Laser induced fluorescence kinetics

Figure 8.12 Laser-induced fluorescence kinetics of dissociation reactions. I, reaction of Nal followed at the absorption wavelength of the excited molecule II, same reaction monitored at the absorption wavelength of the free Na atom III, reaction of NaBr, followed at the absorption wavelength of the excited molecule...

Laser Photolysis-Laser-Induced Fluorescence Kinetic Studies... 

Ide, T., E. Iwasaki, Y. Matsumi, J.-H. Xing, K. Takahashi, and T.J. Wallington (2008), Pulsed laser photolysis vacuum UV laser-induced fluorescence kinetic study of the reactions of Cl( P3/2) atoms with ethyl formate, n-propyl formate, and n-butyl formate, Chem. Phys. Lett., 467, 70-73. 

In addition to measuring total recombination coefficients, experimentalists seek to determine absolute or relative yields of specific recombination products by emission spectroscopy, laser induced fluorescence, and optical absorption. In most such measurements, the products suffer many collisions between their creation and detection and nothing can be deduced about their initial translational energies. Limited, but important, information on the kinetic energies of the nascent products can be obtained by examination of the widths of emitted spectral lines and by... 

It is now possible to design the experiments using molecular beams and laser techniques such that the initial vibrational, rotational, translational or electronic states of the reagent are selected or final states of products are specified. In contrast to the measurement of overall rate constants in a bulk kinetics experiment, state-to-state differential and integral cross sections can be measured for different initial states of reactants and final states of products in these sophisticated experiments. Molecular beam studies have become more common, lasers have been used to excite the reagent molecules and it has become possible to detect the product molecules by laser-induced fluorescence . These experimental studies have put forward a dramatic change in experimental study of chemical reactions at the molecular level and has culminated in what is now called state-to-state chemistry. 

Houcine et al. (64) used a non-intrusive laser-induced fluorescence method to study the mechanisms of mixing in a 20 dm CSTR with removable baffles, a conical bottom, a mechanical stirrer, and two incoming liquid jet streams. Under certain conditions, they observed an interaction between the flow induced by the stirrer and the incoming jets, which led to oscillations of the jet stream with a period of several seconds and corresponding switching of the recirculation flow between several metastable macroscopic patterns. These jet feedstream oscillations or intermittencies could strongly influence the kinetics of fast reactions, such as precipitation. The authors used dimensional analysis to demonstrate that the intermittence phenomenon would be less problematic in larger CSTRs. 

P. Ho, M.E. Coltrin, and W.G. Breiland. Laser-Induced Fluorescence Measurements and Kinetic Analysis of Si Atom Formation in a Rotating Disk Chemical Vapor Deposition Reactor. J. Phys. Chem., 98(40) 10138—10147,1994. 

Experimental Techniques A absorption CIMS = chemical ionization mass spectroscopy CK = competitive kinetics DF discharge flow EPR = electron paramagnetic resonance FP = flash photolysis FT = flow tube FTIR Fourier transform intra-red GC = gas chromatography, UF = laser induced fluorescence LMR = laser magnetic resonance MS = mass spectroscopy PLP = pulsed laser photolysis SC = smog chamber SP = steady (continuous) photolysis UVF = ultraviolet flourescence spectroscopy... 

Gas phase kinetic studies of the reactions of hydroxyl radical are most conveniently carried out with direct monitoring of the OH radical with time using laser induced fluorescence (111. The low absorption coefficient of the aqueous hydroxyl radical ( 188nm 540 M 1 cm-1, (12)) precluded the direct measurement of this reactant species by its absorbance. Also, the absence of a readily observable product species for the reaction of OH + MSA at the wavelength range (275-575 nm) easily accessible in our experiments, has lead us to monitor the concentration of OH in solution indirectly by competition kinetics (13), measuring the absorption of the thiocyanate radical anion (ejsonm = 7600 M cm 1 (12)). 

Taylor, P.H., D angelo, J.A., Martin, M.C., Kasner, J.H., Dellinger, B. (1989) Laser photolysis/laser-induced fluorescence studies of reaction rates of OH with CH3C1, CH2C12, and CHC13 over an extended temperature range. Int. J. Chem. Kinet. 21, 829-846. Tewari, Y.B., Miller, M.M., Wasik, S.P., Martire, D.E. (1982) Aqueous solubility and octanol/water partition coefficient of organic compounds at 25.0°C. J. Chem. Eng. Data 27, 451 —454. 

