Flash kinetic spectrophotometry

Fermi golden rule, 268 Filipescu, N., 291 Fisch, M. H., 307 Fischer, F., 379 Flash photolysis, 80-92 of aromatic hydrocarbons, 89, 90 determination of jsc, 228-230 determination of triplet lifetime, 240-242 energy of higher triplet levels, 219-220 flash kinetic spectrophotometry, 82, 83 measurement of triplet spectra, 81,82 nanosecond flash kinetic apparatus, 89 nanosecond flash spectrographic apparatus, 88... 

Selected entries from Methods in Enzymology [vol, page(s)] Absorption spectroscopy, 24, 3 flash kinetic spectrophotometry, 24, 25 ion transport (H+, K+, exchange phenomena), 24,... 

Figure 10.10 Instrumentation for flash photolysis. (A) Flash spectroscopy (B) flash kinetic spectrophotometry.

Figure 4.2 Microsecond flash apparatus, (a) Schematic diagram of apparatus for microsecond flash spectropho-tography. (b) Schematic diagram of apparatus for microsecond flash kinetic spectrophotometry. After Ref. [2,b].

The purpose of this study is to explore the fate of OH radicals and the identity and chemistry of their progeny in seawater. This paper presents some of the experimental evidence concerning radical formation and behavior in seawater and artificial seawater obtained by the fast-reaction kinetics technique of flash photolysis-kinetic spectrophotometry (4) supplemented by pulse radiolysis ( ). The companion paper which follows presents results on related reactions and rates observed in media simpler than seawater and applies them to partially explain the data reported here using a simple reaction-mechanistic model. 

Zafiriou, 0. C. True. M. B. Flash photolysis - kinetic spectrophotometry of seawater and related solutions Data acquisition, processing, and validation," UHOI Tech. Memo. 1-77, Woods Hole Oceanographic Institution, 1977. 

Application of Spectrophotometry to the Study of Catalytic Systems H. P. LeftinandM. C, Hobson,Jr. Hydrogenation of Pyridines and Quinolines Morris Freifelder Modern Methods in Surface Kinetics Flash, Desorption, Field Emission Microscopy, and Ultrahigh Vacuum Techniques Gert Ehrlich... 

The pulse radiolysis method has been described in detail in some of the early papers (22, 22), in a brief review of the subject (23), and in a current comprehensive review (14). It is, in brief, a fast reaction method in which the external perturbation applied to the system is a microsecond pulse of electrons. The current is sufficiently high to produce an instantaneous concentration of transient species high enough to be observed by fast measurement of the optical absorption. Spectra may be recorded either photographically or spectrophotometrically. The kinetics are studied by fast spectrophotometry. Since a perturbing pulse as short as 0.4 /xsec. has been used, the time resolution has approached 10-7 sec. The flash photolysis method used in some of the other studies (27, 15) has been reviewed in detail (24). 

The kinetic behavior of solvated electrons has been followed directly using flash radiolysis (44, 45, 58) or flash photolysis technique (62, 94, 107). The former method is more universally applicable owing to the high absorption coefficient of e soiv in a spectral region where most reactants contribute little to the overall optical density. Stopped-flow spectrophotometry has also been applied in the specific case of the eaq + H20 reaction (43), but it is not applicable to reactions where the e soiv half-life is below 0.1 msec. 

The formation of 2(NO) was too rapid to monitor by standard stopped-flow spectrophotometry, but the kinetics could be accessed using a low-temperature stopped-flow accessory. Second-order rate constants varied little in the range of —40 to —70 °C, but a significantly negative (—118 J/(mol K)) value of the entropy of activation was obtained. The reverse reaction, dissociation of NO, could be characterized kinetically, but some doubt was expressed about the accuracy of the parameters. Laser flash photolysis could be applied in the ambient temperature range and the value of AS for the forward reaction of Equation 7.48 was confirmed at... 

Laser flash photolysis has been used to generate a hypervalent Mn =O species, which has an extremely high reactivity [569], related to that observed earlier in stopped-flow spectrophotometry experiments [32]. In contradiction to the Michaelis-Menten kinetics, it was found to react by second-order kinetics and to yield a TOF of 5.2 x 10 s , which is orders-of-magnitude higher than that obtained by standard methods with a Mn porphyrin [214]. It is concluded that the species obtained by flash photolysis is different from the intermediate involved in standard epoxidation with PhlO [214]. 

In 1976, Closs and Rabinow made the first measurement of rate constants for the reaction of a carbene. They used a flash photolysis technique. A brief flash of radiation generated carbene intermediates, and their decay was then monitored spectrophotometri-cally. In this way the rate constant for reaction of diphenylcarbene with 1,1 -diphenylethylene was found to be 4.8 x 10 s h Using this technique, measurements were possible on a microsecond time scale. With the advent of laser flash photolysis techniques, the resolution time was reduced to nanoseconds, causing a resurgence of interest in the kinetics of carbene reactions. The early results of use of this technique have been discussed by Griller and coworkers. ... 

Iodine in inert solvents such as hexane or carbon tetrachloride forms violet solutions containing I2 molecules which absorb in the visible. When such a solution is subjected to a microsecond flash, the absorbance decreases during the flash and afterwards returns to its original value at a rate that is easily monitored by microsecond flash spectrophotometry. Typical oscilloscope traces, representing optical intensity changes with time, are similar in form to that shown in Figure 4.9(b) above. The return follows second-order kinetics. The evidence indicates that iodine molecules are dissociated by the flash into atoms (which do not absorb visible light) and that the atoms subsequently combine ... 

The principle of the pulse method is the following. The initial substance, a source of radicals, is irradiated for a short time with a powerful flash of light or particles, which results in the formation of a sufficiently high nonequilibrium radical concentration. Their consumption is monitored by the method of high-performance spectrophotometry, and the consumption kinetics allows one to understand in which reaction and with which rate constant radicals are consumed. 

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