Experimental flash techniques

For very low temperatures (to —180 °C) the cell is immersed in a bath cooled by liquid nitrogen. 

Once the absorption peaks of the transient species have been located, the changes of concentration with time can be determined much more accurately by flash kinetic spectropho- 

In the microsecond time range, several other monitoring techniques besides those of kinetic spectrophotometry can be used [9]. Conductivity methods may be useful where charged particles are involved, especially where changes of absorbance are small they are even more sensitive than spectrophotometry, and can be used for times down to picoseconds. Fast polarography, by pulse techniques, which monitor current-time curves immediately 

Attempts to extend flash-photolysis experiments to faster reactions was limited by the fact that flash lamps do not produce enough power unless their duration is at least a microsecond. With the advent of pulsed lasers (see below) in the 1960 s, it became possible to produce flashes with duration in the nanosecond range. 

Lasers exploit the phenomena of stimulated emission of energy. The principle is as follows. Consider a population of fluorophore molecules A (i.e., molecules capable of fluorescing when excited) in a bath of photons of uniform energy hv, produced, e.g., by a light-source. 

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A laser flash technique can be used to generate radical-cations and follow the kinetics of their reaction with nucleophiles [68, 69]. The experimental technique... 

The role of quinones in photobiological reactions involving chlorophyll has also been investigated (405,406). Despite the great effort and the multidisciplinary approach, progress in this field is slow because of the enormous complexity involved in photosynthetic systems. Hopefully the recent advances in experimental ESR technique, including the coupling of rapid scan ESR to flash photolysis, will help to elucidate the nature of the physical and chemical processes in photosynthesis. 

The difficulties associated with free radical absorption spectroscopy are almost entirely experimental ones and in the first part of this paper a brief account is given of the limitations of the existing methods with particular reference to the flash technique developed by the author. The second part deals with a few of the radical spectra which have been obtained by this method. 

Molecular orbital calculations indicate that cyclo C-18 carbyne should be relatively stable and experimental evidence for cyclocarbynes has been found [25], Fig. 3B. Diederich et al [25] synthesised a precursor of cyclo C-18 and showed by laser flash heating and time-of flight mass spectrometry that a series of retro Diels-Alder reactions occurred leading to cyclo C-18 as the predominant fragmentation pattern. Diederich has also presented a fascinating review of possible cyclic all-carbon molecules and other carbon-rich nanometre-sized carbon networks that may be susceptible to synthesis using organic chemical techniques [26]. 

The flash desorption technique is applied usually in ultrahigh vacuum conditions. Then all the mentioned contributions to S and F should be accounted for in the evaluation of the experimental desorption curves. The effect of Sw on the results of desorption measurements is discussed in... 

The optical absorption spectra of sulfonyl radicals have been measured by using modulation spectroscopy s, flash photolysis and pulse radiolysis s techniques. These spectra show broad absorption bands in the 280-600 nm region, with well-defined maxima at ca. 340 nm. All the available data are summarized in Table 3. Multiple Scattering X, calculations s successfully reproduce the experimental UV-visible spectra of MeSO 2 and PhSO 2 radicals, indicating that the most important transition observed in this region is due to transfer of electrons from the lone pair orbitals of the oxygen atoms to... 

Photoinitiation can be switched on and off extremely rapidly. For example, the time of laser flash can be as short as 1 psec (10-12 s) and shorter. The practical absence of time inertia of photoinitiation lies in the timescales of the experimental techniques for studying fast free radical reactions (flash photolysis, rotating sector technique, photo after-effect [109]). 

Flash photolysis techniques were unsuitable for measuring the slow off reactions for the iron(II) model complexes such as Fen(TPPS)(NO), since the experimental uncertainties in the extrapolated intercepts of kohs vs. [NO] plots were larger than the values of the intercepts themselves. When trapping methods were used to evaluate NO labilization from FeII(TPPS)(NO), k(,n values were found to be quite small but were sensitive to the nature of the trapping agents used. Lewis bases that could coordinate the metal center appeared to accelerate NO loss. More reliable estimates for the uncatalyzed off reaction were obtained by using Ru(edta)- as an NO scavenger, and the koS values listed in Table I were obtained in this manner (21c). The small kQ values found for Fe(II) models are consistent with the trend observed for the ferro-heme proteins discussed above. 

