Optical flash experiments

A recent electron paramagnetic resonance study (13) has shown that I also degrades by a photoreduction pathway in the presence of alcohols or water, through the intermediacy of the corresponding ketyl radical. Since corresponding ketyl radicals and triplet states often have overlapping absorption spectra, it is important to emphasize that the transient observed in our optical flash photolysis experiments is the triplet state (not the ketyl radical), the key evidence being that both the yield and the lifetime of the transient are decreased by triplet quenchers. An important question... 

Optical flash or laser photolysis experiments in which the optical absorption spectrum of the system is recorded as a function of delay time after a short photolysis laser pulse could... 

Due to the fact that lasers can be focused into a very small volume, small slits can be used together with a fast rotating disk to make the time resolution in the one-slit experiment in the tens of nanoseconds when using very sensitive detection techniques and samples with good Raman enhancements. This technique will probably be most useful in the microsecond time regime. Fig. 1 shows the results of this technique when used in the measurement of the time development of the bands characteristic of the intermediates produced in the bacteriorhodopsin photosynthetic cycle (8). Using optical flash photolysis (17) techniques, the rise time of the intermediate having a Raman band at 1570 cm l is known to be in the microsecond time scale. 

The diagnostics applied to shock experiments can be characterized as either prompt or delayed. Prompt instrumentation measures shock velocity, particle velocity, stress history, or temperature during the initial few shock transits of the specimen, and leads to the basic equation of state information on the specimen material. Delayed instrumentation includes optical photography and flash X-rays of shock-compression events, as well as post-mortem examinations of shock-produced craters and soft-recovered debris material. 

Two kinds of flash photolysis experiments on the interfacial reactions of electrons and holes have been reported. In the first case, the electrons and holes themselves were detected by their optical absorptions, and in the second case the products of their reactions were traced. 

The chemical properties of BA have been studied in detail (Lapin et al., 1984). Low temperature epr spectroscopy shows clearly that the ground state of BA is the triplet (3BA). The zero field parameters (Table 3) reveal some details of this structure. When the irradiation is performed at 4.6 K in a 2-methyltetrahydrofuran glass no epr signals from radical species are apparent. The optical spectrum under these conditions shows absorptions (Table 4) which disappear when the glass is warmed. From these findings the absorption bands are assigned tentatively to 3BA. This conclusion is strongly supported by results from laser flash photolysis experiments. 

The experiments on CO forms of the synthetic compounds were done with sample concentrations adjusted to 50 pM to give an absorbance of 1 over a 1 mm optical path at the Soret maximum. For the oxygen experiments, the samples were prepared as the CO complexes and covered with a balloon containing 3 1 mixture of CO to O. An additional photographic strobe lamp having a flash duration... 

Confusion over the matrix and gas-phase optical spectroscopy of PN spilled over to the liquid phase. Initial flash photolysis experiments involving phenyl azide gave conflicting results, with different authors favoring the presence of triplet phenylnitrene, " benzazirine BZ, or cyclic ketenimine as the carrier of the transient spectra. 

Figure 1. Schematic Diagram of the Apparatus and Optical Arrangement used for Diffuse Reflectance Laser Flash Photolysis 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. 

One common complication in flash photolysis studies is the production from the excited state of radicals which may be confused with or inhibit observation of the triplet state. The radical species, which usually has a longer lifetime than that of the triplet state, may be identified by separate experiments (Jackson and Porter, 1961) but such species may themselves show interesting acid base properties (e.g. Lindqvist, 1960 Lindqvist and Kasche, 1965 Simic and Hoffman, 1972 Neta, 1975). Their optical density contributions may therefore change with acidity in the range studied. 

To see this let us consider a very simple optical experiment. Let a flash of light be sent out from a point S half way between two points A and B on one of the two earths. The observer who does this experiment will consider that the light from S arrives at A and B at the same time because, according to Einstein s theory, any motion of his earth through spaces makes no difference so that the results he obtains will be the same as if his earth was at rest. 

The absorption spectra of the dyes were measured with a Shimadzu UV-3101 PC spectrophotometer (Japan) in a cell with a 1-cm optical path length. The fluorescence and fluorescence excitation spectra were studied with the use of a Shimadzu RF-5301 PC spectrofluorimeter. To study the triplet state of the dyes, apparatuses of flash photolysis with xenon lamp excitation (with an energy of 50 J and a pulse length at half maximum of xi/2 = 7 ps) [6] was used. To detect the triplet state of the dyes, the solutions were deoxygenated using a vacuum unit or purged with argon for experiments on the laser flash photolysis apparatus. A... 

Detection of transients by their U V and visible spectra is usually performed by the laser flash photolysis. Such experiments have been performed for more than 35 years, and much of the data on spectra of transients, especially radicals, and on their decay kinetics has been accumulated and has been summarized in reference books. Figures 12.11 and 12.12 demonstrate optical spectra of P-centered radicals. 

NaCl2 molecules.6 The existence of these molecules indicates an attractive exit channel for the reaction M + X2—MX + X. Optical and e.s.r. spectroscopy of y-irradiated KZnF3 crystals has shown7 that three types of perturbed F2 species are produced at 77 K. The mechanisms of the reactions of the radical ions Cl2, Br2, and I2, produced by nanosecond pulse radiolysis of the aqueous halides, have been established by y-radiolysis and flash photolysis experiments.8... 

ESR experiments were used to measure the kinetics of several types of reactions, those that can be monitored only by ESR, such as proton exchange or electron exchange reactions of radicals, and some that can be measured by other techniques as well, e.g. decay kinetics. Although most decay kinetics of phenoxyl radicals were followed by pulse radiolysis or flash photolysis by monitoring optical absorption, kinetics for some long-lived radicals were frequently monitored by ESR. For example, the second-order decay rates of 4-alkyl-2,6-di-f-butylphenoxyl radicals were measured to be 2200, 500 and 2 s ... 

---