Access to the final states is achieved by the use of standard techniques such as angular, time-of-flight mass spectrometry for kinetic energy and angular distribution, and laser-induced fluorescence or multiphoton ionization for the internal degrees of freedom. 

In the following we present an application of laser induced fluorescence to a study of the chemistry of sulfur in rich hydrogen/oxygen/nitrogen (H2/O2/N2) flames and demonstrate a simple rationale for taking quench effects into account. Fluorescence measurements for S2, SH, S02, SO, and OH along with measurements of flame temperature and H-atom (in sulfur free flames) have been employed to develop a kinetic model for the highly coupled flame chemistry of sulfur. The kinetic aspects of the study already have been presented in considerable detail (6). 

Kinetic measurements were made by monitoring the laser-induced fluorescence of CH following the excitation in the (0-0) band of the X — A transition as a function of the time delay after the ArF laser dissociation. In the absence of any added reactants, CH had a decay time of 100 to 300 /isec at a total pressure of 30 to 100 torr (CHBr3 pressures of 1 to 10 mtorr) which can be attributed mainly to the CH + CH reaction. The addition of the reactants listed in Table I shortened the CH radical decay times considerably, indicative of some removal process involving a bimolecular mechanism since the total pressure was always maintained constant. Least squares plots of the inverse lifetimes of CH radicals versus the partial pressure of the added reactant yielded... 

In this work, we have demonstrated that the CH radical can be generated with sufficiently high concentrations by means of the multiphoton dissociation of CHBr at 193 nm for kinetic measurements. The formation and decay of the CH radical was monitored by the laser-induced fluorescence technique using the (A2 b — X2ir) transition at 430 nm. Several rate constants for the reactions relevant to high temperature hydrocarbon combustion have been measured at room temperature. One of the key reactions, CH + N2, has been shown to be pressure-dependent, presumably due to the production of the CHN2 radical at room temperature. 

Measurement of DR branching ratios is perhaps the most problematic and contentious topic in experimentally-based interstellar chemistry. As the chief means of positive ion neutralization, DR is crucial in determining the eventual outcome of most, if not all, sequences of synthetic ion/molecule steps. Two fundamentally different techniques have been used for DR product analysis. The FALP technique of Smith and Adams, used with considerable success in the study of ion/electron recombination kinetics [171,176,177], has been adapted to permit subsequent neutral product detection by LIF (laser-induced fluorescence) or VUV (vacuum ultraviolet) spectroscopy, as shown in Fig. 13. Such studies, first... 

Donohoue et al. [31] has reported two other kinetic data sets for Cl and Br reactions using a pulsed laser photolysis-pulsed laser induced fluorescence spectroscopy. These data sets are obtained using pseudo-first order conditions with respect to halogens or mercury and experiments were performed at a broad range of temperatures. The authors of these studies indicate an uncertainty estimation of 50% in the rate coefficients due to the determination of absolute concentrations of chlorine and bromine atoms [31]. Sumner et al. [20] reinvestigated both reactions using a 17.3 m environmental chambers equipped with fluorescent lamps and sun lamps to mimic environmental reactions, and evaluated the rate constants... 

Spectroscopic methods are required for free radical intermediates. Laser induced fluorescence of hydroxyl radicals has been used successfully to determine elementary rate parameters associated with the isomerization reaction RO2 QOOH [113]. Laser perturbation of hydroxyl radical concentrations in stabilized cool-flames has been used to obtain global kinetic data for chain-branching rates at temperatures of importance to the low-temperature region [79]. These methods appear to be most suited at present to combustion studies in flow systems. There are also several studies of the relative intensity from OH radical fluorescence during oscillatory cool-flames [58,114]. 

T. Ueda, F. Kitamura, R. Mitchell, T. Metcalf, T. Kuwana, and A. Nakamoto, Chiral separation of naph-thalene-2,3-dicarboxaldehyde-labeled amino-acid enantiomers by cyclodextrin-modified micellar electro-kinetic chromatography with laser-induced fluorescence detection, Chem. 63 2919 (1991). 

On the experimental side, the coupling of crossed molecular beam techniciues with sophisticated detection techniques (Laser Induced Fluorescence, CARS or REMPI spectroscopy, Rydberg tagging photoionisation using synchrotron radiation or U lasers) has improved considerably the detailed study of chemicid reactivity. It is now possible to prepare reactants in a well defined state and to analyze the reaction products at a fixed scattering angle for a. collision at a well defined kinetic energy [1]. 

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