Instrumentation laser flash photolysis

This work was sponsored in part by the Office of Naval Research. In addition, acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. Acknowledgment is also made to NSF for assistance in purchasing the laser flash photolysis unit (Grant CHE-8A11829-Chemical Instrumentation Program). 

A diagram of a kinetic, ns, laser flash photolysis apparatus is shown in Figure 7.31. Transient absorption changes are similar to those obtained on conventional is instruments but the time-scales are of course much shorter. 

The decay of phosphorescence emissions can be observed easily with conventional flash photolysis instruments, since they last between ms and seconds. However, fluorescence lifetimes are of the order of ns and such kinetics can be measured only by laser flash photolysis or by time-resolved single photon counting. 

Laser Flash Photolysis. The instrumental set-up is similar to that used for transient absorption, except that there is no need for a monitoring beam. Figure 7.32 shows the rise and decay of the pyrene excimer in solution. 

Janata E (1992b) Instrumentation of kinetic spectroscopy. 10. A modular data acquisition system for laser flash photolysis and pulse radiolysis experiments. Radiat Phys Chem 40 437-443 Janata E, Lilie J, Martin M (1993) Instrumentation of kinetic spectroscopy. 11. An apparatus for AC-conductivity measurements in laser flash photolysis and pulse radiolysis experiments. Radiat Phys Chem 43 353-356... 

When the carbocations are generated by Laser flash photolysis, the ion pair collapse with the nucleophilic counterion Cl- is so fast [136] that the decay cannot be followed with the instrumentation used for these experiments, i.e., only those carbocations which manage to escape from the [Aryl2CH + Cl ] ion pair can be observed. Consequently, all rate constants determined for the Laser photolytically produced carbocations refer to the reactions of the nonpaired entities. 

Janata E. (1992) Instrumentation of kinetic spectroscopy-7. A precision integrator for measuring the excitation in laser flash photolysis and pulse radiolysis experiments. Radiat Phys Chem 39 315-317. 

Fig. 7. The instrumental set up at XI1 /DORIS for time resolved data collection with a linear detector for CO myoglobin following laser photolysis of the ligand. A section of the diffraction pattern with stationary crystal, stationary detector is recorded with a linear detector and (b) shows the time course of three reflections before and after the laser flash (from Bartunik et al. 1982)...

The other main instrumental development involving lasers has been the evolution of lasers pulsed to picosecond or even femtosecond time intervals by mode-locking. These yield rather weak pulses and are normally used in association with devices enabling data accumulation. The Nobel prize for chemistry for 1999 was awarded to Ahmed Zewail for his work in extending flash photolysis... 

Very fast reactions can be studied by flash photolysis, in which the sample is exposed to a brief flash of light that initiates the reaction and then the contents of the reaction chamber are monitored spectrophotometrically. Biological processes that depend on the absorption of Ught, such as photosynthesis and vision, can be studied in this way. Lasers can be used to generate nanosecond flashes routinely, picosecond flashes quite readily, and flashes as brief as a few femtoseconds in special arrangements. Spectra are recorded at a series of times following the flash, using instrumentation described in Chapter 12. 

The conditions which determine whether flash photolysis can be used to smdy a given chemical system are (i) a precursor of the species of kinetic interest has to absorb light (normally from a pulsed laser) (ii) this species is produced on a timescale that is short relative to its lifetime in the system. Current technical developments make it easy to study timescales of nanoseconds for production and analysis of species, and the use of instrumentation with time resolution of picoseconds is already fairly common. In certain specific cases, as we will see in the last part of this chapter, it is possible to study processes on timescales greater than a few femtoseconds. Once the species of interest has been produced, it is necessary to use an appropriate rapid detection method. The most common technique involves transient optical absorption spectroscopy. In addition, luminescence has been frequently used to detect transients, and other methods such as time-resolved resonance Raman spectroscopy and electrical conductivity have provided valuable information in certain cases. 

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