Fluorescence laser-excited

However, with the advent of lasers, the teclmique of laser-induced fluorescence (LIF) has probably become the single most popular means of detennining product-state distributions an early example is the work by Zare and co-workers on Ba + FLT (X= F, Cl, Br, I) reactions [25]. Here, a tunable laser excites an electronic transition of one of the products (the BaX product in this example), and the total fluorescence is detected as a... 

This teclnhque can be used both to pennit the spectroscopic detection of molecules, such as H2 and HCl, whose first electronic transition lies in the vacuum ultraviolet spectral region, for which laser excitation is possible but inconvenient [ ], or molecules such as CH that do not fluoresce. With 2-photon excitation, the required wavelengdis are in the ultraviolet, conveniently generated by frequency-doubled dye lasers, rather than 1-photon excitation in the vacuum ultraviolet. Figure B2.3.17 displays 2 + 1 REMPI spectra of the HCl and DCl products, both in their v = 0 vibrational levels, from the Cl + (CHg) CD reaction [ ]. For some electronic states of HCl/DCl, both parent and fragment ions are produced, and the spectrum in figure B2.3.17 for the DCl product was recorded by monitoring mass 2 (D ions. In this case, both isotopomers (D Cl and D Cl) are detected. 

It has been shown in Chapter 5, the fluorescence quenching of the DPA moiety by MV2 + is very efficient in an alkaline solution [60]. On the other hand, Delaire et al. [124] showed that the quenching in an acidic solution (pH 1.5-3.0) was less effective (kq = 2.5 x 109 M 1 s 1) i.e., it was slower than the diffusion-controlled limit. They interpreted this finding as due to the reduced accessibility of the quencher to the DPA group located in the hydrophobic domain of protonated PMA at acidic pH. An important observation is that, in a basic medium, laser excitation of the PMAvDPA-MV2 + system yielded no transient absorption. This implies that a rapid back ET occurs after very efficient fluorescence quenching. 

We will first consider possible assignments for the fluorescing states in laser-excited PuF6(g) based on available energy level structure and thermodynamic information. We will then consider some of the implications of the long-lived PuF6 fluorescence we have observed in terms of potential photochemical separation processes. 

A variety of formats and options for different types of applications are possible in CE, such as micellar electrokinetic chromatography (MEKC), isotachophoresis (ITP), and capillary gel electrophoresis (CGE). The main applications for CE concern biochemical applications, but CE can also be useful in pesticide methods. The main problem with CE for residue analysis of small molecules has been the low sensitivity of detection in the narrow capillary used in the separation. With the development of extended detection pathlengths and special optics, absorbance detection can give reasonably low detection limits in clean samples. However, complex samples can be very difficult to analyze using capillary electrophoresis/ultraviolet detection (CE/UV). CE with laser-induced fluorescence detection can provide an extraordinarily low LOQ, but the analytes must be fluorescent with excitation peaks at common laser wavelengths for this approach to work. Derivatization of the analytes with appropriate fluorescent labels may be possible, as is done in biochemical applications, but pesticide analysis has not been such an important application to utilize such an approach. 

Itoh, M. Adachi, T. Transient absorption and two-step laser excitation fluorescence studies of the excited-state proton transfer and relaxation in the methanol solution of 7-hydroxyflavone. J. Am. Chem. Soc. 1984, 106, 4320 -324. 

Itoh, M. Hasegawa, K. Fujiwara, Y. Two-step laser excitation fluorescence study of the ground- and excited-state proton transfer in alcohol solutions of 7-hydroxyisoflavone. J. Am. Chem. Soc. 1986, 108, 5853-5857. 

Richmond, M. D. and Yeung, E. S., Development of a laser-excited indirect fluorescence detection for high-molecular weight polysaccharides in capillary electrophoresis, Anal. Biochem., 210, 245, 1993. 

