Flash duration

Flash experiments lo ) show completion of mNP formation within the flash duration time (20 (xs). Therefore the short-lived species (Amax 340—350 nm and 490—510 nm, lifetime 40 ms in presence of OH and 0,3 ms in presence of methylamine) observed cannot be an intermediate and is probably the radical anion of mNA i h... 

To test the first hypothesis, solutions of 3,5-dinitroanisole and hydroxide ions were flashed and the absorption spectra at different time intervals after excitation were compared. The absorption ( max 400-410 nm) that remains after all time-dependent absorptions have decayed can be shown to be due to 3,5-dinitrophenolate anion, the photosubstitution product of 3,5-dinitroanisole with hydroxide ion. When the absorption band of the 550-570 nm species is subtracted from the spectrum of the solution immediately after the flash, there remains an absorption at 400-410 nm, which can also be ascribed to 3,5-dinitrophenolate anion. The quantity of this photoproduct does not increase during the decay of the 550-570 nm species. Therefore the 550-570 nm species cannot be intermediate in the aromatic photosubstitution reaction of 3,5-dinitroanisole with hydroxide ion to yield 3,5-dinitrophenolate. Repetition of the experiment with a variety of nucleophiles on this and other aromatic compounds yielded invariably the same result nucleophilic aromatic photosubstitution is, in all cases studied, completed within the flash duration (about 20jLts) of our classical flash apparatus. 

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... 

A 300-Kvolt X-ray FEXITRON unit was used, the flash duration being 0.12/isec and t he opt ical magnification 1.1. Ionization probes pi aced on t he axis of t he chge wer e... 

The stationary state concentrations of intermediates in a static system are very small to be detected by common spectroscopic techniques. But if a strong flash oflight is used, a large concentration of the intermediates may be generated which can easily be subjected to spectroscopic analysis. Essentially it is a relaxation method, the flash duration must suitably match the decay constant of the intermediates. The technique was developed by Norrish and Porter in early fifties. 

The energy released as heat in the course of the nonradiative decay of P to the ground state and detected as a pressure wave by laser-induced optoacoustic spectroscopy (LIOAS) exhibits positive deviations (i.e., a> 1 cf. Eq. (1)) from the values which were calculated on the basis of the absorption spectrum of Pr alone (Figure 15) [90,115]. This indicates that already within the 15-ns duration of the excitation flash, one or several intermediates must have been formed. These in turn, within the same interval, may again absorb light from an intense laser flash and (at least in part) dissipate heat upon their return to the ground state of the same species (internal conversion) and/or to Pr (photochemical back reaction). The formation of primary photoproducts within the nanosecond flash duration was of course to be expected in view of the much shorter lifetimes of the photochromic fluorescence decay compo-... 

Substrate Slow Fast Within flash duration Slow Fast Within flash duration G (ketoses) G (aldoses)... 

We therefore conclude that no directly formed precursor of the V-band is seen in our experiments, and the photoregeneration reaction must occur in times shorter than the flash duration (3 /zsec). 

The flash photolysis of COS has also been reported. A plot of the relative concentration of S2 from reaction (3) as a function of reaction time is shown in Figure 3. The cuiwe was obtained by flashing 50 torr of COS with a 1000-joule lamp of short flash duration, and taking the absorption spectrum of the irradiated mixture after various time delays. 

Flash Duration.—It has become clear that the flash time of i msec, is longer than the lifetime of some of the radicals studied and it will be necessary to use the following methods of reducing its duration. 

D13. Demas, J. N., and Crosby, G. A., Photoluminescenoe decay curves analysis of the effects of flash duration and linear instrumental distortions. Anal. Chen. 42, 1010-1017 (1970). 

Figure 18. Dependence of colorability (O) and fatigue resistance ( ) on spiropyran (SP) concentration. At concentration values higher than 2 x 10 6A/, the flash duration was increased from microseconds to milliseconds. Spiro[benzodithiolane-3-phenyl-6-nitro-8-methoxybenzopyran] (218, Table 17) was dissolved (5 x 10 5 M) in anhydrous toluene. (Reprinted from Ref. 33 with permission of Comite van Beheer van het Bulletin v.z.w.)...

Fig. 2.8 Time-of-flight experiment performed with poly(methyl phenyl silylene). Photocurrent trace recorded with a positively biased ITO electrode at F = 2.5xl0 V m" d=2 im, 2exc=347 nm, flash duration 20 ns. Adapted from Eckhardt et al. [36] with permission from Taylor Francis Ltd.

Fig. 11. Phosphorescence emission spectra of the stereoregular phenylcyclosiloxanes [PhSiO(OSiMe3)] where n = 4, 6, 8, and 12, measured together with the tetraphenyl-l,3-disiloxanediol, as solids (powder) at liquid nitrogen temperature, 7" = 77 K. Excitation wavelength = 320 nm, spectral bandwidth A/l= 8 ntn, edge filter (50% transmission at 370 nm). Excitation flash duration 3 ps (at half maximum), delay time /ji = 30 ps, sampling time window fsmp = 10 ms, cumulative emission from 10 flashes per point. Legend D2 - tetiaphenyl-disiloxanediol D4-D12 - phenylcyclosiloxane, ring sizes of n = 4-12 as indicated. Note the large maximum obtained from D6 (n = 6) at 460 nm.

OGP core complexes (d,e,f) Effects of calcium depletion d) BB Y inach membranes e)Ca + reconstituted OCa " depleted. (g,h,i) Effects of replacing CT with F" g) spinach OGP core complexes h)Cl -reconstitued i)F -substituted.All data at 0-C except c)20-C flash interval 2 secs flash duration 300 ns. 

Fig. 6.4 Benzylic radicals generated by a much shorter flash duration than in Fig. 6.3a from benzyl bromide, photographic trace and diagram of the spectrum by permission from [20]...

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