Decay kinetics, nanosecond laser flash photolysis

Phenyltrimethyldisilene (15) and (E)- and (Z)-l,2-dimethyl-l,2-diphenyldisilene (16) were also observed as transient absorption spectra by laser flash photolysis of the precursors in methylcyclohexanes28. The absorption band at 380 nm, assigned to the disilene 15, reached maximum intensity at ca 10 ns after the excitation and then started to decrease. The half-life assigned to 15 was 700 ns. The logarithm of the decay profile of the transient absorption at 380 nm versus time shows a very good linear relationship, indicating that the decay of the transient absorption fits first-order kinetics. This result shows that intramolecular isomerization or proton abstraction from the solvent is the origin for the decay of the disilene 15, which survives in solution only for several nanoseconds. 

All of the molecules in this study have triplet states which are easily detectable by the technique of nanosecond transmission laser flash photolysis. (11) The triplet state of acetoveratrone has a lifetime in excess of 15 ps in ethanol (Figure 2) under conditions of laser excitation the decay involves a mixture of first and second order kinetics, with the latter dominating at high laser powers. This second order decay demonstrates that the triplet state is decaying at least partly by triplet-triplet annihilation. 

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. ... 

Flash techniques were originally developed for the study of gas reactions [2,g] but were soon applied to solutions [2,h]. By the mid-60s, apparatus with a time-resolution of a few microseconds, using gas flash-lamps, had come into common use. With such equipment it was possible to identify transient species in solution from their spectra, and to determine their rates of decay and other processes. Excited states became recognised as distinct chemical species. The first study in which the spectra of the initial excited state, of the products and of some radical intermediates, were all detected in solution, and the kinetics investigated, was published in 1958 [2,k], Nanosecond pulses became available after the invention of the laser in 1960, but were not applied in flash photolysis until the problem of synchronising the analysing ( probe ) flash with the initiating ( pump ) flash was solved... 

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