Laser induced fluorescence spectroscopy

If a particle resonantly absorbs a photon from the laser beam, the particle is left in an excited energy state. Such a state is unstable and will decay spontaneously, emitting a photon again. As has been discussed earlier, the excited state of finite lifetime emits its photon on return to a lower energy level in random directions. It is this fact that allows one to measure an absorption signal directly, as outlined in Chapter 6. Conveniently, the fluorescence is observed at 90° to a collimated laser beam. In principle, a very small focal volume Vc may be defined in the imaging set-up, resulting in spatial resolution of the laser-particle interaction volume note that spatial resolution cannot normally be realized in an experiment, which measures the absorption directly. 

In a sense, the method of LIF may be seen as a fancy way of measuring the absorption of a species, but with a bonus. Absorption spectroscopy, which detects the transmitted light, has (in many experimental implementations) a limited sensitivity. The problem is that one has to detect a minute amount of missing light in 

As is obvious from the above picture, two radiative transitions are involved in the LIF process. First, absorption takes place, followed by a second photon-emission step. Therefore, when planning a LIF experiment one should always bear in mind that LIF requires considerations associated with absorption spectroscopy. Any fancy detection equipment is merely used to detect the consequences of the absorption, with the additional information on how much was absorbed where. 

One major caveat with fluorescence measurements is that they are no longer associated with a simple absolute measure of the absorbed amount of radiation (and therewith particle concentration). Too many difficult-to-determine or outright unknown factors influence the observed signal. Amongst these factors are spectroscopic quantities, such as quenching, and experimental quantities, such as observation angle and optics transmission, to mention 

Despite this analytical shortcoming, its extreme sensitivity accounts for the popularity of LIF in many fields, including the investigation of chemical processes, and for many decades LIF has been one of the dominant laser spectroscopic techniques in the probing of unimolecular and bimolecular chemical reactions. 

---

COLORANTS FORFOOD, DRUGS, COSTffiTICS AND TffiDICAL DEVICES] (Vol 6) -analysis using laser-induced fluorescence [SPECTROSCOPY, OPTICAL] (Vol 22) -drinkingwater [ANALYTICALTffiTHODS - PTYPHENATED INSTRUTffiNTS] (Vol 2)... 

The hydrolysis of the uranyl(VI) ion, UO " 2> has been studied extensively and begins at about pH 3. In solutions containing less than lO " M uranium, the first hydrolysis product is the monomeric U02(OH)", as confirmed using time-resolved laser induced fluorescence spectroscopy. At higher uranium concentrations, it is accepted that polymeric U(VI) species are predominant in solution, and the first hydrolysis product is then the dimer, (U02)2(0H) " 2 (154,170). Further hydrolysis products include the trimeric uranyl hydroxide complexes (U02)3(0H) " 4 and (1102)3(OH)(154). At higher pH, hydrous uranyl hydroxide precipitate is the stable species (171). In studying the sol-gel U02-ceramic fuel process, O nmr was used to observe the formation of a trimeric hydrolysis product, ((U02)3( -l3-0)(p.2-0H)3) which then condenses into polymeric layers of a gel based on the... 

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

Kim NJ, Jeong G, Kim YS, Sung J, Kim SK, Park YD (2000) Resonant two-photon ionization and laser induced fluorescence spectroscopy of jet-cooled adenine. J Chem Phys 113 10051... 

Bublitz [62] used time resolved laser induced fluorescence spectroscopy and fibre optics to determine polyaromatic hydrocarbons in oil polluted soils. The detection limit was 5mg kgy1 oil in soil. 

Hofzumahaus, A., U. Aschmutat, M. HeBling, F. Holland, and D. H. Ehhalt, The Measurement of Tropospheric OH Radicals by Laser-Induced Fluorescence Spectroscopy during the POPCORN Field Campaign, Geophys. Res. Lett., 23, 2541-2544 (1996). 

Monitoring the formation of Na atoms by laser induced fluorescence spectroscopy (at another wavelength) as a function of time shows a stepwise growth in the Na atom intensity, following the implications of the periodic vibration of the [Na—I] species exactly. Thus product formation depends on both a vibrational mode and a curve crossing both play a role along the reaction coordinate. 

Gabriel, U., Charlet, L., Schlapfer, C. W., Vial, J. C., Brachmann, A. Geipel, G. 2001. Uranyl surface speciation on silica particles studied by time-resolved laser-induced fluorescence spectroscopy. Journal of Colloid and Interface Science, 239, 358-368. 

Nevin A, Cather S, Anglos D, Fotakis C (2006) Analysis of protein-based binding media found in paintings using laser induced fluorescence spectroscopy. Anal Chim Acta 573-574 341-346. 

Silylenes are short-lived intermediates, and their detection requires fast methods such as ultraviolet (UV)24 or laser-induced fluorescence spectroscopy.25 The characteristic absorption maxima in the UV-visible spectra of these species, which are assigned to n - p transitions of electrons at the silicon atom, were used as a fingerprint to prove the occurrence of silylenes in matrices or solution. In addition, these transient species, which under normal conditions are too short lived to be observed by a slow detection method such as infrared (IR) spectroscopy, can be isolated in inert hydrocarbon or noble gas matrices, thus allowing the accurate measurement of their IR spectra. 

Gonzalez-Perez, M., Milori, D. M. B. P, Colnago, L. A., Martin-Neto, L. A., and Melo, W. J. (2007). Study of organic matter in Brazilian Oxisol under different tillage systems by laser induced fluorescence spectroscopy. Geoderma 138,20-24. 

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 purpose of this paper is to review the use of laser induced fluorescence spectroscopy (LIFS) for studying combustion processes. The study of such processes imposes severe constraints on diagnostic instrumentation. High velocities and temperatures are common, as well as turbulent inhomogeneities, and there is a need to make space and time resolved species concentration and temperature measurements. The development of LIFS has reached the point where it is capable of making significant contributions to experimental combustion studies. 

Chan, C. Daily, J. W. "Measurement of OH Quenching Cross-Sections in Flames using Laser Induced Fluorescence Spectroscopy," Western States Section/Combustion Institute Paper No. 79-20. 

Chan, C. "Measurement of OH in Flames using Laser Induced Fluorescence Spectroscopy" Ph.D. thesis, Department of Mechanical Engineering, University of California, Berkeley, 1979. 

Laser-Induced Fluorescence Spectroscopy Applied to the Hydroxyl Radical in Flames... 

PCAH have been observed in a flame using laser induced fluorescence spectroscopy by injecting individual species into the post-reaction zone. While the spectra are broadened by the elevated temperature, the spectra are qualitatively similar to low temperature (100 C) spectra and are indicative of the particular species injected. Thus, the injection procedure appears to be a feasible method of determining PCAH spectra at flame temperatures. These spectra will be used as a data base to determine individual PCAH concentrations in flames from their LIF spectra. 

---