Nanosecond instrumental techniques kinetics

The wealth of structural and kinetic information obtained from TRIR spectroscopy makes it a powerful tool for establishing important structure-reactivity relationships for short-lived reactive intermediates. As demonstrated by the investigations discussed in this review, recent technical advances have greatly expanded the applicability of this technique so that organic systems can now be routinely examined in the nanosecond timescale. As the necessary instrumentation becomes even more refined and easier to implement, application of TRIR spectroscopy to a range of photochemical and photobiological problems with... 

As in all potentiostatic techniques, the double layer charging is a parallel process to the faradaic reaction that can substantially attenuate the photocurrent signal at short-time scale (see Section 5.3)" . This element introduces another important difference between fully spectroscopic and electrochemical techniques. Commercially available optical instrumentation can currently deliver time resolution of 50 fs or less for conventional techniques such as transient absorption. On the other hand, the resistance between the two reference electrodes commonly employed in electrochemical measurements at the liquid/liquid interfaces and the interfacial double layer capacitance provide time constants of the order of hundreds of microseconds. Consequently, direct information on the rate of heterogeneous electron injection from/to the excited state is not accessible from photocurrent measurements. These techniques do allow sensitive measurements of the ratio between electron injection and decay of the excited state under pho-tostationary conditions. Other approaches such as photopotential measurements, i.e. relative changes in the Fermi levels in both phases, can provide kinetic information in the nanosecond regime. 

The ILIT program at Brookhaven National Laboratory has been terminated. We have demonstrated that our best instrumentation can now attain nanosecond and possibly subnanosecond time resolution (Sec. IV.E Fig. 6). The holy grail for those who study ultrafast interfacial kinetics remains the development of a pump-probe technique whose time resolution would be limited only by the operative physical chemical processes (see Sec. V.E). We did not achieve that goal, but ILIT could be a component in... 

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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