Laser-induced fluorescence technique

George, L.A. Development of a Direct, Low Pressure, Laser-induced Fluorescence Technique for NO2, Ambient Measurements and Urban NO, Ph.D. Thesis, Portland State University Portland, OR, 1991,1-135. 

The primary photochemical reaction for nitromethane in the gas phase is well supported by experiments to be the dissociation of the C—N bond (equation 98). The picosecond laser-induced fluorescence technique has shown that the ground state NO2 radical is formed in <5 ps with a quantum yield of 0.7 in 264-nm photolysis of nitromethane at low pressure120. The quantum yield of NO2 varies little with wavelength, but the small yields of the excited state NO2 radical increase significantly at 238 nm. In a crossed laser-molecular beam study of nitromethane, it was found that excitation of nitromethane at 266 nm did not yield dissociation products under collision-free conditions121. 

Table 3. Molecules studied by laser-induced fluorescence technique...

All three of these retro-Diels-Alder reactions give excited diene intermediates that decay in comparable times the x values range from 150 to 230 fs. The exact structural characteristics of these intermediates is currently unclear. Perhaps this issue could be addressed using femtosecond spectroscopic studies applying laser-induced fluorescence techniques, or through theory-based approaches. 

Unger, D.R. 8t Muzzio, F.J., Laser-induced fluorescence technique for the quantification of mixing in impinging jets AIChE J. 1999, 45(12), 2477-2486. 

By means of the sub-microsecond time resolution achieved by employing a pulsed laser photolysis/pulsed laser induced fluorescence technique, Hynes and wine are able to obtain k14a, k.14a and k and obtain an overall expression for kj4 as a function of temperature and [O2]- Their result in 700 torr of air is again substantially lower than the result from three competitive studies (Table IV)... 

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. 

Limits of detection for the nitrite ion are 0.02 nM. Reproducibility of the method is satisfactory. The method is simple, is rapid, and demonstrates advantages over the high-sensitivity laser-induced fluorescent techniques,... 

We have employed a pulsed laser photolysis - pulsed laser induced fluorescence technique to carry out direct, real time studies of OH reactions with DMS and DMS-dfc in N2, air, and O2 buffer gases. Both temperature and pressure dependencies have been investigated. We find that the observed rate constant (kQbs = d[0H]/[0H] [DMS]dt) depends on the O2 concentration. Our results are consistent with a mechanism which includes an abstraction route, a reversible addition route, and an adduct + O2 reaction which competes with adduct decomposition under atmospheric conditions. 

Also of significance is the fact that the laser itself has been utilized to determine relaxation rates of interest through the use of g-switching techniques. Kovacs, Flynn, and Javan [60] and Flynn, Kovacs, Rhodes, and Javan [61] first reported use of this technique to determine rates of importance to the C02 laser systems. Yardley and Moore [62] initially employed laser induced fluorescence techniques to determine V-V rates in methane. This technique was subsequently employed quite successfully in the determination of many important rates for the C02 laser system [63-65], The review by Moore [66] presents a critique of the potential of this experimental technique and an interpretation of results. 

Little has been published concerning the photodynamics of PuFe gas, although this compound was first synthesized in 1942. We recently reported the first observation of fluorescence from electronically excited PuFe and found its behavior to differ significantly from that of UFe or NpFe The photophysics of PuFe excited at 532 nm and at 1064 nm has now been observed in detail using laser-induced fluorescence techniques. The fluorescence emission spectra recorded are the same at both excitation wavelengths with the fluorescence intensity peaking at about 2300 nm. The fluorescence decay of PuFe gas excited at 532 nm was found to be laser-fluence dependent and a mechanism is proposed which accounts for this observation. Net photodecomposition of PuFe was rapid at a fluence of 5 J/cm at 532 nm (7 ns pulse width). 

The spectacular success of gene sequencing projects is largely due to the application of automated sequencing methods using laser-induced fluorescence techniques. The automation refers to the read-out process the preparation and loading of... 

In a recent crossed molecular beam experiment. Alagia et til. [36] measured the total DCS and product translational energy distribution for the N( D)+D2 reaction at 165 and 220 meV collision energies. They found an exact forward-backward symmetry which is consistent with an insertion mechcuiism rmd the existence of an intermediate complex. Using a laser-induced fluorescence technique. Umemoto [37] measured nascent rotational distributions and concluded that only the insertion mcchtmism is important in the N( D)- -H2 reaction at low mid medium cnergj-. This result has been recentlj confirmed by the measure of the product vibrational population for NH(f = 0,1.2,3,4) [38]. 

Miniaturization and integration of sensors into lab on a chip total analysis systems are likely to have very important implications in medical diagnostics and microanalysis of complex mixtures. The breakthroughs here have involved microfluidics and laser-induced fluorescence techniques, as well as ultramicroelectrodes tor ultrasensitive electrochemical analysis. 

Recently, Zewail and co-workers [54] applied the degenerate four-wave mixing (DFWM) technique with an ultrashort pulsed laser to a study of chemical reactions (unimolecular dissociation reaction of Nal, bimolecular reaction Na + H2) in the gas phase. Comparing this technique with other means, they showed that the wave packet motion extracted from the DFWM is in excellent agreement with results obtained from the laser induced fluorescence technique. 

Matsumoto j, Hieokawa j, Akimoto H and Kajii Y (2001) Direct measurement o/NOj in the marine atmosphere by laser-induced fluorescence technique. Atmos Environ 35 2803-2814. 

Lin Y W, ChiuT C, Chang HT (2003). Laser-induced fluorescence technique for DNA and proteins separated by capillary electrophoresis. J. Chromatog. B. 793 37-48. 

The latest experiments have used tunable laser induced fluorescence techniques to monitor the CN(A 2 ) fragments produced through photodissociation of ICN by a frequency quadrupled NdrYAG laser (X = 266.2 nm) or a flash lamp (X > 220 nm) ). They each concluded that virtually all the CN(JT) fragments are formed without vibrational excitation (Nj,=o-No= i > 1.00 0.02) but that they are rotationally excited with a distriTtution characterised approximately by a rotational temperature of 3000 K. While earlier time-of-fl ht photofragment spectroscopy experiments had been rationalised by assuming that CN(A 2 ) and CN(4 II)... 

Applying lasers as the excitation source and either a scanning monochromator coimected to a boxcar integrator or, better, an optical multichannel analyzer for the experimental setup has given rise to the development of the laser-induced fluorescence technique which can be used for diagnostic pmposes in many contexts [37]. Pulsed UV lasers like nitrogen, frequency-tripled Nd YAG or excimer lasers serve as the light source. 

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