The systems that we investigated in collaboration with others involved intermolecular and intramolecular electron-transfer reactions between ruthenium complexes and cytochrome c. We also studied a series of intermolecular reactions between chelated cobalt complexes and cytochrome c. A variety of high-pressure experimental techniques, including stopped-flow, flash-photolysis, pulse-radiolysis, and voltammetry, were employed in these investigations. As the following presentation shows, a remarkably good agreement was found between the volume data obtained with the aid of these different techniques, which clearly demonstrates the complementarity of these methods for the study of electron-transfer processes. 

The vivid interest in hydrazine as a powerful propellant has stimulated many investigations both of its thermal decomposition and of its oxidation. Although hydrazine decomposes much more readily than ammonia, the study of its homogeneous decomposition by classical means using a static system is complicated considerably by wall catalysis. Thus, other experimental techniques have had to be applied, e.g. decomposition flames, flash photolysis, studies of explosion characteristics and the shock-tube technique. 

The kinetics of myoglobin oxidation and reduction have been studied by a variety of experimental techniques that include stopped-flow kinetics, pulse radiolysis, and flash photolysis. In considering this work, attention is directed first at studies of the wild-type protein and then at experiments involving variants of Mb. 

The way to improve LIBS analytical abilities is double pulse LIBS. It is conducted by using two pulses from the same laser source operated at a single wavelength. By utilizing a modified commercial laser, a proper external trigger circuit allows us to extract two laser pulses with adjustable delay from the same flash-lamp emission, and a relative time delay variable from a few to several tenths of microseconds. The experimental results show that such a technique... 

Electron attachment to solutes in nonpolar liquids has been studied by such techniques as pulse radiolysis, pulse conductivity, microwave absorption, and flash (laser) photolysis. A considerable amount of data is now available on how rates depend on temperature, pressure, and other factors. Although further work is needed, some recent experimental and theoretical studies have provided new insight into the mechanism of these reactions. To begin, we consider those reactions that show reversible attachment-detachment equilibria and therefore provide both free energy and volume change information. 

It is useful to briefly discuss some of the common and, perhaps, less common experimental approaches to determine the kinetics and thermodynamics of radical anion reactions. While electrochemical methods tend to be most often employed, other complementary techniques are increasingly valuable. In particular, laser flash photolysis and photoacoustic calorimetry provide independent measures of kinetics and thermodynamics of molecules and ion radicals. As most readers will not be familiar with all of these techniques, they will be briefly reviewed. In addition, the use of convolution voltammetry for the determination of electrode kinetics is discussed in more detail as this technique is not routinely used even by most electrochemists. Throughout this chapter we will reference all electrode potentials to the saturated calomel electrode and energies are reported in kcal mol. ... 

The rapid formation of molecular iodine following the flash photolytic dissociation of CHgl has been observed by time-resolved mass spectrometry.60 This has been attributed to the reaction (20) rather than to slow termolecular recombination. The experimental difficulties associated with sampling by this technique have been discussed by Meyer.61 This reaction is further discussed in Section IX.D on reaction of I(52Py2) with alkyl iodides. 

The 77-SCF MO Cl method has also been used446 to interpret spectral transitions of a series of possible intermediates in the reaction of uracil and cytosine with the solvated electrons eaq, produced by radiolysis of water. Experimentally this reaction has been investigated by Hayon,447 who used the technique of flash radiolysis. Hayon measured the optical-absorption spectra of the transient species in the UV range to obtain information on the site of attack of eaq on the pyrimidine base. At pH 5.0 the solvated electrons react with the pyrimidine molecules mainly at the C-2 and C-4 carbonyls, and the intermediates are rapidly protonatcd to give the corresponding ketyl radicals. For uracil Hayon found two absorption maxima (at 305 and < 280 nm) at pH 5.1 and one peak at 310 nm at pH 11.7. In this last case, on ionization of one of the chromophores the ketyl radical anion of the other nondissociated carbonyl is formed. Several species, 44, 45, 46, have been suggested by... 

In an entirely different experimental approach the unsymmetrical mixed-valence ion shown in equation (76) was subjected to laser flash photolysis.100 Excitation was carried out into the MLCT absorption band of the Ru11 -> 7t (pz) chromophore. Following excitation, one of the deactivation channels leads to the unstable mixed-valence isomer and its subsequent relaxation to the final, stable oxidation state distribution was observed directly using picosecond laser techniques. 

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