ) (2 ) particularly with molecular species and moreover the very strong laser pulses required may cancel the non-perturbing characteristic of the method. Therefore precise knowledge of the quenching is necessary in some experimental circumstances. 

ALAN GELB—Physical Sciences Incorporated, Woburn, MA 01801 DAVID R. CROSLEY—SRI International, Menlo Park, CA 94025 Optical Saturation in Laser-Induced Fluorescence... 

Laser-induced fluorescence is a sensitive, spatially resolved technique for the detection and measurement of a variety of flame radicals. In order to obtain accurate number densities from such measurements, the observed excited state population must be related to total species population therefore the population distribution produced by the exciting laser radiation must be accurately predicted. At high laser intensities, the fluorescence signal saturates (1, 2, 3 ) and the population distribution in molecules becomes independent of laser intensity and much less dependent on the quenching atmosphere (4). Even at saturation, however, the steady state distribution is dependent on the ratio of the electronic quenching to rotational relaxation rates (4, 5, 6, 7). When steady state is not established, the distribution is a complicated function of state-to-state transfer rates. 

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... 

Laser induced fluorescence is particularly well suited to combustion chemistry, as a sensitive "in-situ" probe for free radicals in flames or under more controlled conditions in laboratory flash photolysis, discharge flow tube, or shock tube experiments. Using laser-saturation fluorescence previous studies from this laboratory (J ) have shown that C2(a3n ) is present in high concentrations in the hot region of an oxy-acetylene flame. C2(a-,n and X1 ) reacts with 0 .(2,3 4) One of the products of this reaction (and/or the reaction of C2H+02) is CC0.(2) In the present study, we report C20()rn.-f, i 7 fluorescence excitation spectra, A"3 , lifetimes and quenching rate constants, and... 

Although the 0( P)- H2 reaction has been Intensively studied by the VADW method, as well as by other theoretical techniques (see In addition Table Mi). no state-to-state experimental measurements have yet been made for It. However, several molecular beam experiments of the reaction of 0( P) with saturated hydrocarbons HR. using laser induced fluorescence detection of the product OH molecule, have been reported (Andresen and Luntz (21. Dutton et al. (321). These experiments have... 

Fig. 5.6 (a) Decrease of molecular orientation with increasing pump intensity from saturation in the lower and upper state (b) schematic level scheme monitored through the polarization Pq of laser-induced fluorescence (c)... 

R. Altkorn, R.Z. Zare, Effects of saturation on laser-induced fluorescence measurements. Annu. Rev. Phys. Chem. 35, 265 (1984)... 

Forster R, Frost M, Fulle D, Hamann HF, SchlepegreU A, Troe J (1995) High pressure range of the addition of HO to HO, NO, NO2, and CO. I. Saturated laser induced fluorescence measurements at 298 K. J Chem Phys 103 2949... 

SATURATION IN LASER-INDUCED FLUORESCENCE DETECTION OF NASCENT PRODUCT ROTATIONAL ANGULAR MOMENTUM ALIGNMENT... 

In a recent paper [1] we described an analytical method of extracting alignment information on nascent product rotation angular momentum distributions using laser-induced-fluorescence under arbitrary saturation conditions. A variety of... 

In a recent paper by Vigud and co-workers [7] coherent saturation effects in laser-induced-fluorescence are discussed. Coherent effects should be considered in situations where the laser coherence time is comparable to the time the molecule interacts with the laserfield. In the saturation regime where the Rabi ffequency co/ is much larger than the total decay rate out of the excited state the fluorescence intensity turns out to be proportional to (O/j, i.e. proportional to the square root of the laser power. 

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