The use of near-IR-laser excited FT-SERS eliminates the disturbing fluorescence of impurities found with visible excitation, and provides SERS enhancement factors that are about 20 times larger than those found for excitation at 514.5nm [792]. For a strong Raman scatterer (fluorene), a typical detection limit of 500 ng is found for a 3-mm diameter spot. For weak scatterers, the detection limits may be in the high- xg region, which means that some compromise between chromatographic... 

Electron diffraction spectroscopy ETA LEAFS Electrothermal atomisation laser-excited atomic fluorescence... 

LEAFS Laser-excited atomic fluorescence scattering... 

Several characteristics of the metal beam have been studied in detail. It is well known that metal clusters and metal oxides are formed as a result of the ablation process. However, these potentially interfering species have been studied in detail130 and it has been concluded that they do not introduce any doubt as to the validity of the experimental results. Much more important than cluster or oxide formation are the atomic electronic state populations of the metal beams. For each metal reactant, these have been characterized using laser-induced fluorescence (LIF) excitation spectroscopy. For Y, only the two spin-orbit states of the ground electronic state (a Dz/2 and a D-3,/2) were observed.123... 

Time-resolved IR spectra of similar peptides following a laser-excited temperature jump showed two relaxation times, unfolding 160 ns and faster components <10 ns (Williams et al., 1996). These times are very sensitive to the length, sequence, and environment of these peptides, but do show that the fundamental helix unfolding process is quite fast. These fast IR data have been contrasted with Raman and fluorescence-based T-jump experiments (Thompson et al., 1997). Raman experiments at various temperatures have suggested a folding in 1 /xs, based on an equilibrium analysis (Lednev et al., 2001). But all agree that the mechanism of helix formation is very fast. 

Personov RI, AT shits LA, Bykovskaja LA (1972) The effect of fine structure appearance in laser-excited fluorescence spectra of organic compounds in solid solutions. Opt Commun 6 169-173... 

Fig. 2 LSCM view of dry and moistened (15 min) fluorescent vegetative microspore of Equisetum arvense under three laser excitation. 1 - channel 488 nm 2 - channel 533 nm 3- channel 633 nm 4-summed image with mixed (in a superposition pseudocolours. 1 bar = 20 pm.

Yuzefovsky et al. [241] used Cis resin to preconcentrate cobalt from seawater prior to determination at the ppt level by laser-excited atomic fluorescence spectrometry with graphite electrothermal atomiser. 

Laser-excited atomic fluorescence spectrometry has been used to determine down to 1 ng/1 of lead in seawater [359]. 

Cobalt Co(III) adsorbed on C18 bonded silica Laser excited atomic fluorescence spectrometry - [241]... 

Diazirine fluorescence provides additional support for RIES.22 Excited di-alkyldiazirines (but not alkylhalodiazirines) fluoresce, and the fluorescence intensity increases with decreasing temperature, suggesting the existence of a barrier to nitrogen loss from the excited diazirine.22 For example, dimethyl-diazirine (35) and 35-df fluoresce upon pulsed laser excitation, with emission due to the excited diazirines. 

The dynamics of the SHL intensity after subpicosecond UV laser excitation of RuC18B LB films is shown in Figure 32[115,116]. The SHL intensity decreased to 70 % of its initial value upon excitation and returned to almost the initial value within several hundred picoseconds as shown by a bold line. The fluorescence decay of RuC18B LB films measured by the single photon-counting... 

Figure 1.14 Raman spectra from a 0.1 wt% Mo03/y-AI203 catalyst obtained by using different (488, 325, and 244 nm) laser excitation energies [108], The UV-Vis absorbance spectrum is reported in the inset to indicate that while the catalyst does not absorb light in the visible region, it does show two UV absorption peaks at 290 and 220 nm. The data clearly illustrate the advantage of using ultraviolet (244 nm) light for Raman excitation, since the spectrum obtained with visible (488 nm) radiation is dominated by the fluorescence of the solid. (Reproduced with permission from Elsevier.)...

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