Flash Photolysis Systems

As the name implies, this technique relies on flash photolysis to generate the reactive species A. In one of the most common configurations, resonance or induced fluorescence is used to monitor the decay of A—hence the name flash photolysis-resonance fluorescence (FP-RF). Since lasers are now frequently used as the photolysis source, the term laser flash photolysis-resonance fluorescence (LFP-RF) is also used. 

The limitations on the total pressure in the FP-RF cell are far less severe than those for FFDS. The lower end of the pressure range that can be used is determined by the need to minimize diffusion of the reactants out of the viewing zone. The upper end is determined primarily by the need to minimize both the absorption of the flash lamp radiation by the carrier gas and the quenching of the excited species being monitored by RF. In practice, pressures of 5 Torr up to several atmospheres are used. The kinetic analysis is again typically pseudo-first-order with the stable reactant molecule B in great excess over the reactive species as outlined earlier. Table 5.5 gives some typical sources of reactive species used in FP-RF systems. 

An example of the use of FP-RF to study the kinetics of an atmospherically relevant reaction is found in Fig. 5.8 (Stickel et al., 1992). Chlorine atoms were 

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Capellos and Suryanarayanan (Ref 28) described a ruby laser nanosecond flash photolysis system to study the chemical reactivity of electrically excited state of aromatic nitrocompds. The system was capable of recording absorption spectra of transient species with half-lives in the range of 20 nanoseconds (20 x lO sec) to 1 millisecond (1 O 3sec). Kinetic data pertaining to the lifetime of electronically excited states could be recorded by following the transient absorption as a function of time. Preliminary data on the spectroscopic and kinetic behavior of 1,4-dinitronaphthalene triplet excited state were obtained with this equipment... 

Laser flash photolysis experiments48,51 are based on the formation of an excited state by a laser pulse. Time resolutions as short as picoseconds have been achieved, but with respect to studies on the dynamics of supramolecular systems most studies used systems with nanosecond resolution. Laser irradiation is orthogonal to the monitoring beam used to measure the absorption of the sample before and after the laser pulse, leading to measurements of absorbance differences (AA) vs. time. Most laser flash photolysis systems are suitable to measure lifetimes up to hundreds of microseconds. Longer lifetimes are in general not accessible because of instabilities in the lamp of the monitoring beam and the fact that the detection system has been optimized for nanosecond experiments. 

Figure 18.1. Laser flash photolysis system developed hy Lindqvist. (Adapted from Ref. 2.)...

A Ruby Laser Nanosecond Flash Photolysis System, 1,4-Dinitronaphthalene , PATR 4445... 

One important difference in the design of a is conventional flash photolysis apparatus and the ns laser flash photolysis system is the size of the sample. The energy of laser pulses is usually very much lower than that of photographic flashes, typically 0.1 J as against 103 J. For this reason the laser light must be focussed on very small samples (0.1 ml for example). 

Spectroscopic techniques are often linked to chromatograph columns for separation of components, or to flow systems, flash photolysis systems, shock tubes, molecular beams and other techniques for following reaction. 

Figure 12.3 Block diagram of the laser flash photolysis system used in our laboratory.

Wong (1981) studied the competition between the self-reaction of t-C4H90 radicals and the reaction of r-C4H90 with several hydrocarbons in solution at 293 K. He used a flash-photolysis system with electron-spin-resonance detection of the radicals to measure the competitive reactions. Based on his earlier results for the hydrocarbon rate coefficients (Wong, 1979), he deduced the rate coefficient for the self-reaction to be (1.3 0.5) x lO A/ -sec at 293 K. The hydrocarbons used in the competitive experiments were cyclo-pentane, anisole, methyl-terr-butylether, and methanol, with respective rate coefficients for reaction with I-C4H90 of 3.4 X 10 , 7.2 x lO, 2.43 x 10 , and 1.29 x 10 M -sec . ... 

Kinetic Overview. The observations may best be introduced in summary by the kinetic map depicted in FIGURE 1. The kinetic behavior of the sample may be resolved into four distinct regions, temporally. Two (II, III) have been spectrally characterized and match those observed by Fischer et al (6). Third, there emerges (IV) a featureless "black" background transient absorbance, which may be traced from its origin in early nanosecond to its decay in the later microsecond domain. The final component (I) remained inaccessible to detailed study on both flash photolysis systems used owing to its appearance in an awkward tine domain. 

These principles were put into practice some 30 years latter by Porter and Norrish, who, however, were physical chemists, not biochemists. The early work was therefore directed to chemical ends, particularly the study of the triplet state - for which they shared the Nobel prize. There is a serious difficulty in all attempts to describe flash photolysis apparatus and experiments. It is that no single design of apparatus has ever been replicated in many laboratories. Rather, each group of experimenters have evolved their own equipment, tailoring its characteristics to suit the system under study. For the sake of concreteness, the properties of some of the principal elements of practical flash photolysis systems will be discussed, bearing in mind that cost is a meaningful laboratory parameter. 

Studies of the self-reaction between NO3 radicals have been conducted using both the stopped-flow and the slow laser-flash photolysis system. We prefer the results from the photochemical system, and these are reported in the table. The data are of importance in the analysis of laboratory data, especially for the slower reactions of NO3. 

Time resolved fluorescence measurements were carried out using a single photon counting apparatus. Transient absorption data were taken with a flash photolysis system as described by Durrant et al. 1989a. 

Modem techniques of fast kinetics make it possible to study almost any reaction. Many of the techniques are relatively similar some are quite complicated. The major difficulty is in properly defining the problem so that the results of the study are significant. Modem equipment and modem techniques can not create order out of chaos. Presently it is possible to purchase commercially much of the equipment necessary to make many of these fast measurements. Flash photolysis systems are commercially available. Temperature jump, pressure jump, stopped flow, fast flow and quenched flow instruments can be bought. In fact attachments to stopped flow equipment are commercially available to do many additional types of experiments. 

Fig. 8.1 Scheme of a nanosecond flash-photolysis system for transient absorption (PMT photomultiplier tube)... 

Figure 3.18 Schematic diagram of a flash photolysis system.

The rate coefficient for reaction of OH with methoxybenzene has been studied over a range of temperatures by Perry et al. (1977b), and near 298 K by Ohta and Ohyama (1985). Data are summarized in table III-F-1, and plotted in figure III-F-1. The data of Perry et al. (1977b) point to a complex reaction mechanism. At T < 330 K, the dominant reaction channel is irreversible formation of an OH/methoxybenzene adduct (adduct lifetime > 10 ms or so). At higher temperatures (T > 390 K), adduct decomposition is rapid on the timescale of the flash photolysis system and OH loss is dominated by a slower abstraction process (occurring at the methyl group). At intermediate temperatures, the OH temporal behavior is nonexponential as equilibration of the OH/methoxybenzene adduct occurs on the timescale of the flash photolysis experiment. 

The study of bimolecular gas reaction rate coefficients has been one of the primary subjects of kinetics investigations over the last 20 years. Largely as a result of improved reaction systems (static flash photolysis systems, flow reactors, and shock tubes) and sensitive detection methods for atoms and free radicals (atomic and molecular resonance spectrometry, electron paramagnetic resonance and mass spectrometry, laser-induced fluorescence, and laser magnetic resonance), improvements in both the quality and the quantity of kinetic data have been made. Summarizing accounts of our present knowledge of the rate coefficients for reactions important in combustion chemistry are given in Chapters 5 and 6. 

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