Kinetic flash photolysis

One of the main difficulties encountered in the determination of recombination rate coefficients is the minimization of the wall contribution. The utilization of fast flow systems with poisoned walls or the condition of short observation times relative to diffusion rates attained in the kinetic flash photolysis methods have been employed successfully in recent years. 

An apparatus to measure the gas phase recombination rate of bromine atoms over a range of temperatures by kinetic spectroscopy [29] is shown 

Experiments are performed with mixtures that are diluted several hundred fold with inert gas in order to control the temperature increase in the reaction cell caused by the exothermic recombination event. The magnitude of the thermal effects depends upon the fraction of molecules dissociated by the initial flash, the energy of the average absorbed quantum, the fraction of the cell volume sampled by the analysis beam, and the length of the observation period. Many of the difficulties may be overcome by recording data for a short time interval following the initiating flash [30]. 

A common procedure in mechanistic studies is to follow the growth or decay rate of a transient intermediate as a function of the concentration of some other species such as a sensitizer, quencher, trapping agent, acid or buffer. Bimolecular rate constants are then obtained from the concentration dependence of the observed first-order rate constants, /cobs (see Section 3.9.7). 

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Figure 10.7 Nanosecond kinetic flash photolysis apparatus...

Figure 10.7 shows a nanosecond kinetic flash photolysis apparatus. The absorbance of the sample is monitored, using a photomultiplier, by the change in the transmittance of the sample to the xenon arc lamp continuous light source. 

Using kinetic flash photolysis, the decay of the transient species can be determined as a function of time at the appropriate single wavelength found by the spectroscopic method. 

Scheme I. A summary of the reactions of RhCl(PPH3)2 as studied by the kinetic flash photolysis of RhCl(CO)(PPh )2-...

Methyl radicals were indeed detected at short delay times in flash photolysis , and ESR studies have demonstrated the presence of CHj and NO2 radicals . In addition, large yields of CH are formed in the photolysis, presumably by abstraction reactions of CHj . Recently, experiments by kinetic flash photolysis have shown that the primary step leads to C-N cleavage and that the formation of methyl nitrite is explained by step (b). In contrast to previous results the... 

The results of recent kinetic flash photolysis studies are consistent with this interpretation, but indicate that other processes may also be important. For example, flash photolysis of (CpMo(CO)3)2 in cyclohexane, THF and acetonitrile, with either visible (X > 460 nm) or ultraviolet (340 < X < 405 nm) light results in the formation of two transient products, both of which were detected at the earliest experimental observation times (t 50 ps) (110,115). Two subsequent thermal processes result in quantitative regeneration of the dimer complex. 

Let us generalize the quantum yield of a multi-step process x is equal to the product of the efficiencies of all steps required to complete that process. This allows us to determine rate constants by measuring quantum yields and lifetimes. Kinetic measurements such as time-resolved fluorescence or kinetic flash photolysis yield observed rate laws for the decay of excited states or of reactive intermediates. When the decay of an intermediate x obeys first-order kinetics, as is frequently the case, then the observed lifetime t = l/kobs is equal to the inverse of the sum of the rate constants of all processes contributing to the decay of the species observed.1... 

The Stem Volmer Equation 3.36 can be used for any photophysical or photochemical single-step process x. Moreover, because quantum yields, x ) / xq, is equal to that of the lifetimes of the reactive excited state or intermediate in the absence and presence of quencher (Equation 3.38). This relation provides a stringent test for the assignment of an intermediate x that is observed by time-resolved methods [fluorescence lifetime measurement (Section 3.5) or kinetic flash photolysis (Section 3.7.1)]. Assignment of on observed intermediate to the one that... 

The availability of software for the on-line analysis and documentation of data for transients from kinetic flash photolysis will be useful for examining the now very extensive compilation of data. 

The advantage of a TOF mass spectrometer is its rapidity in scanning a spectrum. The device is extremely useful in monitoring fast gas-phase kinetics, flash photolysis, and shock-tube experiments, and can be used in GCMS applications. It can be used in routine analytical applications, and the best designs have a resolution of around 500 with an upper mass limit of 1000. 

M. Eigen, R.G.W. Norrish, G. Porter Kinetics, flash photolysis... 

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