Pump probe techniques chemistry

Pump-probe techniques use similar pulses to excite the sample to create the chemistry and to observe the reaction (see figure 5). The observation pulse is delayed so that time can be varied (see figure). The technique can be used both for absorption and emission (up conversion). The time resolution is then only limited by the width of the pulse that does the excitation. Multiple experiments must be done because each experiment will only measure one time. The approach can be used both for flash photolysis and for pulse radiolysis. 

A double-potential-step chronoamperometry (DPSC) experiment consists of two CA experiments. The potential of the second step is normally adjusted so that the R molecules formed upon reduction of O in the first step is reoxidized to O in a diffusion-controlled process, but it might also be adjusted to other values with the purpose of detecting other species formed [7]. In contrast to the CA technique, DPSC is a reversal technique, where the intermediates/products formed during the first step are probed directly in the second step. In this sense, it corresponds to a pump/probe experiment in photochemistry. While CA, in general, provides little if any information about follow-up chemistry, DPSC is a very strong tool for distinguishing between different mechanisms such as for example E, ECj, and DIMl. It is also a good tool for the determination of the relevant rate eonstants. 

Transient intermediates are most commonly observed by their absorption (transient absorption spectroscopy see ref. 185 for a compilation of absorption spectra of transient species). Various other methods for creating detectable amounts of reactive intermediates such as stopped flow, pulse radiolysis, temperature or pressure jump have been invented and novel, more informative, techniques for the detection and identification of reactive intermediates have been added, in particular EPR, IR and Raman spectroscopy (Section 3.8), mass spectrometry, electron microscopy and X-ray diffraction. The technique used for detection need not be fast, provided that the time of signal creation can be determined accurately (see Section 3.7.3). For example, the separation of ions in a mass spectrometer (time of flight) or electrons in an electron microscope may require microseconds or longer. Nevertheless, femtosecond time resolution has been achieved,186 187 because the ions or electrons are formed by a pulse of femtosecond duration (1 fs = 10 15 s). Several reports with recommended procedures for nanosecond flash photolysis,137,188-191 ultrafast electron diffraction and microscopy,192 crystallography193 and pump probe absorption spectroscopy194,195 are available and a general treatise on ultrafast intense laser chemistry is in preparation by IUPAC. 

The detailed knowledge of the different steps of biological processes on a molecular level is one of the ambitious goals of molecular biology. The importance of this field was underlined by the award of the Nobel Prize in chemistry in 1988 to J. Deisenhofer, R. Huber, and H. Michel for the elucidation of the primary steps in photosynthesis and the visual process [1511]. This subsection illustrates the importance of time-resolved Raman spectroscopy in combination with pump-and-probe techniques (Sect. 6.4) for the investigation of fast biological processes. 

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