Flash or Laser Photolysis

This involves the application of a pulse of high intensity light of short duration to a solution containing one or more species. In the original Nobel prize winning studies a flash lamp of a few microseconds duration was used. Now a laser pulse is more often utilized and times as short as picoseconds or less may be attained. Several set-ups have been described. Their complexity and cost are related to the time resolution desired. An inexpensive system using 

The photolyzing light pulse is produced by a dye laser and enters the sample at about 10° to the axis of the sample beam. The observation beam originates from a 75-W xenon arc lamp. The apparatus is supplied by OLIS, Athens, Georgia USA. Reproduced with permission from C. A. Sawicki and R. J. Morris, Flash Photolysis of Hemoglobin, in Methods in Enzymology (E. Antonini, L. R. Bernardi, E. Chiancone eds.), 76, 667 (1981). 

The very rapid reaetion (3.15) with a large — AG can thus be measured. We therefore have an effective method for generating very rapidly in situ a powerful reducing or oxidizing agent. One of the most impressive applications of these properties is to the study of internal electron transfer in proteins. 

The Ru(NH3)j+ moiety can be attached to histidine-83 on the azurin surface. It can then be oxidized to Ru(III) without altering the conformation of the protein. This ruthenated protein is mixed with Ru(bpy)3+ and laser irradiated. The sequence of events which occurs is shown in the scheme 

In the first step, considerable amounts of the final product are produced as well as smaller amounts of a transient in which the oxidation states are incorrect . Internal electron transfer redresses this imbalance. The species Ru(bpy)3+ produced must be removed rapidly (by scavenging with edta) so that it cannot oxidize the Ru(II) protein and interfere with the final step.See Sec. 5.9. Some other examples of the application of the photolytic method to a 

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Ultrasonics Flash or laser photolysis Pulse radiolysis nmr ... 

J.V. Michael. Measurement of Thermal Rate Constants by Flash or Laser Photolysis in Shock Tubes Oxidation of H2 and D2. Prog. Energy Combust. Sci., 18 327-347, 1992. 

Large perturbations using flash or laser photolysis and shock tubes require a new equilibrium situation to be set up which is far from the initial equilibrium state. These methods are generally used in gas phase studies, and small perturbations are used for solutions, though there is nothing constraining the techniques in this way. 

The identity of the radicals formed from the initially excited molecule can be studied spectroscopically. If conventional radiation sources are used, the radicals will be formed in steady state concentrations and their rates of formation and removal cannot be measured. If, however, flash or laser photolysis is used the radicals are formed in much larger concentrations and their concentration-time profiles can be determined spectroscopically see Sections 2.1.4 and 2.5.2. From this, rate constants for the overall formation and removal of these radicals can be found. 

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... 

Experiments are performed behind reflected shock waves where the hot gas is effectively stagnant and not flowing. Flash or laser photolysis occurs after the reflected shock wave has traversed the spectroscopic observation station. Transient species are observed radially across the shock tube. Reflected shock pressure and temperature are kept low so that thermal decomposition is minimized. The initial transient species concentration is initiated by photolysis, and its decay is then totally determined by bimolecular reaction. Diffusion out of the viewing zone is negligibly slow on the experimental time scale. This experiment is then an adaptation of the static kinetic spectroscopy experiment with the reflected shock serving as a source of high temperature and density i. e., shock heating is equivalent to a pulsed furnace. 

In the laser photolysis experiments the aromatic compound (4-10" M) and the nucleophile (0 04 M ) in acetonitrile-water (1 1) were irradiated with the frequency doubled pulse (100 mj, 6 ns, 347 nm) of a ruby laser. Only time-dependent absorption changes were measured (double pulsed xenon flash lamp with 10 /is continuous output as light source) absorption spectra were constructed from these measurements at 12 or 25 nm intervals. 

Perhaps the most striking new result is that, in all the various reactions investigated so far by flash photolysis, the end products of the photosubstitution are formed within a period of 10 s or less. Free radical anions are formed in some of the systems they have lifetimes of the order of 10 -10 s and they do not contribute significantly to substitution product formation. Evidendy in order to trace intermediates of the substitution reaction we have to resort to still faster methods (laser photolysis. Section 4). 

The eventual products in reaction (1) have been identified as SO and MSA from experiments involving the steady photolysis of mixtures of DMS and a photolytic precursor of OH (4-91 Absolute measurements of lq have been obtained using the discharge-flow method with resonance fluorescence or electron paramagnetic resonance (EPR) detection of OH (10-141. and the flash photolysis method with resonance fluorescence or laser induced fluorescence (LIF) detection of OH (14-181. Competitive rate techniques where Iq is measured relative to the known rate constant for a reaction between OH and a reference organic compound (18-211 have also been employed to determine k at atmospheric pressure of air. 

For many ketone-amine photoinitiator systems, the ketyl radicals or their anions have been obtained through flash photolysis experiments or laser spectroscopy. Also, the results of CIDNP or ESR measurements and spin trapping experiments support the reaction pathway given in Scheme 8. 

As most of the free radicals are short-lived, direct monitoring of their reactions is not an easy task and powerful tools based on fast reaction techniques are required to follow such processes.Thus, fast reaction techniques utilize either short pulses of high-intensity flash of light or laser (in flash photolysis), or short pulses of charged particles and high-energy photons from accelerators (in pulse radiolysis). 

Figure 2.20. Absorption (black line) and fluorescence spectra of the AXFF in Ar-saturated cyclohexane at room temperature. Fluorescence spectra were obtained during the 266- and 355-nm (dark gray line) or 266- and 532-nm (light gray line) two-color two-laser flash photolysis. The absorption spectrum was obtained during one-laser photolysis (266-nm, black line) of AX (4.0 x 10 4M). The second laser irradiation was at 1 ps after the first laser pulse. All the fluorescence spectra of AXFb were normalized with the corresponding absorption peaks. Inset Kinetic traces of the fluorescence intensity of AXH- at 460 and 645 nm.

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