Picosecond spectroscopy chemical systems

Picosecond Spectroscopy and Applications to Chemical and Biological Systems... 

Picosecond spectroscopy provides a means of studying ultrafast events which occur in physical, chemical, and biological processes. Several types of laser systems are currently available which possess time resolution ranging from less than one picosecond to several picoseconds. These systems can be used to observe transient states and species involved in a reaction and to measure their formation and decay kinetics by means of picosecond absorption, emission and Raman spectroscopy. Technological advances in lasers and optical detection systems have permitted an increasing number of photochemical reactions to be studied in. greater detail than was previously possible. Several recent reviews (1-4) have been written which describe these picosecond laser systems and several applications of them... 

There is still a gap between our models of liquid-state reactions and the often bewildering complexity of real chemical systems. Progress in shortening the gap will probably come only from the application of a variety of methods to this problem. The full promise of picosecond spectroscopy techniques for studying the details of the dynamics of reactive events in liquids has yet to be realized. How deeply can these methods probe the dynamics Computer simulations, another source of experimental information in reacting systems, are only beginning to be exploited. "" The description by direct computer simulation of both primary and secondary recombination dynamics, for example, would yield a wealth of information that could be used to test theories. 

Automation In data acquisition and treatment can be aimed at a variety of objectives inherent in the above-mentioned factors. It should be pointed out that physical and physico-chemical kinetic factors play a decisive role in the reduction of human Intervention. The acquisition of data at a high rate imposed by the technique Itself (e.g. picosecond spectroscopy [19]) or by the system Investigated (e.g. meaurements of rates of reactions with half-lives of the order of a few milliseconds by the stopped-flow methodology [20,21]) demand the use of a computerized system without which application of the particular spectroscopic technique or method would not be feasible. On the other hand, the so-called microprocessor-controlled spectroscopy , widely commercialized at present, broadens the scope and facilitates the operator s work by eliminating various sources of error. 

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. 

Since there are a large number of different experimental laser and detection systems that can be used for time-resolved resonance Raman experiments, we shall only focus our attention here on two common types of methods that are typically used to investigate chemical reactions. We shall first describe typical nanosecond TR spectroscopy instrumentation that can obtain spectra of intermediates from several nanoseconds to millisecond time scales by employing electronic control of the pnmp and probe laser systems to vary the time-delay between the pnmp and probe pnlses. We then describe typical ultrafast TR spectroscopy instrumentation that can be used to examine intermediates from the picosecond to several nanosecond time scales by controlling the optical path length difference between the pump and probe laser pulses. In some reaction systems, it is useful to utilize both types of laser systems to study the chemical reaction and intermediates of interest from the picosecond to the microsecond or millisecond time-scales. 

MV2+ acceptors and SCN electron donors in solution [43], Colloidal semiconductor particles, typically of ca. 10-100 nm diameter, in aqueous sols may be treated as isolated microelectrode systems. Steady-state RRS experiments with c.w. lasers can be used to study phototransients produced at the surfaces of such colloidal semiconductors in flow systems [44], but pulsed laser systems coupled with multichannel detectors are far more versatile. Indeed, a recent TR3S study of methyl viologen reduction on the surface of photoex-cited colloidal CdS crystallites has shown important differences in mechanism between reactions occurring on the nanosecond time scale and those observed with picosecond Raman lasers [45]. Thus, it is apparent that Raman spectroscopy may now be used to study very fast interface kinetics as well as providing sensitive information on chemical structure and bonding in molecular species at electrode surfaces. 

Light is an ideal source of energy to drive artificial molecular machines. First, it can be used to monitor the state of the machine using various spectroscopy techniques. Second, the response is very fast, even on nano- or picosecond scales. Third, photo-driven processes in such systems are usually reversible, resulting in photo-driven molecular machines having an autonomous operation that is very important for the construction of molecular devices. And last, photo-driven molecnlar machines are clean and convenient compared with chemically driven machines and... 

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